Near-infrared Spectra of Chamaeleon I Stars

T HE A STRONOMICAL J OURNAL ,121:1163È1170,2001February(2001.The American Astronomical Society.All rights reserved.Printed in U.S.A.HUBBL E SPACE T EL ESCOPE NICMOS SPECTROSCOPY OF CHARONÏS LEADINGAND TRAILING HEMISPHERESC HRISTOPHED UMAS AND R ICHARD J.T ERRILEJet Propulsion Laboratory,Mail Stop 183-501,California Institute of Technology,4800Oak Grove Drive,Pasadena,CA 91109R OBERT H.B ROWNLunar and Planetary Laboratory,University of Arizona,1629East University Boulevard,Tucson,AZ 85721G LENN S CHNEIDERSteward Observatory,University of Arizona,933North Cherry Avenue,Tucson,AZ 85721ANDB RADFORD A.S MITHInstitute for Astronomy,University of Hawaii,2680Woodlawn Drive,Honolulu,HI 96822Received 2000August 1;accepted 2000November 6ABSTRACTWe used the near-infrared camera and multiobject spectrometer NICMOS on the Hubble Space Telescope to obtain 1.1È2.4k m,low-resolution (R D 200)slitless grism spectrophotometry of the individ-ual members of the Pluto-Charon system.Water ice is present in its crystalline state on both the leading and trailing hemispheres of Charon.A 2.21k m absorption band,possibly due to the presence of ammonia hydrate in surface,is detected in the reÑectance spectrum of its leading side only.Geological activity on Charon or implantation of ions escaped from PlutoÏs atmosphere could account for the for-mation of species such as locally on the satellite.We also measured a slightly higher geomet-NH 3ÉH 2O ric albedo for Charon than reportedfrom the mid-1980s observations of the mutual events.Key words:planets and satellites:individual (Charon)Ètechniques:spectroscopic1.INTRODUCTIONAlthough PlutoÏs discovery was made 70years ago,most of the information we have collected on its surface composi-tion has been obtained only very recently.Sensitive tele-scope instrumentation is now available to carry out spectrophotometric observations of the Pluto-Charon system at visible and,particularly,at near-infrared wave-lengths,a spectral range that provides an excellent diagnos-tic of the physical state of the ices present on the surface.The surface of Pluto was Ðrst suggested to be primarily covered with methane ice (Cruikshank,Pilcher,&Morrison 1976).Additional photometric observations,carried out at various rotational phases (Buie &Fink 1987),showed that methane is not distributed uniformly over the surface.Owen et al.(1993)reported that ice is in fact the mostN 2abundant compound on Pluto,followed by and CO.CH 4Currently,the planet is moving away from the Sunand itsatmosphere is slowly condensing on the surface (Stern,Trafton,&Gladstone 1988).While gas should also beN 2the main constituent of PlutoÏs atmosphere (Owen et al.1993),only methane gas has already been detected,from the band at 1.67k m (Young et al.1997),and CO gas awaits 2l 3detection.PlutoÏs surface displays patches of bright N 2-richregions and dark nitrogen-depleted areas (Jewitt 1994).N2ice may be concentrated in the polar regions at tem-peratures ¹40K (Tryka et al.1994),while ices with lower volatility should cover the equatorial regions.The surface temperature varies locally,and recent Infrared Space Obser -vatory measurements (Lellouch et al.2000)conÐrmed the presence of cold-bright regions and warm-dark regions.The visible albedo distribution across Pluto was determined from light-curve inversion techniques (Buie,Tholen,&Horne 1992;Young et al.1999)and direct imaging using the Hubble Space Telescope (HST )Faint Object Camera (Stern,Buie,&Trafton 1997).Both methods conÐrmed the presence of bright polar regions,while the equatorial zone displays a darker but highly variegated surface.Because of the small angular separation between Pluto and its satellite (from to our knowledge of 0A .30A .9),CharonÏs surface composition could,until recently,only be deduced from the analysis of the mid-1980s occultation events.These observations showed that Charon is mostly covered with ice (Buie et al.1987;Marcialis,Rieke,&H 2O Lebofsky 1987;Fink &DiSanti 1988).Later analysis by Roush (1994)and Roush et al.(1996)of the mid-1980s photometric measurements suggested that the surface of Charon could also be covered with patches of iceCO 2mixed with large-grained ice in addition to CH 4H2O.Dumas et al.(1999)reported HST Near Infrared Camera and Multi-Object Spectrometer (NICMOS)observations revealing for the Ðrst time the crystalline state of water ice over Charon.This result was conÐrmed by independent NICMOS observations of Charon made by Buie &Grundy (2000),as well as by ground-based observations carried out at Keck Observatory by Brown &Calvin (2000).Brown &Calvin also reported the presence of a 2.21k m band in their spectrum of Charon,which they attributed to the presence of ammonia ice on the surface.This paper presents the results of a reanalysis of the HST NICMOS observations obtained by Dumas et al.(1999).We conÐrm the presence of a 2.21k m absorption feature in the spectrum of Charon;however,this absorption appears to be detected only in the spectrum of the leading hemi-sphere.This result suggests that resurfacing processes,pos-11631164DUMAS ET AL.Vol.121TABLE1G EOMETRY OF THE P LUTO-C HARON S YSTEM DURING THE1998J UNE““C AMERA3C AMPAIGNÏÏProperty Jun11,UT1301a Jun14,UT1456aHeliocentric distance(AU)................30.068430.0692Geocentric distance(AU).................29.104929.1190Sub-Earth latitude(deg)...................[21.4[21.3PlutoÏs sub-Earth longitude(deg)........99.6(max.light curve)273.2(min.light curve)CharonÏs sub-Earth longitude(deg)......279.7(trailing side)93.2(leading side)Separation(arcsec).........................0.9080.917CharonÏs position angle(deg)............169.95347.65Spectral separation(pixels)............... 2.55(\0A.517) 2.70(\0A.548)a Time at beginning of G206series.sibly involving Pluto,are currently occurring on Charon.A mechanism is discussed that involves bombardment of CharonÏs surface by ions escaping from PlutoÏs atmosphere.2.OBSERVATIONSWe report spectroscopic observations of the Pluto-Charon system obtained in1998on June11and14(HST program GTO7223)at CharonÏs maximum elongation using HST and its near-infrared camera and multiobject spectrometer NICMOS(Thompson et al.1998).The details of the geometry of the Pluto-Charon system are described in Table1.Planetocentric coordinates were calculated for both dates using the1994IAU model(Davies et al.1995). NICMOS has three256]256HgCdTe low-noise detec-tors covering a0.8È2.4k m spectral range with pixel scales ranging from43mas pixel~1(camera1)to203mas pixel~1 (camera3).Camera3(NIC3)is equipped with a set of three grisms,an optical setup combining a prism and a grating, providing low-resolution(R D200)slitless spectro-photometric capabilities.In this paper,we discuss the results obtained using NIC3and the G141and G206 grisms,spanning a spectral range from 1.1to 2.4k m. Though NIC3normally produces slightly afocal images, our data were obtained during the1998June““camera3 campaign,ÏÏduring which HSTÏs secondary mirror was adjusted to achieve optimal optical performance.Pluto and Charon were separated by for the dates of our obser-0A.9vations.However,because of HST scheduling constraints, the line joining the binary system could not be oriented perpendicular to the dispersion direction;hence,the spatial separation of the spectra was reduced to(Table1).0A.5Table2summarizes the exposure parameters for the G141 and G206grisms.Because the wavelength solution of each spectrum is a function of the position of the science target on the detector,we initially obtained an image of Pluto-Charon through the F150W broadÐlter.Two spectra of Pluto-Charon(320s per exposure)were then recorded with the G141grism.A small telescope o†set was implemented between each exposure to collect data from di†erent loca-tions on the detector and to reduce the contamination ofTABLE2G RISM O BSERVATION P ARAMETERSParameter G141G206Spectral coverage(k m)...... 1.1È1.9 1.4È2.5Number of exposures (212)Total exposure time(s)......639.9767.4NICMOS sequence..........STEP64STEP16our data fromÑat-Ðeld residual e†ects,as well as to sample around bad pixels.The last step was to acquire the long-wavelength spectra using the G206grism.Because the G206 background level was D150times higher than for the G141 grism,shorter exposures were required in order to avoid saturation.We recorded a series of12spectra(64s each)at four di†erent locations on the detector(three images per position of the telescope).3.DATA REDUCTIONThe extraction and absolute photometric calibration of the spectra of Pluto and Charon were achieved using the ““NICMOSlookÏÏsoftware(Pirzkal&Freudling1998; McCarthy et al.1999),developed at the Space Telescope European Coordinating Facility for reduction of the NICMOS spectroscopic data.3.1.Separation of Pluto and CharonTheÐrst step in data reduction was to subtract the back-ground from the G206images.The background was esti-mated from a median combination of the set of four-position dithered G206images.The low background level for the G141grism was simplyÐtted during the extrac-tion.Because the spectra of the two objects partially overlap,we estimated their individual contributions and created,for each original spectrum,two images containing separate synthetic spectra of Pluto and Charon.Figure1 shows the result ofÐtting the contribution of Pluto and Charon along one detector column.Because the pixel~10A.2scale of NIC3oversamples the point-spread function(PSF), each image wasÐrst rebinned to aÐner grid(rebin factor\8)via bicubic convolution interpolation(Park& Schowengerdt1983).The angle of dispersion of the spectra with respect to the detectorÏs horizontal axis was measured, as well as the location where the spectrum was illuminating the exact center of one pixel.From the knowledge of these two parameters,we could estimate the width of PlutoÏs and CharonÏs PSFs.The separated synthetic spectra of Pluto and Charon were obtained by adjusting the amplitude and position of the two PSFs in order to minimize the variance of the di†erence between the original and the synthetic pro-Ðles(Fig.1).This measurement was made over the range of pixels covered by the two objects,and a goodÐt was reached after only three iterations.The process returned two images corresponding to the separated spectra of Charon and Pluto,which were rebinned back to the orig-inal resolution.The maximum count rate for Pluto in a single G206exposure was near330e~s~1,while the residual count rate was in the^13e~s~1range(NIC3 gain\6.5e~ADU~1).The residual image(original imageNo.2,2001NEAR-IR SPECTROSCOPY OF CHARON 1165F IG .1.ÈCross section of a rebinned G206spectrum of Pluto-Charon along the spatial direction (y -axis of the detector).The original proÐle is noted with diamonds,while the synthetic proÐle of Pluto plus Charon is indicated by the red line.The residual of the Ðt (original proÐle minus synthetic)is represented with a black line.For each original image,two synthetic images are produced by Ðtting the individual contributions of Pluto and Charon along the detector columns (three iterations of the Ðtting algorithm).The synthetic images are then used to extract the indi-vidual spectra of Pluto and Charon.minus synthetic)was used to improve the models of PlutoÏs and CharonÏs spectra.The total Ñux in a detector pixelF T due to Pluto and Charon is simply (F P )(F C )F T \F P ]F C,and the residual value for one particular pixel is res \F origwhere is the Ñux of the original image for this[F T ,F orig pixel.Then the corrected Ñuxes for the Ðts of Pluto and Charon are respectively and F P @\F P ](F P /F T )res F C@\This correction,performed individually F C ](F C /F T )res.forall pixels,allows us to conserve the original total Ñux of Pluto and Charon while correcting the models for the uncertainties in the individual pixel response and the exact position of the PSF with respect to the center of the pixel.3.2.Extraction of the SpectraAt this stage,the data have been processed in such a way that we can extract and photometrically calibrate the G141and G206spectra of Pluto and Charon.Version 2.9.2of NICMOSlook (Freudling &Pirzkal 1998;Pirzkal &Freudling 1998)was used for this purpose.The wavelength solution for all NICMOS grisms as a function of the posi-tion of the object on the detector has been parameterized.The only adjustment we made consisted of accounting for the planetÏs apparent motion,which had the e†ect of modifying the original position measured from the F150W image.The NICMOS detectors do not have spatially Ñat responses across their pixels.Thus,the Ñux measured in an undersampled pixel depends upon the location of the PSF core with respect to the pixel center.It was therefore neces-sary to extract the spectra using a model of the intrapixel response function (IPRF;see Lauer 1999for a detailed description).The average e†ect of the IPRF was larger for ““camera 3campaign ÏÏobservations than for observations carried out when the PSF was not exactly in focus for NIC3.Similarly,because the PSF is better sampled at longer wavelengths,the correction for the G206grism data was smaller than for the G141grism.We used the obser-vations of the solar analog P330-E collected during the 1998June ““camera 3campaign ÏÏ(HST program 7959)to determine the amplitude of a characteristic IPRF correction to apply to our G206and G141spectra.The scatter of the data points was measured before and after applying the IPRF correction.For this purpose,we deÐned the mean spectrum for each grism and subtracted it from the individ-ual spectra to calculate the means of the standard devi-ations over a deÐned range of wavelengths.After applying the IPRF,we measured a decrease in the means of the standard deviations of 10%and 40%,respectively,for the G206and G141grisms.3.3.Calibration3.3.1.Spectral CalibrationGiven that the wavelength solution for each spectrum is a function of the target location on the detector,the estima-tion of this location (from the F150W image and the appar-ent motion of the object)provides an additional source of error for the spectral calibration.The wavelength sampling of the spectra after extraction is 10nm pixel ~1(G206grism).Therefore we Ðrst rebinned the individual spectra to a Ðner grid prior to co-registering them (this process was similarly applied to the spectrum of the solar analog P330-E).The spectra of P330-E and Pluto-Charon were then co-registered,using the absorption bands visible in both spectra as Ðducial.Because of the lower signal-to-noise ratio (S/N)of the G206data,and the deep bands of methane and water present at these wavelengths,the co-registration process returned a less accurate wavelength calibration for the G206grism than for G141.The accuracy of the wavelength calibration of the spectra is of the order of 20nm for the G206grism,slightly better at shorter wave-lengths (G141grism).This is illustrated by the small shifts (¹20nm)in wavelength that can be measured for the 1.85k m band of methane and the 2.05k m band of water in Figure 2.Smaller wavelength shifts can also be noticed for the G141grism.Such shifts result from the uncertainty in the wavelength calibration of our data set and do not rep-resent any real hemispheric variations in the physical state of the surface ices.3.3.2.Radiometric CalibrationFor each pixel,the response function in wavelength used to Ñat-Ðeld the spectra was determined by interpolating the narrowband Ñat Ðelds (*j /j D 1%)obtained as part of the standard NICMOS Cycle 7calibration program.The inverse sensitivity curves (used by NICMOSlook to convert the spectral Ñux into millijanskys)were obtained for each grism from observations of the calibrator stars P330-E and G191-B2B collected during the 1998June ““camera 3cam-paign.ÏÏThe intensities of the individual G206spectra were adjusted to minimize the scatter in our data set,and the 1p errors represent the residual dispersion after parison of the G141and G206spectra shows a mis-match in the overlap region,and we estimate the G206Ñux to be 12%higher than the G141Ñux after ing recent ground-based observations of Pluto carried out by Owen et al.(1993)and recalibrated in geometric albedo by Roush et al.(1996),we Ðnd that the G141grism calibration is best matched by the ground-based results,giving geometric albedos of p D 0.55and p D 0.7at 1.9k m for the minimum and maximum of PlutoÏs light curve,respectively.The precision of the Ñux calibration returned1166DUMAS ET AL.Vol.121by the G141grism is higher because its lower background makes the extraction of the photometric calibrators more accurate.We therefore selected the G141grism as the refer-ence for our photometric calibration.Our spectra of Pluto and Charon were then divided by the spectra of the solar analog P330-E (which were extracted using NICMOSlook as described above).After correction of our spectra for the solar spectrum,we calibrated the spectra of Pluto and Charon in geometric albedo using the solar Ñux given by Labs &Neckel (1968)and the radii for Pluto (1151^4km)and Charon (591^5km)determined by Reinsch,Burwitz,&Festou (1994).An independent reference for the solar Ñux (Thekekara 1973)was used and returned the same cali-bration values.4.RESULTS AND DISCUSSIONFigure 2shows the calibrated NICMOS spectra of Pluto and Charon obtained for the two dates 1998June 11and 14.The spectra have been slightly smoothed (convolution with a 2pixel wide Gaussian)in this Ðgure in order to make their comparison easier.The original spectra with error bars are presented below,along with the modeling results in °4.2.2.4.1.PlutoThe focus of this paper being Charon,we will not provide a detailed model and discussion for the case of Pluto (Fig.2a )and will refer to et al.(1999)for recent modeling Douteof PlutoÏs spectra.Nevertheless,we will note,as a proof of the validity of the extraction procedure described above,that (1)the two grism spectra are self-consistent in the 1.60È1.85k m overlap region and (2)there is excellent agreement between our NICMOS spectra and the higher resolutionspectra obtained by et al.(1999).The strong bands of Doutemethane ice are clearly visible,as well as the 2.15k m dip due to particularly for the spectrum that corresponds toN 2,minimumof PlutoÏs light parison of our spectra at both light-curve minimum and maximum with the higher resolution ground-based spectra obtained at UKIRT by Cruikshank et al.(1997)also shows an excellent agreement between the two data sets for the 1.9k m ““continuum ÏÏregion.At shorter wavelengths,we note that our determi-nation of PlutoÏs albedo is slightly higher than that reported by the UKIRT measurements,but comparison of the rela-tive 1.22k m/1.9k m ratio of the continuum between CH 4our calibrated NICMOS spectra and recent ground-based data obtained at Keck (Brown &Calvin 2000)conÐrms that the continuum level for the blue end of the spectra is higherF IG .2.ÈG141and G206spectra of (a )Pluto and (b )Charon obtained during the ““camera 3campaign ÏÏof 1998June.The species responsible for the absorption bands detected in our spectra are noted on the Ðgures.Near-infrared spectroscopy of Charon shows that the 1.65k m feature of crystalline water ice is visible for both the leading and trailing sides.The 2.21k m absorption reported from Keck observations of Charon by Brown &Calvin (2000)is visible in the HST NICMOS spectrum,but for the leading hemisphere only.Whatever compound is responsible for(NH 3?)this absorption,it is more abundant on the leading sideof the satellite than on its trailing side.than previously reported.Additional discussion about the Ñux calibration of our NICMOS data is presented in °4.2.2.Table 3presents the broadband photometric measurements derived from our spectroscopic data set for both obser-TABLE 3N EAR -I NFRARED P HOTOMETRY OF P LUTO -C HARON P LUTO (mag)C HARON (mag)D ATE (1998)J a H b K c J a H b K c Jun 11......12.69^0.0112.66^0.0113.09^0.0414.74^0.0314.70^0.0214.81^0.12Jun 14......12.83^0.0212.81^0.0213.19^0.0414.70^0.0414.63^0.0414.81^0.10N OTE .ÈThese measurements are derived from the spectra in Fig.3.The error bars in magnitude reÑect only the 1p uncertainty plotted in the Ðgure.The absolute photometric accuracy of the grisms in NIC3is itself evaluated to be in the 5%È10%range.a J :1.15È1.35k m;0mag \1576.2Jy for k m.j eff \1.25b H :1.50È1.80k m;0mag \1018.6Jy for k m.jeff \1.65c K :2.00È2.40k m;0mag \672.8Jy for k m.j eff\2.20No.2,2001NEAR-IR SPECTROSCOPY OF CHARON 1167vation dates.We measure a magnitude di†erence *KD 0.1between maximum and minimum of PlutoÏs light curve,which agrees with previous ground-based observations.4.2.CharonFigure 2b compares the calibrated spectra of the leading and trailing hemispheres of Charon and shows that (1)water ice appears to be in its crystalline state on both hemi-spheres,(2)the HST NICMOS geometric albedo is slightly higher than reported from analysis of the mid-1980s mutual occultation events,and (3)spectral features characteristic of species such as hydrogen cyanide (HCN)or,more probably,ammonia hydrate are present in the spectrum(NH 3ÉH 2O)of CharonÏs leading side.4.2.1.Crystalline Ice versus AmorphousThe spectra of Figure 2b show the 1.65k m spectral feature characteristic of crystalline water ice for both the trailing and leading hemispheres of the satellite.Water ice has also been found in its crystalline state on the surfaces of the large satellites of Uranus (Grundy et al.1999).This suggests that resurfacing processes can occur in the outer solar system at a faster rate than required for water ice to turn into its amorphous state under the action of solar irradiation (Strazzulla et al.1992).A possible mechanism,proposed by Brown &Calvin (2000),implicates vapor-ization of the outermost layers of water ice on Charon by micrometeorite bombardments and recondensation of this ice into its crystalline state over the surface of the satellite.Figure 2b also shows di†erences between the spectral response of CharonÏs leading and trailing hemispheres.In particular,the spectral slope at short wavelengths is stron-ger for the leading side,which could be directly linked to the nature of the neutral absorber in surface.Also,the depth of the 1.55k m water band is shallower for the leading side,which might indicate hemispheric variations in the grain size distribution of water ice,although we would expect a similar behavior for the 2.02k m water band.The 2.21k m feature in the spectrum of CharonÏs leading side will be discussed below in °4.2.3.4.2.2.Geometric Albedo of CharonTable 3presents the broadband photometric measure-ments derived from the spectra of CharonÏs trailing and leading sides.The K -band magnitude di†erence between the hemispheres is within the uncertainty of our measure-ments,conÐrming that the leading and trailing sides of Charon have a similar brightness.The calibration of our HST NICMOS spectra returns a value for CharonÏs geo-metric albedo of p D 0.42^0.05in the 1.8k m water contin-uum region (Fig.2b ),which is slightly higher than the p D 0.34^0.05measurement (Roush et al.1996)derived from the mid-1980s mutual events (although both error bars overlap).The di†erence in CharonÏs geometric albedo reported in this paper is small,but larger than the uncer-tainty of the NICMOS grism calibration.The latter is esti-mated to be a few percent higher than the 5%È10%range reported by Freudling &Pirzkal (1998),mainly because of the processes applied to our data to separate the contribu-tions of Pluto and Charon.Furthermore,the good agree-ment between the NICMOS determination of PlutoÏs geometric albedo (Fig.2a )and earlier ground-based mea-surements supports the absolute calibration of our data.This sensibly higher determination of CharonÏs albedocannot be produced by residual contamination from the planet.Indeed,the immediate consequence would be to underestimate PlutoÏs albedo,whereas our measurements agree with the ground-based results.The slightly higher NICMOS albedo is consistent (within the error bars)with the earlier results and might simply reÑect larger uncer-tainties than previously estimated in CharonÏs diameter and in the photometric calibration of the mutual-event data.Nevertheless,we cannot entirely rule out the possibility that the di†erences measured in the albedos of Pluto and Charon could result from systematic errors in the radio-metric calibration returned by the NICMOSlook package.If the increase in albedo is real,then it might suggest time-variable phenomena over Charon,such as seasonal changes occurring while the system is moving away from the Sun.Also,the satellite is now observed with an aspect angle 10¡larger than during the 1987mutual events,presenting a slightly larger apparent cross section of its southern polar region.The present geometry could contribute to an increase in albedo if the southern polar region is made of material that exhibits a high reÑectivity at these parison with other icy satellites of the outer solar system is possible.Near-infrared spectroscopy of the large satellites of Saturn returns geometric albedo measure-ments in the range p D 0.2(Hyperion)to p D 1.0(Mimas).The NICMOS observations show that CharonÏs albedo of p D 0.4at 1.8k m is very similar to the albedo of IapetusÏs trailing side,which is typical of a ““dirty ice ÏÏcomposition.Figure 3shows the result of modeling the reÑectance spectra of CharonÏs leading and trailing hemispheres with an intimate mixture (D 90%)of crystalline water ice (Grundy &Schmitt 1998)and D 10%of a spectrally blue component,the latter being required to match the contin-uum level at short wavelengths.In this model,the water ice is made of 30k m grains while the blue component corre-sponds to 10k m particles having a reÑectance spectrum decreasing linearly from 8%to 6%over the 1.0È2.7k m region.Both spectra of Charon are reasonably well matched by the spectrum of pure water ice,except for the 2.21k m band and the short-wavelength region (CharonÏs trailing side).The modeling results do not require the addi-tion of ice or ice to improve the Ðnal Ðt.CO 2CH 44.2.3.T he L eading Hemisphere of CharonThe HST NICMOS spectrum of CharonÏs leading side(Fig.3b )is identical to the spectrum of the same side of the satellite obtained at Keck by Brown &Calvin (2000).It displays the same 2.21k m spectral feature that was attrib-uted to the presence of ammonia hydrate on the surface of the satellite.This additional and independent detection of the 2.21k m band suggests that this spectral feature is real and rules out the possibility that it could be produced by residual contamination from Pluto.The Ðrst obvious candidate for this 2.21k m feature is ice itself.Although we do not expect methane ice to CH 4still be present over CharonÏs surface,we nevertheless modeled the spectrum of CharonÏs leading side (Fig.4)with an intimate mixture of water ice (D 78%;30k m grain size),methane ice (D 15%;30k m grain size),and D 7%of the spectrally blue compound described above.The model pre-sented in Figure 4shows that adding methane ice to the mixture does not provide a good match to the features observed in the spectrum of CharonÏs leading side.The small amounts of methane used in the model can reproduce。

J.Dairy Sci.90:1122–1132©American Dairy Science Association,2007.Evaluating Mid-infrared Spectroscopy as a New Technique for Predicting Sensory Texture Attributes of Processed CheeseC.C.Fagan,*1C.Everard,*C.P.O’Donnell,*G.Downey,†E.M.Sheehan,‡C.M.Delahunty,§andD.J.O’Callaghan ʈ*Biosystems Engineering,UCD School of Agriculture,Food Science and Veterinary Medicine,University College Dublin,Earlsfort Terrace,Dublin 2,Ireland†Teagasc,Ashtown Food Research Centre,Dublin 15,Ireland‡Department of Nutritional Sciences,University College Cork,Cork,Ireland§Department of Food Science,University of Otago,PO Box 56,Dunedin 9015,New Zealand ʈTeagasc,Moorepark Food Research Centre,Fermoy,Co.Cork,IrelandABSTRACTThe objective of this study was to investigate the po-tential application of mid-infrared spectroscopy for de-termination of selected sensory attributes in a range of experimentally manufactured processed cheese samples.This study also evaluates mid-infrared spectroscopy against other recently proposed techniques for pre-dicting sensory texture attributes.Processed cheeses (n =32)of varying compositions were manufactured on a pilot scale.After 2and 4wk of storage at 4°C,mid-infrared spectra (640to 4,000cm −1)were recorded and samples were scored on a scale of 0to 100for 9attributes using descriptive sensory analysis.Models were devel-oped by partial least squares regression using raw and pretreated spectra.The mouth-coating and mass-form-ing models were improved by using a reduced spectral range (930to 1,767cm −1).The remaining attributes were most successfully modeled using a combined range (930to 1,767cm −1and 2,839to 4,000cm −1).The root mean square errors of cross-validation for the models were 7.4(firmness;range 65.3),4.6(rubbery;range 41.7),7.1(creamy;range 60.9),5.1(chewy;range 43.3),5.2(mouth-coating;range 37.4),5.3(fragmentable;range 51.0),7.4(melting;range 69.3),and 3.1(mass-forming;range 23.6).These models had a good practical utility.Model accuracy ranged from approximate quantitative predic-tions to excellent predictions (range error ratio =9.6).In general,the models compared favorably with previously reported instrumental texture models and near-infrared models,although the creamy,chewy,and melting models were slightly weaker than the previously reported near-infrared models.We concluded that mid-infrared spec-troscopy could be successfully used for the nondestruc-tive and objective assessment of processed cheese sen-sory quality.Received April 12,2006.Accepted October 30,2006.1Corresponding author:colette.fagan@ucd.ie1122Key words:descriptive sensory analysis,processed cheese,mid-infrared spectroscopy,chemometricsINTRODUCTIONOver 18million tonnes of cheese were produced world-wide in 2004,and processed cheese is an important seg-ment of this market (Wohlfarth and Richarts,2005).The United States,the largest producer of processed cheese (where 20%of all cheese consumed is processed cheese),produced 1,092,000tonnes in 2003(Wohlfarth and Ric-harts,2005).In the same year,the 25countries of the European Union produced 655,000tonnes of processed cheese (Wohlfarth and Richarts,2005).Consumer preference for a food product is principally determined by its sensory characteristics.Accurate mon-itoring and control of sensory properties will facilitate the production of high-quality products.A number of factors determine the final quality and sensory proper-ties of processed cheese (Caric´and Kala ´b,1993).These include the processing conditions used during manufac-ture,the composition of the ingredients,and the propor-tions of those ingredients added to the blend.Sensory profiling allows various quality attributes to be identified and their intensity determined (Brown et al.,2003).Sensory attributes are traditionally assessed by descriptive sensory evaluation using trained panel-ists.However,this is a time-consuming and expensive process that may lack objectivity (Blazquez et al.,2006).Although instrumental techniques such as texture pro-file analysis (TPA )and the 3-point bend test are avail-able for determining the texture attributes of food prod-ucts,these laboratory-based techniques are time-con-suming and require the use of skilled personnel in their execution (Blazquez et al.,2006).Therefore,considerable interest exists in the development of instrumental tech-niques to enable more objective,faster,and less expen-sive assessments of cheese quality to be made,including sensory aspects (Downey et al.,2005).Such a technique would assist producers to maximize yields,increasePREDICTION OF CHEESE SENSORY TEXTURE ATTRIBUTES1123throughput and efficiency,reduce labor costs,and opti-mize product quality,consistency,and customer satisfac-tion.Critical points in the manufacturing process could be monitored to ensure that the final product would meet required specifications.Recently,Kealy (2006)examined cream cheese using TPA,one of the main instrumental techniques for tex-ture measurement,and compared the results with those of a trained taste panel.Although a reasonably strong correlation was found between the taste panel results and TPA-derived hardness and adhesiveness parame-ters,the correlation for cohesiveness was not straightfor-ward.Everard (2005)also investigated the prediction of sensory attributes of processed cheese from instrumen-tal texture attributes derived from TPA,a compression test,and a 3-point bend test.He could predict the texture attributes of firmness,rubbery,creamy,chewy,frag-mentable,and mass-forming with a good level of accu-racy (Everard,2005).Spectroscopic analysis in combination with predictive mathematical models,developed using multivariate data analysis techniques such as partial least squares (PLS )regression,have potential use in controlling and monitoring the quality of raw materials through to the final product in food processing.In particular,infrared spectroscopy has been applied as an objective and nonde-structive technique to provide a rapid and real-time anal-ysis of both composition and quality (Downey,1998;Lefier et al.,2000;Ozen and Mauer,2002;Blazquez et al.,2004).Blazquez et al.(2006)modeled the sensory attributes of processed cheese using near-infrared re-flectance spectroscopy and PLS regression.They found that it was possible to model a number of attributes including firmness,melting,rubbery,and creamy.Two other studies have investigated the prediction of sensory attributes in natural cheese.Downey et al.(2005)and Sørensen and Jepsen (1998)successfully demonstrated that near-infrared spectroscopy in conjunction with PLS regression can be used to predict several sensory attri-butes of Cheddar and Danbo cheeses,respectively.Mid-infrared spectroscopy has been most widely used for de-termination of the fat and protein contents of cheese (Chen and Irudayaraj,1998).Irudayaraj et al.(1999)also investigated the use of mid-infrared spectroscopy to follow texture development in Cheddar cheese during ripening.They demonstrated that springiness could be successfully correlated with a number of bands in the mid-infrared spectra.Research has shown that mid-in-frared spectroscopy is a useful technique for characteriz-ing the changes in proteins during cheese ripening (Ma-zerolles et al.,2001).Pillonel et al.(2003)also found that mid-infrared spectroscopy may be successfully applied to the discrimination of Emmental cheese based on geo-graphic origin.Journal of Dairy Science Vol.90No.3,2007Table 1.Quantity of ingredients (g/kg)used in the production of experimental processed cheese samples Sample Emulsifyingnumber(s)Cheddar Butter Water salt1,10838.70.0161.39.72838.70.0151.619.43,11838.70.0141.929.04,12838.751.6112.99.75,13838.751.6100.019.46,14838.751.690.329.07,15838.7100.061.39.78838.7100.051.619.49,16838.7100.041.929.017,25848.451.6103.29.718,26838.751.6100.019.419,27829.051.6100.029.020,28751.645.2203.29.721,29745.245.2203.219.422,30738.745.2200.025.823,31651.638.7303.216.124,32645.238.7303.222.6No data are currently available on the application of mid-infrared spectroscopy to determine the sensory attributes in processed cheese,or regarding evaluation of mid-infrared spectroscopy in comparison with other technologies in such an application.Therefore,the objec-tives of this study were to investigate the use of mid-infrared spectroscopy in predicting sensory texture attri-butes using a range of experimentally manufactured pro-cessed cheese samples and to compare the models devel-oped with those recently modeled using near-infrared spectra and instrumental texture attributes.These newly presented data allow for the critical evaluation of mid-infrared spectroscopy as a rapid,nondestructive technique for predicting the sensory texture attributes of processed cheese.MATERIALS AND METHODSProcessed Cheese SamplesThirty-two processed cheese batches were manufac-tured in a pilot plant at Moorepark Food Research Cen-tre,Cork,Ireland.The ingredients and formulations are listed in Table 1.The formulations,which were selected to provide samples with compositional ranges that ex-tended beyond those used commercially by processed cheese manufacturers,provided samples with a wide range of sensory characteristics.The ingredients were mixed for 1min in a jacketed cooker (Stephan UMM/SK5Universal cooker;Stephan u So ¨hne GmbH &Co.,Hameln,Germany).The blend was cooked at 80°C for 2min by indirect steam heating.During cooking,the blend was stirred constantly using a knife at 300rpm and a baffle mixer at 80rpm.The cooked blend was stored in food-grade plastic containers (225g capacity),FAGAN ET AL.1124Table2.Vocabulary of sensory attributes,their definitions,and mastication phases used to carry out the sensory analysis of processed cheese samplesSensoryattribute Definition Mastication phaseFirmness The extent of the initial resistance offered by the cheese,Phase1:Judged on thefirst chew using the front teeth ranging from“soft”to“firm”Rubbery The extent to which the cheese returns/springs to its Phase2:Assessed during thefirst2to3chews initial form after biting,ranging from“a little”to“a lot”Creamy The texture associated with cream that has been whipped,ranging from“a little”to“a lot”Chewy The effort needed to break down the structure of the cheese,Phase3:Judged in the middle phase of mastication ranging from“a little”to“a lot”Mouth-coating The extent to which the cheese clings to the insideof the mouth(roof,teeth,tongue,gums),ranging from“a little”to“a lot”Fragmentable Breaks down to smaller versions of itself,ranging from Phase4:Probably judged toward the end of the chewing “a little”to“a lot”Melting The extent to which the cheese melts in the mouth;smooth,velvet fullness in the mouth,ranging from“a little”to“a lot”Mass-forming The extent to which the cheese form a bolus or massin the mouth after chewing,ranging from“a little”to“a lot”Greasy/oily The extent to which a greasy/oily residue is deposited Phase5:Judged at the end of the chewing sequence in the mouth after the cheese is broken down,rangingfrom“a little”to“a lot”which were lidded,allowed to cool,and placed in storage at4°C for4wk.The independent compositional variables for samples1to16were fat and emulsifying salt,and the variables for samples17to32were moisture and emulsifying salt.Descriptive sensory analysis and mid-infrared spectroscopy were carried out at2and4wk postmanufacture.Sensory AnalysisA panel of10assessors(9females and1male),aged 35to55yr old,were selected and recruited in1998and 2000according to international standards(International Organization for Standardization,1993).The panel de-veloped a vocabulary of9texture terms:firmness,rub-bery,creamy,chewy,mouth-coating,fragmentable, melting,mass-forming,and greasy/oily(Table2),which they used to assess each sample.Samples were prepared for analysis in duplicate by removing them from storage and preparing two5-g cubes.These samples were left to equilibrate to room temperature(21°C).Each equili-brated sample was presented to assessors in a glass tumbler covered with a clock glass and labeled with a randomly selected3-digit code.Assessors were provided with deionized water and unsalted crackers to cleanse their palate between tastings.The assessors scored the samples for each attribute by marking on unstructured 100-mm line scales labeled at both ends with extremes of each attribute.The intensity of each of the descriptive terms was recorded using the Compusense v.4.0sensory data acquisition software(Guelph,Ontario,Canada).At each time point,the descriptive sensory analysis took Journal of Dairy Science Vol.90No.3,2007place over2d.The order of tasting was balanced to account for the order of presentation and carryover ef-fects(MacFie et al.,1989).All assessments were con-ducted in individual booths at the sensory laboratory at University College,Cork,which complies with interna-tional standards for the design of test rooms(Interna-tional Organization for Standardization,1988).Mid-Infrared SpectroscopyMid-infrared spectra were collected over the range of 4,000to640cm−1,with a resolution of8cm−1,using an ATI Mattson Infinity Series Fourier transform spectro-photometer(ATI Mattson,Madison,WI)controlled by WinFirst software(ATI Mattson).The sample accessory used for sample presentation was an attenuated total reflectance ZnSe crystal(Graseby Specac Ltd.,Kent, UK),with an incidence angle of45°and6internal reflec-tions.Sixty-four interferograms were coadded before Fourier transformation.Prior to mid-infrared analysis, samples were removed from storage and left to equili-brate to room temperature.This was confirmed prior to analysis using a digital thermocouple(Sensor-Tech Ltd., Co.Louth,Ireland).Processed cheese samples were wiped across the attenuated total reflectance crystal to ensure even and immediate contact.Triplicate spectra were captured for each sample and replicate spectra were averaged prior to data analysis.Multivariate Data AnalysisMultivariate data analysis was carried out using The Unscrambler software(v.8.0;Camo A/S,Oslo,Norway).PREDICTION OF CHEESE SENSORY TEXTURE ATTRIBUTES1125Figure 1.Typical mid-infrared spectrum of processed cheese.Principal component analysis of the spectra was used to examine the spectral data set for any possible outliers.Models for the prediction of sensory attributes were de-veloped using PLS regression and confirmed by cross-validation.Prior to PLS regression,spectra were pre-treated using multiplicative scatter correction (MSC ),first derivative (Savitzky-Golay,2data points each side),second derivative (Savitzky-Golay,4data points each side),and each derivative plus MSC (Geladi et al.,1985).The potential of the models to predict the sensory attri-butes was evaluated using the root mean square error of cross-validation (RMSECV ),correlation coefficient (r )and the number of PLS loadings (#L ).The range error ratio (RER )was used to determine the practical utility of the models (Williams,1987).It was calculated by di-viding the range in the reference data of a given attribute by the prediction error for that attribute.The ratio of prediction error to deviation (RPD )was calculated by dividing the standard deviation of the reference data by RMSECV.RESULTS AND DISCUSSIONMid-Infrared SpectraA number of studies have assigned the main cheese constituents (fat,protein,moisture)to specific bands in the mid-infrared spectra (Chen and Irudayaraj,1998;Chen et al.,1998;Irudayaraj and Yang,2000;Mazerolles et al.,2001).The positions of these bands are indicated in a typical mid-infrared spectrum of processed cheese from this study (Figure 1).The results of principal component analysis of the spectra were investigated to determine whether any in-fluential outliers were present in the data set.An influ-ential outlier is a sample that has both a high residual and high leverage.A high residual means that the model,Journal of Dairy Science Vol.90No.3,2007Table 3.Statistical summary of sensory attributes (n =64)Sensory attribute Mean Range SD Firmness 37.6 5.6–70.921.9Rubbery 23.3 2.9–44.614.4Creamy 35.710.7–70.722.6Chewy24.8 2.8–46.114.6Mouth-coating 33.117.4–54.810.0Fragmentable 25.6 1.4–52.420.0Melting40.113.2–82.523.6Mass-forming 10.5 1.8–25.4 5.5Greasy/oily36.628.5–43.83.9which nevertheless fits the other samples quite well,poorly describes the sample.Leverage measures the dis-tance from the projected sample to the center or mean point.If a sample has a high leverage,it is exerting a stronger influence on the model than the remaining samples.According to these criteria,no outlier was found.Previous research has recommended that prior to analysis,a portion of the mid-infrared spectra (1,800to 2,700cm −1)might be omitted because of its low signal-to-noise ratio (Pillonel et al.,2003).This approach was used in this study,with the region 1,775to 2,830cm −1having a low signal-to-noise ratio,and was therefore omitted from analysis.In a preliminary investigation of the spectra,the region 640to 923cm −1was found to be of limited use in predicting sensory attributes and was also omitted.Therefore,only spectral data in the ranges of 930to 1,767cm −1and 2,839to 4,000cm −1were used for the multivariate data analysis.Predication of Sensory Texture Attributes by Mid-infrared SpectroscopyA summary of the values scored by the taste panel for each of the 9sensory attributes is shown in Table 3.The table highlights the high degree of variability in the data,which should support the development of robust models.Models were developed using 1)the combined spectral ranges of 930to 1,767cm −1and 2,839to 4,000cm −1,and 2)930to 1,767cm −1.The spectra were used in a number of forms:raw,MSC,first derivative,second derivative,and MSC plus each derivative step,giving 12models for each sensory attribute.A second derivative step offered no improvement in model accuracy for any attribute;hence,those prediction results are not shown.The RMSECV,r,and #L values obtained from the models developed are given in Table 4for the combined spectral range or the 930to 1,767cm −1range.These parameters allow for assessment of model strength.The preferred predictive model for an attribute (highlighted in bold in Table 4)was that which produced the lowest RMSECVFAGAN ET AL.1126Table4.Summary of partial least squares prediction results for sensory attributes using mid-infrared spectra1Scatter correctedRaw data Scatter corrected First derivative+first derivative Sensoryattribute Spectral range,cm−1r RMSECV#L r RMSECV#L r RMSECV#L r RMSECV#L Firmness930–1,767and2,839–4,0000.8810.570.947.4110.928.8100.9010.07 Rubbery930–1,767and2,839–4,0000.92 5.540.95 4.590.95 4.650.95 4.65 Creamy930–1,767and2,839–4,0000.947.840.947.860.957.150.947.55 Chewy930–1,767and2,839–4,0000.92 5.770.93 5.460.89 6.550.94 5.17 Mouth-coating930–1,7670.84 5.4100.85 5.3110.84 5.4100.85 5.211 Fragmentable930–1,767and2,839–4,0000.95 6.190.96 5.390.94 6.540.96 5.87 Melting930–1,767and2,839–4,0000.947.940.957.560.957.750.957.47 Mass-forming930–1,7670.83 3.1100.83 3.1100.83 3.190.63 4.34 Greasy/oily930–1,767and2,839–4,0000.56 3.230.54 3.320.59 3.240.56 3.23 1Preferred model in bold.RMSECV=root mean square error of cross-validation;#L=number of partial least squares loadings.and highest r values.It was also desirable for the pre-ferred model to incorporate the lowest#L possible. The results showed that only2of the models(mouth-coating and mass-forming)were improved when the re-duced spectral range(930to1,767cm−1)was used.None of the preferred models was developed using raw spectral data(i.e.,accuracy was improved by the application of a pretreatment).Thefirmness and fragmentable attri-butes were most successfully modeled using MSC spec-tra.The application of afirst derivative step resulted in the preferred models of rubbery,creamy,mass-forming, and greasy/oily.The most accurate models for the chewy, melting,and mouth-coating attributes were achieved when the spectra were subjected to scatter correction and afirst derivative.In conjunction with the RMSECV,r,and#L,the prac-tical utility of the models can also be assessed using the RER.Models with RER of less than3have little practical utility;RER values of between3and10indicate limited to good practical utility;and values above10show that the model has a high utility value(Williams,1987).The preferred models for predicting thefirmness,rubbery, creamy,chewy,mouth-coating,fragmentable,melting, and mass-forming attributes(shown in bold in Table4) had RMSECV values of between3.1and7.4and resulted in corresponding RER values of between7.2and9.6, indicating that the models had good practical utility. Therefore,these attributes had the potential to be pre-dicted by mid-infrared spectroscopy and multivariate data analysis.The greasy/oily attribute was not success-fully modeled(RER=4.8),possibly because of the small range displayed by the samples analyzed,and will there-fore not be discussed further.A graphical display of the preferred regression model for each attribute(high-lighted in bold in Table4)is shown in Figure2A to2H. Figure2shows that there is minimal scatter in the plots, as indicated by the high r values(0.83to0.96),and that the regression lines also have slopes close to1(0.77to 0.96)and low intercepts(1.0to5.9),demonstrating a Journal of Dairy Science Vol.90No.3,2007goodfit(Figure2).The accuracy of each model can be evaluated using the coefficients of determination(R2) between the predicted and measured values,as stated by Williams(2003).The models for mass-forming and mouth-coating both provided approximate quantitative predictions because their R2lay in the range of0.66to 0.81.Good predictions were achieved for the attributes firmness,rubbery,creamy,and chewy,with R2values of between0.82and0.90.The fragmentable model was considered to be excellent,having an R2greater than 0.91.The#L must also be taken into account.This ranged from5to11for the selected models.The models forfirmness,fragmentable,mouth-coating,and mass-forming incorporated a relatively high number of load-ings(9to11),which may have implications for their robustness,because the lower#L,the more robust the model.Thefirst3loadings of the models,which accounted for greater than90%of the variation in the spectral data,are plotted in Figure3A to3H.Although a number of preferred models were developed using spectra pre-treated with afirst derivative step,interpretation of the loadings associated with these models was difficult.This was because the observed peaks and valleys did not follow the raw spectral pattern.However,second deriva-tive steps were very helpful in spectral interpretation because in this form,band intensity and peak location were maintained with those in the raw spectral pattern. Therefore,although the second derivative step did not improve any of the prediction models,the loadings pre-sented for rubbery,creamy,and mass-forming were ob-tained using second derivative spectra and those for chewy,mouth-coating,and melting were obtained using MSC second derivative spectra.The loading plots pre-sented forfirmness and fragmentable were obtained us-ing MSC spectra.Figure3A to3H shows the relation-ships among the loadings used in the prediction model and the different wavenumbers.If a wavenumber had a large positive or negative loading,this meant that thePREDICTION OF CHEESE SENSORY TEXTURE ATTRIBUTES1127Figure2.Linear regression plots of actual vs.predicted sensory attributes of(A)firmness,(B)rubbery,(C)creamy,(D)chewy,(E)mouth-coating,(F)fragmentable,(G)melting,and(H)mass-forming.RER=range error ratio.Journal of Dairy Science Vol.90No.3,20071128FAGAN ET AL.Figure3.Loading plots for partial least squares models of the sensory attributes of(A)firmness,(B)rubbery,(C)creamy,(D)chewy, (E)mouth-coating,(F)fragmentable(G)melting,and(H)mass-forming.Journal of Dairy Science Vol.90No.3,2007PREDICTION OF CHEESE SENSORY TEXTURE ATTRIBUTES1129wavenumber was important for the attribute concerned.Therefore,they assisted in summarizing the relationship between the spectra and the predicted attribute and provided an aid to interpreting the molecular basis for predicting an attribute.The important loadings were distributed across the full spectral range used in predicting each attribute (Fig-ure 3).There was considerable structure present in all of the loading plots.In comparing the plots produced using similar spectral treatments,that is,MSC (Figure 3A and 3F),second derivative (Figure 3B and 3C),and MSC plus second derivative (Figure 3D and 3G),it was apparent that differences existed in the relative impor-tance of various regions of the spectra in predicting the different sensory attributes.For example,the region around 3,200to 3,900cm −1was shown to be of greater importance in predicting the attributes of firmness (Fig-ure 3A),chewy (Figure 3D),and melting (Figure 3G)than for the other attributes.This region of the spectra corresponded with a broad moisture absorption peak.The loadings incorporated into the firmness model ex-plained the variation across almost the full spectral range used (Figure 3A).Peaks and valleys occurred at around 1,095,1,160,and 1,269to 1,396cm −1,associated with the vibration of C–H,C–O bonds of carbohydrates;1,739,2,846,and 2,931cm −1,associated with lipids;and around 1,554,1,604,and 1,646cm −1,which are known to correspond with amides I and II.The amide I and amide II regions of the spectra were also important in predicting chewy,with peaks observed in the loading plot in the region of 1,504,1,547,1,639,and 1,655cm −1(Figure 3D).The loadings for the chewy model were also found to explain variation in the moisture absorp-tion region (3,359to 3,907cm −1),the peaks associated with lipids (1,739and 2,927cm −1),and the region around 1,080cm −1.The most important regions of the spectra for predicting mouth-coating were the regions associated with the vibration of C–H,C–O bonds of carbohydrates (987,1,072to 1,095,1,176,and 1,334to 1,427cm −1)and lipids (1,732,1,739,and 1,751cm −1;Figure 3E).Peaks were also observed in the amide I and II region (1,542,1,562,and 1,655cm −1).A number of major peaks were clearly identified in the fragmentable loading plot (Fig-ure 3F).These were 1,110,1,169,1,242,and 1,462cm −1,and 1,743,2,854,and 2,924cm −1,which corres-ponded with the vibration of the C–H,C–O bonds of carbohydrates and lipids,respectively.Emulsifying salts chelate calcium,which plays an important role in the 2-dimensional structure of processed cheese.They also aid in the dispersion of proteins,which contributes to the emulsification of fat.In this study,the emulsifying salt used was disodium phosphate.The effect of increasing the phosphate concentration was 2-fold,namely,an in-creasing ability to chelate calcium and an incremental Journal of Dairy Science Vol.90No.3,2007increase in the pH of cheese.The interaction between these 2effects (emulsifying salt and pH)will result in increased firmness of the cheese.However,this result is also dependent on the moisture content because mois-ture acts as a plasticizer in processed cheese and de-creases the concentration of the dispersed phase,hence decreasing the firmness of the processed cheese.Greater firmness is also attributed to a higher concentration of protein,and increases in the fat and water contents weaken the protein structure,thereby decreasing the firmness of the processed cheese.This explains the im-portance of the fingerprint (991to 1,400cm −1),lipid,amide,and moisture-absorption regions in predicting the firmness,chewy,fragmentable,and mass-forming attributes.The regions of the spectra that were most important in predicting the attributes of rubbery (Figure 3B)and creamy (Figure 3C)were all associated with lipids (1,739,1,743,2,846,2,858,2,916,2,919cm −1).Minor peaks were also observed in the 1,079to 1,173,1,542,and 3,556to 3,907cm −1spectral regions,which are associated with the vibration of the C–H,C–O bonds;amide II;and moisture absorption,respectively.These peaks were of particular importance in predicting the rubbery and creamy attributes,because fat in cheese has the effect of preventing the protein network of the cheese matrix from forming a tough,dense structure (Lawlor et al.,2001).The loadings for the melting model (Figure 3G)ex-plained variation in a number of different regions,in-cluding the amide I and II regions (1,547,1,654cm −1),lipid regions (1,751,2,927cm −1),and moisture-absorp-tion region (3367to 3907cm −1).All factors that influence either the content or distribution of fat,or the strength of the protein network are known to influence cheese meltability (Lefevere et al.,2000).This accounts for the significance of the moisture,amide,and lipid regions of the spectra in predicting melting.The regions of the spectra that were the most im-portant in predicting mass-forming were found to be 1,092and 1,130cm −1(C–H,C–O bond vibrations);1,535,1,547,1,646,and 1,647cm −1(amides I and II);and 1,736,1,743,and 1,751cm −1(lipids;Figure 3H).This indicates the role that the fat content and protein structure has in determining the mass-forming potential of pro-cessed cheese.These results highlight the importance of different regions across the entire spectral range used in pre-dicting the sensory textural attributes of processed cheese.The importance of different spectral regions in predicting sensory attributes is related to the effects of the formulation and composition on processed cheese texture.Changes in the formulation and composition of。

a r X i v :a s t r o -p h /0510662v 1 22 O c t 2005Draft version February 5,2008Preprint typeset using L A T E X style emulateapj v.6/22/04DISCOVERY OF A WIDE LOW-MASS BINARY SYSTEM IN UPPER SCORPIUSK.L.Luhman 1Department of Astronomy and Astrophysics,The Pennsylvania State University,University Park,PA 16802;kluhman@.Draft version February 5,2008ABSTRACTUsing the near-infrared spectrometer SpeX and its slit-viewing camera at the IRTF,I have resolved a low-mass member of the Upper Scorpius OB association into a double star.From K -band images of the pair,DENIS-P J161833.2-251750.4A and B,I measure a separation of 0.′′96and a magnitude difference of ∆K =0.42mag.I present resolved 0.8-2.5µm spectroscopy of the two objects,both of which exhibit signatures of youth in the shape of their H -and K -band continua,demonstrating that both are members of Upper Scorpius rather than field stars.In addition,through a comparison to optically-classified pre-main-sequence objects,I derive a spectral type near M5for each component,corresponding to a mass of ∼0.15M ⊙with the evolutionary models of Chabrier and Baraffe.The probability that this pair is composed of unrelated M-type members of Upper Scorpius is ∼10−5.When added to the recent discoveries of other wide,easily disrupted low-mass binaries,this new system further establishes that the formation of low-mass stars and brown dwarfs does not require ejection from multiple systems.These observations also indicate that wide low-mass binaries can form in OB associations as well as in smaller clusters where the previously known wide pairs have been found.Thus,the available data show no perceptible effect of star-forming environment on the prevalence of loosely bound low-mass systems.Subject headings:infrared:stars —stars:evolution —stars:formation —stars:low-mass,browndwarfs —binaries:visual –stars:pre-main sequence1.INTRODUCTIONThe multiplicity of stars is influenced by the spe-cific characteristics of the star formation process,and thus has been the subject of extensive measure-ments.Similarly,in an attempt to gain insight into the formation of brown dwarfs,recent studies have heavily scrutinized the binary properties of objects near and below the hydrogen burning mass limit in the solar neighborhood (Koerner et al.1999;Mart´ın et al.1999;Gizis et al.2003;Reid et al.2001;Close et al.2002a,b,2003;Bouy et al.2003;Burgasser et al.2003;Freed et al.2003;Siegler et al.2005;Forveille et al.2005;Liu &Leggett 2005;Gelino &Kulkarni 2005),open clusters (Mart´ın et al.1998,2000,2003),and young clusters and associations (Neuh¨a user et al.2002;Bouy et al.2004;Chauvin et al.2004,2005;Luhman 2004;Luhman,McLeod,&Goldenson 2005;Kraus,White,&Hillenbrand 2005).Most of the binary low-mass stars and brown dwarfs in these surveys exhibit maximum separations of a ∼20AU,which is consistent with the predictions of the embryo ejection scenario for the formation of low-mass bodies (Reipurth &Clarke 2001;Boss 2001;Bate et al.2002).However,a small but growing number of low-mass binaries have been found at wider projected separations that range from 33-41AU (Harrington et al.1974;Mart´ın et al.2000;Chauvin et al.2004;Phan-Bao et al.2005;Mamajek 2005)to beyond 200AU (Gizis et al.2000;Luhman 2004;Bill`e res et al.2005).Because these wide binaries are weakly bound and extremely fragile,it is unlikely1Visiting Astronomer at the Infrared Telescope Facility,which is operated by the University of Hawaii under Cooperative Agreement no.NCC 5-538with the National Aeronautics and Space Admin-istration,Office of Space Science,Planetary Astronomy Program.they that have been subjected to violent dynamical interactions,indicating that some low-mass stars and brown dwarfs are able to form without the involvement of ejection.In fact,the role of ejection may be minor based on other aspects of the multiplicity of low-mass objects (Maxted &Jeffries 2005).To fully utilize multiplicity as a probe of the for-mation of low-mass bodies,it is necessary to con-sider populations spanning a range of ages and star-forming conditions.OB associations represent an area of this phase space that remained unexplored until Kraus,White,&Hillenbrand (2005)obtained HST im-ages of low-mass members of Upper Scorpius,which has an age of 5Myr (de Geus,de Zeeuw,&Lub 1989;Preibisch &Zinnecker 1999)and is the nearest OB as-sociation at a distance 145pc (de Zeeuw et al.1999).Among their 12targets,they discovered three bina-ries,all with projected separations less than 18AU.Kraus,White,&Hillenbrand (2005)tentatively sug-gested that this absence of wide systems in Upper Sco relative to lower density clusters like Chamaeleon I and TW Hya (Chauvin et al.2004;Luhman 2004)might in-dicate a variation in the formation process of low-mass binaries with star-forming environment.In this Letter,I report the discovery of a widely-separated low-mass binary star in Upper Sco,which was found serendipitously during a spectroscopic survey of the low-mass members of this association.I present near-infrared (IR)images and spectroscopy of the components of the pair,measure their spectral types and assess their membership in a binary system,and discuss the implica-tions of this new pair.2.OBSERVATIONS2LuhmanWhile conducting a spectroscopic survey of known low-mass members of the Upper Sco association on 2005June16with SpeX(Rayner et al.2003)at the NASA Infrared Telescope Facility(IRTF),I found that one of the targets,DENIS-P J161833.2-251750.4 (Mart´ın,Delfosse,&Guieu2004),appeared as a∼1′′double on the slit-viewing camera.To further investi-gate the nature of this pair,I performed the imaging and spectroscopy described in this section.2.1.PhotometryTo measure theflux ratio of the components of DENIS-P J161833.2-251750.4(hereafter DENIS1618-2517),I used the slit-viewing camera on SpeX.This camera con-tained a512×512Aladdin2InSb array and had a plate scale of0.′′12pixel−1(Rayner et al.2003).Through a Kfilter,I obtained18dithered one-second exposures. These images were median combined to produce aflat field image,which was then divided into each original exposure.The12frames with the best image quality were registered and combined.A5′′×5′′subsection of the resulting image surrounding DENIS1618-2517is shown in Figure1.The components of this pair exhibit FWHM=0.′′5in this image.I extracted photometry for each object with the task PHOT under the IRAF pack-age APPHOT using a radius of three pixels,arriving at a magnitude difference of∆K=0.42±0.03mag.To measure astrometry for the pair,Ifirst determined the rotation between the cardinal directions and the array axes using the2MASS coordinates of DENIS1618-2517 and the one other2MASS source appearing in the im-age(2MASS16183428-2518067).With this information, I measured a position angle of50.3±1◦for the sec-ondary relative to the ing the plate scale from Rayner et al.(2003),I then measured a separation of0.′′96±0.′′04for the pair.2.2.SpectroscopySpectra were obtained of the components of DE-NIS1618-2517using SpeX in the prism mode with a 0.′′5slit.This configuration produced a wavelength cov-erage of0.8-2.5µm and a resolution of R∼200.After adjusting the position angle of the slit to align it with the axis connecting the pair,I selected an integration time of 30sec and obtained a total of18exposures during a se-quence of dithers between two positions on the slit.The 10frames with the best image quality were reduced with the Spextool package(Cushing,Vacca,&Rayner2004). The data were corrected for telluric absorption with the method described by Vacca et al.(2003).3.ANALYSIS3.1.Spectral ClassificationI now use the photometry and spectroscopy from the previous section to examine the spectral types and extinctions of the components of DENIS1618-2517. Mart´ın,Delfosse,&Guieu(2004)found that the Hαemission and Na I absorption in an unresolved optical spectrum of the pair were indicative of youth and thus membership in Upper Sco.In the SpeX data,Ifind ev-idence of youth in both objects.As shown in Figure2, each spectrum exhibits a triangular H-band continuum, which is a distinguishing characteristic of cool pre-main-sequence objects(Lucas et al.2001).The depths of the optical and IR molecular absorption bands,primarily TiO,VO,and H2O,are also very similar between the two objects,demonstrating that the two objects have the same spectral type to within∼0.5subclass.In com-parison,two young low-mass stars at the same age with a K-band magnitude difference of0.42are predicted to have∆T∼70K(Baraffe et al.1998),which corresponds to∼0.5subclass(Luhman et al.2003).Thus,the rela-tive spectral types and magnitudes of the components of DENIS1618-2517are consistent with the coevality ex-pected for a binary system.Although the absorption band strengths agree between the two objects,the secondary is redder beyond1.7µm. This is difficult to explain as a difference in spectral type or extinction,and instead probably is due to ex-cess emission from circumstellar material around the sec-ondary,an imperfect separation of the light from the two objects during extraction of their spectra,or the combination of misalignment of the slit along the bi-nary axis and differential refraction.Meanwhile,the IR spectra of both DENIS1618-2517A and B are signifi-cantly redder than most of the other low-mass members of Upper Sco that were observed during the same run. The same trend is present in the2MASS J−H and H−K s colors of these objects and is consistent with ex-tinction.Relative to the bluest(i.e.,unreddened)SpeX data of members of Upper Sco and other young clus-ters at M5-M6(Luhman,McLeod,&Goldenson2005), the spectrum of DENIS1618-2517A+B exhibits an ex-tinction of A V∼ 3.The presence of significant red-dening is additional evidence that these objects are not foregroundfield dwarfs.2Based on the strength of the molecular absorption bands,DENIS1618-2517A and B are earlier than most of the other Upper Sco members observed in this run,which have optical spectral types that range from M5to M9(Ardila,Mart´ın,&Basri 2000;Mart´ın,Delfosse,&Guieu2004).This suggests that DENIS1618-2517A+B may be closer to M5than the value of M6measured by Mart´ın,Delfosse,&Guieu (2004).Ifind the same result when I compare the pair to optically-classified members of other young clusters.For instance,as shown in Figure2,the molecular absorption bands of DENIS1618-2517A+B agree better with those of MHO6than with MHO5,which have optical types of M4.75and M6,respectively(Brice˜n o et al.2002).The relatively large reddening toward DENIS1618-2517may have caused Mart´ın,Delfosse,&Guieu(2004)to arrive at a spectral type that was slightly too bin-ing a spectral type of M5with the temperature scale of Luhman et al.(2003)and the evolutionary models of Baraffe et al.(1998)implies a mass of∼0.15M⊙for each component of DENIS1618-2517.3.2.Evidence of BinarityThe results from the previous section and from Mart´ın,Delfosse,&Guieu(2004)demonstrated that the components of DENIS1618-2517are young members of Upper Sco.I now examine if they are likely to comprise a true binary system rather than a pair of unrelated mem-bers of the association that happen to have a small pro-2The relatively high extinction toward DENIS1618-2517could be caused by local circumstellar dust or cirrus clouds in the general vicinity of DENIS1618-2517(Meyer,Wilking,&Zinnecker1993).Wide Binary in Upper Scorpius3jected separation.The surface density of M-type mem-bers of Upper Sco is∼5deg−2and the total area of the association is∼100deg2(Ardila,Mart´ın,&Basri2000; Preibisch,Guenther,&Zinnecker2001;Preibisch et al. 2002;Mart´ın,Delfosse,&Guieu2004).Across the en-tire association,the probability offinding a pair of M-type members with a projected separation of a≤1′′is 3×10−4.The observations that discovered this pair con-sidered only a small fraction of the membership(a few dozen)and were not designed to detect close pairs.As a result,the relevant probability for DENIS1618-2517 is lower by at least an order of magnitude.Based on the very low value of this probability,I conclude that DENIS1618-2517A and B comprise a binary system rather than two unrelated members of Upper -ing the average distance of145pc for the association (de Zeeuw et al.1999),the projected separation of0.′′96 of this pair corresponds to140AU.Demonstrating a common proper motion for a pair of stars is the conventional method of conclusively es-tablishing binarity in the solar neighborhood,but it is less useful for young populations because even un-related cluster members are comoving at the level of precision typically available for these measurements. For instance,the common proper motions measure-ments for the recently discovered substellar companions to GQ Lup(Neuh¨a user et al.2005)and2M1207-3932 (Chauvin et al.2005)are not precise enough to distin-guish between true binaries and comoving unbound clus-ter members seen in projection near each other.Thus, for GQ Lup,2M1207-3932,previously known pairs in Upper Sco(Kraus,White,&Hillenbrand2005),and in-deed the vast majority of visual doubles in young clusters and associations,available evidence of binarity is based on the same statistical considerations presented in this work for DENIS1618-2517.Only the value of the proba-bility differs among these binaries,which depends on the separation of a given pair and the surface density of clus-ter members.The probability of unrelated clusters mem-bers with small separations is exceedingly low for sparse populations like Upper Sco,Chamaeleon,Lupus,and TW Hya(Kraus,White,&Hillenbrand2005;Luhman 2004;Neuh¨a user et al.2005;Chauvin et al.2004)but non-negligible for dense clusters like the Trapezium in Orion(Prosser et al.1994).4.DISCUSSIONAs with the brown dwarf pair in Chamaeleon from Luhman(2004),the binarity of DENIS1618-2517was discovered serendipitously and thus cannot be used at this time in a multiplicity measurement for low-mass members of Upper Sco.However,the mere existence of this binary system has important implications.First, DENIS1618-2517is one of only a few known low-mass binaries with very wide separations(a>100AU, Gizis et al.2000;Luhman2004;Bill`e res et al.2005). These wide pairs are fragile and easily disrupted,and thus demonstrate that some low-mass stars and brown dwarfs can form without the assistance of ejection.In addition,with these observations of DENIS1618-2517, wide low-mass binaries are now known across a wider range of star-forming conditions,from small,sparse clus-ters(Chamaeleon)and to rich OB associations(Upper Sco).Thus,in the measurements available to date,the formation of wide low-mass binaries has no discernible dependence on star-forming environment.K.L.was supported by grant NAG5-11627from the NASA Long-Term Space Astrophysics program.REFERENCESArdila,D.,Mart´ın,E.,&Basri,G.2000,AJ,120,479Baraffe,I.,Chabrier,G.,Allard,F.,&Hauschildt,P.H.1998, A&A,337,403Bate,M.R.,Bonnell,I.A.,&Bromm,V.2002,MNRAS,332,L65 Bill`e res,M.,Delfosse,X.,Beuzit,J.-L.,Forveille,T.,Marchal,L., &Mart´ın,E.2005,A&A,440,L55Boss,A.2001,ApJ,551,L167Bouy,H.,Brandner,W.,Mart´ın,E.L.,Delfosse,X.,Allard,F.,& Basri,G.2003,AJ,126,1526Bouy,H.,et al.2004,A&A,424,213Brice˜n o, C.,Luhman,K.L.,Hartmann,L.,Stauffer,J.R.,& Kirkpatrick,J.D.2002,ApJ,580,317Burgasser,A.J.,Kirkpatrick,J.D.,Reid,I.N.,Brown,M.E., Miskey,C.L.,&Gizis,J.E.2003,ApJ,586,512Chauvin,G.,et al.2004,A&A,425,L29Chauvin,G.,et al.2005,A&A,438,L25Close,L.M.,Potter,D.,Brandner,W.,Lloyd-Hart,M.,Liebert, J.,Burrows,A.,&Siegler,N.2002a,ApJ,566,1095Close,L.M.,Siegler,N.,Freed,M.,&Biller,B.2003,ApJ,587, 407Close,L.M.,Siegler,N.,Potter,D.,Brandner,W.,&Liebert,J. 2002b,ApJ,567,L53Cushing,M.C.,Vacca,W.D.,&Rayner,J.T.2004,PASP,116, 362de Geus,E.J.,de Zeeuw,P.T.,&Lub,J.1989,A&A,216,44 de Zeeuw,P.T.,Hoogerwerf,R.,de Bruijne,J.H.J.,Brown A.G.A.,&Blaauw,A.1999,AJ,117,354Forveille,T.,et al.2005,A&A,435,L5Freed,M.,Close,L.M.,&Siegler,N.2003,ApJ,584,453 Gelino,C.R.,&Kulkarni,S.R.2005,ApJ,submittedGizis,J. 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A.,Bridges,T.,Guenther, E.,& Zinnecker,H.,2002,AJ,124,4044Luhman0.96"NEFig. 1.—K-band5′′×5′′image of the binary system DENIS-P J161833.2-251750.4obtained with the slit-viewing camera on SpeX (FWHM=0.′′5).Preibisch,T.,Guenther,E.,&Zinnecker,H.2001,AJ,121,1040 Preibisch,T.,&Zinnecker,H.1999,AJ,117,2381Prosser,C.F.,Stauffer,J.R.,Hartmann,L.,Soderblom,D.R., Jones,B.F.,Werner,M.W.,&McCaughrean,M.J.1994,ApJ, 421,517Rayner,J.T.,et al.2003,PASP,115,362Reid,I.N.,Gizis,J.E.,Kirkpatrick,J.D.,&Koerner,D.W.2001, AJ,121,489Reipurth,B.&Clarke,C.2001,AJ,122,432Siegler,N.,Close,L.M.,Cruz,K.L.,Mart´ın,E.L.,&Reid,I.N. 2005,ApJ,621,1023Vacca,W.D.,Cushing,M.C.,&Rayner J.T.,2003,PASP,115, 389Wide Binary in Upper Scorpius5Fig.2.—Near-IR spectrum of both components of DENIS1618-2517compared to spectra of Taurus members MHO6(top)and MHO5 (middle).The latter sources have optical spectral types of M4.75and M6(Brice˜n o et al.2002)and have been reddened by A V=2.2and 1.7to match the overall slope of DENIS1618-2517A+B The resolved spectra of A and B exhibit similar spectral types(bottom).The spectra are normalized at1.68µm.。

a r X i v :0803.1019v 1 [a s t r o -p h ] 7 M a r 2008Draft version March 7,2008Preprint typeset using L A T E X style emulateapj v.10/09/06THE DISK POPULATION OF THE CHAMAELEON I STAR-FORMING REGION 1K.L.Luhman 2,L.E.Allen 3,P.R.Allen 2,R.A.Gutermuth 3,L.Hartmann 4,E.E.Mamajek 3,S.T.Megeath 5,P.C.Myers 3,and G.G.F azio 3Draft version March 7,2008ABSTRACTWe present a census of circumstellar disks in the Chamaeleon I star-forming ing the Infrared Array Camera and the Multiband Imaging Photometer onboard the Spitzer Space Telescope ,we have obtained images of Chamaeleon I at 3.6,4.5,5.8,8.0,and 24µm.To search for new disk-bearing members of the cluster,we have performed spectroscopy on objects that have red colors in these data.Through this work,we have discovered four new members of Chamaeleon I with spectral types of M4,M6,M7.5,and L0.The first three objects are highly embedded (A J ∼5)and reside near known protostars,indicating that they may be among the youngest low-mass sources in the cluster (τ<1Myr).The L0source is the coolest known member of Chamaeleon I.Its luminosity implies a mass of 0.004-0.01M ⊙,making it the least massive brown dwarf for which a circumstellar disk has been reliably detected.To characterize the disk population in Chamaeleon I,we have classified the infrared spectral energy distributions of the 203known members that are encompassed by the Spitzer images.Through these classifications,we find that the disk fraction in Chamaeleon I is roughly constant at ∼50%from 0.01to 0.3M ⊙.These data are similar to the disk fraction of IC 348,which is a denser cluster at the same age as Chamaeleon I.However,the disk fraction at M 1M ⊙is significantly higher in Chamaeleon I than in IC 348(65%vs.20%),indicating longer disk lifetimes in Chamaeleon I for this mass range.Thus,low-density star-forming regions like Chamaeleon I may offer more time for planet formation around solar-type stars than denser clusters.Subject headings:accretion disks –planetary systems:protoplanetary disks –stars:formation —stars:low-mass,brown dwarfs —stars:pre-main sequence1.INTRODUCTIONLow-mass stars and brown dwarfs are important sites for studies of planet formation because most stars in the galaxy are low-mass stars while brown dwarfs offer an opportunity to explore planet formation in an extreme environment.Planet formation around low-mass stars and brown dwarfs can be investigated through obser-vations of their primordial circumstellar disks.These observations are most easily performed at mid-infrared (IR)wavelengths (λ∼5-20µm)because this wavelength range provides the best combination of contrast of the disk relative to stellar photosphere,disk brightness,and sensitivity of available telescopes.Molecular clouds that contain ongoing star formation1Based on observations performed with the Magellan Tele-scopes at Las Campanas Observatory,Gemini Observatory,and the Spitzer Space Telescope .Gemini Observatory is operated by AURA under a cooperative agreement with the NSF on behalf of the Gemini partnership:the NSF (United States),the Parti-cle Physics and Astronomy Research Council (United Kingdom),the National Research Council (Canada),CONICYT (Chile),the Australian Research Council (Australia),CNPq (Brazil)and CON-ICET (Argentina).Spitzer is operated by the Jet Propulsion Labo-ratory (JPL),California Institute of Technology under NASA con-tract 1407.Support for this work was provided by NASA through contract 1256790issued by JPL.Support for the IRAC instrument was provided by NASA through contract 960541issued by JPL.2Department of Astronomy and Astrophysics,The Penn-sylvania State University,University Park,PA 16802;kluh-man@.3Harvard-Smithsonian Center for Astrophysics,Cambridge,MA 02138.4Department of Astronomy,The University of Michigan,Ann Arbor,MI 48109.5Department of Physics and Astronomy,The University of Toledo,Toledo,OH 43606.(τ 5Myr)are the natural laboratories for obser-vations of circumstellar disks.Among these regions,Chamaeleon I is arguably the best site for studying disks around low-mass stars and brown dwarfs.It is one of the nearest star-forming regions to the Sun (d =160-170pc,Whittet et al.1997;Wichmann et al.1998;Bertout et al.1999),making its low-mass mem-bers relatively bright.The cluster is young enough that it retains a significant population of primordial disks,but it is old enough that most of its members are no longer highly obscured by dust (A V 5).Because of the relatively low extinction,optical wavelengths are accessible for the spectral classification of the stellar population (Comer´o n et al.2004;Luhman 2004a,refer-ences therein)and for measuring diagnostics of accre-tion (Mohanty et al.2005;Muzerolle et al.2005).Op-tical and near-IR imaging and spectroscopic surveys of Chamaeleon I have produced an extensive census of the stellar and substellar cluster members (Luhman 2007).In particular,this census is unbiased in terms of disks,which is essential for measuring the prevalence of disks.With ∼200known members at masses of 0.01-1M ⊙,Chamaeleon I is sufficiently rich for a statistical anal-ysis of disks around low-mass stars and brown dwarfs.This region is also well-suited for observations of disks at mid-IR wavelengths.The stellar population is sparse enough that current mid-IR telescopes can resolve indi-vidual members,while compact enough for deep imaging of a large fraction of the cluster in a reasonable amount of time.In addition,the cloud exhibits relatively little nebulosity and extended emission at mid-IR wavelengths,which enables accurate photometry of the faint,low-mass members.2Luhman et al.Several studies have searched for evidence of disks in Chamaeleon I through near-and mid-IR photome-try.During pointed observations of known members,Glass (1979)and Jayawardhana et al.(2003)obtained photometry in the JHKL bands (1.2,1.6,2.2,3.4µm)for 17and 15members with spectral types as late as M4and M8,ing a similar set of filters,G´o mez &Kenyon (2001)and Kenyon &G´o mez (2001)performed wide-field imaging of a 0.5deg 2field that en-compassed a large fraction of the Chamaeleon I cloud.Photometry at wavelengths longward of the L -band of-fers more reliable detections of disks because of the bet-ter contrast of cool circumstellar dust relative to warmer stellar photopsheres.To obtain data of this kind for known young stars in Chamaeleon I and to search for new disk-bearing members of the cluster,Baud et al.(1984)observed Chamaeleon I with the Infrared Astronomical Satellite (IRAS )at 12,25,60,and 100µm.They de-tected 70sources,half of which are cluster members.A mid-IR survey with better sensitivity and spatial resolu-tion was later conducted with the Infrared Space Obser-vatory (ISO ,Nordh et al.1996;Persi et al.2000),which provided photometry for 99known members,including a brown dwarf with a spectral type near M8(Natta &Testi 2001;Apai et al.2002).Mid-IR imaging also has been performed toward smaller areas surrounding the three most prominent reflection nebulae in the Chamaeleon I cloud using IRAS (Assendorp et al.1990;Prusti et al.1991)and ISO (Lehtinen et al.2001).The Spitzer Space Telescope (Werner et al.2004)is currently the most advanced telescope operating at mid-IR wavelengths.Because of its high spatial resolution,excellent sensitivity,and large field of view,Spitzer is proficient at identifying disk-bearing members of young clusters (Allen et al.2004;Gutermuth et al.2004;Megeath et al.2004;Muzerolle et al.2004).This capability has been widely utilized through Spitzer surveys of nearby star-forming regions and young associations (τ 10Myr),including Taurus (Hartmann et al.2005;Luhman et al.2006;Guieu et al.2007),Perseus (Luhman et al.2005b;Lada et al.2006;Jørgensen et al.2006;Muench et al.2007;Rebull et al.2007;Gutermuth et al.2007),Lupus (Allen et al.2007),Serpens (Harvey et al.2006,2007;Winston et al.2007),σOri (Hern´a ndez et al.2007;Caballero et al.2007;Zapatero Osorio et al.2007;Scholz &Jayawardhana 2007),λOri (Barrado y Navascu´e s et al.2007),Orion OB1b (Hern´a ndez et al.2006),ηCha (Megeath et al.2005),Chamaeleon II (Young et al.2005;Allers et al.2006;Porras et al.2007),NGC 2362(Dahm &Hillenbrand 2007),Upper Sco (Carpenter et al.2006;Scholz et al.2007),Tr 37,and NGC 7160(Sicilia-Aguilar et al.2006).Extensive Spitzer imaging has been performed in Chamaeleon I as well.Some of these data have been used to examine the properties of the disk population of Chamaeleon I (Luhman et al.2005b;Damjanov et al.2007),identify low-mass brown dwarfs with disks (Luhman et al.2005a,c),and search for outflows (Bally et al.2006).In this paper,we present a comprehensive census of disks in Chamaeleon I using most of the Spitzer imagesbetween3.6and 24µm that have been obtained in this region.We begin by describing the collection and analysis of the Spitzer images (§2)and the identificationof new disk-bearing members of the cluster with these data (§3).We then use the Spitzer photometry of the known members of the cluster to investigate the global properties of the disk population of Chamaeleon I (§4).2.INFRARED IMAGES 2.1.ObservationsFor our census of the disk population in Chamaeleon I,we use images at 3.6,4.5, 5.8,and 8.0µm obtained with Spitzer ’s Infrared Array Camera (IRAC;Fazio et al.2004a)and images at 24µm obtained with the Multi-band Imaging Photometer for Spitzer (MIPS;Rieke et al.2004).The fields of view are 5.2′×5.2′and 5.4′×5.4′for IRAC and the 24µm channel of MIPS,respectively.The cameras produce images with FWHM=1.6-1.9′′from 3.6to 8.0µm and FWHM=5.9′′at 24µm.In this paper,we consider all IRAC and MIPS 24µm observations that have been performed within a radius of 3◦from α=11h 07m 00s ,δ=−77◦10′00′′(J2000)with the exception of the recently executed Legacy pro-gram of L.Allen,which has a program identification (PID)of 30574.These data were obtained through IRAC Guaranteed Time Observations (GTO)for PID=6,36,and 37(G.Fazio),IRAC and IRS GTO for PID=30540(G.Fazio,J.Houck),IRS GTO for PID=40302(J.Houck),Director’s Discretionary Time for PID=248(S.Mohanty),and Legacy programs for PID=139,173(N.Evans),and 148(M.Meyer).The characteristics of the IRAC and MIPS images are summarized in Tables 1and 2,respectively.The boundaries of these images are indi-cated in the maps of Chamaeleon I in Figure 1.2.2.Data ReductionInitial processing of the IRAC images was performed with the Spitzer Science Center (SSC)S14.0.0pipeline.The resulting Basic Calibrated Data (BCD)images were automatically treated for bright source artifacts (jailbar,pulldown,muxbleed,and banding)with customized ver-sions of IDL routines developed by the IRAC instru-ment team (Hora et al.2004;Pipher et al.2004).Mo-saicking of the treated BCDs was performed using R.Gutermuth’s WCSmosaic IDL package,which includes the following features:a redundancy-based cosmic ray detection and rejection algorithm;frame-by-frame dis-tortion correction,derotation,and subpixel offsetting in a single transformation (to minimize data smoothing);and on-the-fly background matching.WCSmosaic was built with heavy dependence on the FITS and WCS ac-cess and manipulation procedures provided by the IDL Astronomy Users Library (Landsman et al.1993).We selected a plate scale of 0.86′′pixel −1for the reduced IRAC mosaics,which is the native scale divided by√Disk Population of Chamaeleon I3in the4.5µm image for areas that were imaged by both channels.Similarly,we rejected STARFIND detections at4.5µm that did not have a corresponding detection at 3.6µm.For the data at5.8and8.0µm,we retained only the sources that were detected in the3.6and4.5µm im-ages,respectively.We marked the sources produced by these criteria on the images and removed remaining spu-rious detections through visual inspection.We measured aperture photometry for the IRAC and MIPS sources using the IRAF task PHOT.We adopted zero point magnitudes(ZP)of19.670,18.921,16.855, 17.394,and15.119in the3.6,4.5,5.8,8.0,and24µm bands,where M=−2.5log(DN/sec)+ZP(Reach et al. 2005;Engelbracht et al.2007).For IRAC,we used an aperture radius of4pixels and inner and outer radii of4and5pixels,respectively,for the sky annu-lus.For MIPS,we adopted radii of3,3,and4pix-els for the aperture and the inner and outer bound-aries of the sky annulus,ing bright,iso-lated stars in the IRAC and MIPS images,we mea-sured aperture corrections between our adopted aper-tures and the larger ones employed by Reach et al.(2005) and Engelbracht et al.(2007).For the MIPS data,we combined our aperture correction with the one estimated by Engelbracht et al.(2007)between their aperture and an infinite one.The total aperture corrections applied to our measurements are0.18,0.17,0.23,0.48,and 0.8mag for3.6,4.5,5.8,8.0,and24µm,respectively. Smaller apertures(and the appropriate aperture correc-tions)were used for sources that were near other stars, including the known members Hn21W and E,the new member2MASS J11062942−7724586and its candidate companion Cha J11062788−7724543,and the new mem-ber2MASS J11084296−7743500,which is close to the protostar IRN.Our quoted photometric errors include the Poisson errors in the source and background emis-sion and the2%and4%uncertainties in the calibra-tions of IRAC and MIPS,respectively(Reach et al.2005; Engelbracht et al.2007).The errors do not include an additional error of±0.05mag due to location-dependent variations in the IRAC calibration.To illustrate the detection and completeness limits of the IRAC data,we consider the two shallow maps (AOR=3960320,3651328)and two of the three deep maps(AOR=3955968,12620032)that are centered on the northern and southern subclusters.The total expo-sure times at a given position in these maps are20.8and 968s,respectively.For each exposure time andfilter,we estimate the completeness limit by measuring the magni-tude at which the logarithm of the number of sources as a function of magnitude departs from a linear slope and begins to turn over.Based on the distributions of IRAC magnitudes,which are shown in Figure2,we arrive at completeness limits of16.25,16.25,14.25,and13.5for the short exposures and17.0,16.75,16.0,and15.25for the long exposures at3.6,4.5,5.8,and8.0µm,respec-tively.The estimates for the short exposures are con-firmed by the comparisons of the histograms for the short and long exposures in Figure2.The improvement in the completeness limits from the short exposures to the long ones is smaller at3.6and4.5µm than at5.8and8.0µm because the short exposures at3.6and4.5µm are already near the depth at which source confusion begins to reduce completeness(Fazio et al.2004b).Thus,the complete-ness limits of the longer exposures at3.6and4.5µm do not scale with√4Luhman et al.2MASS J11081204−7622124,falls outside of the bound-aries of the color-color diagram in Figure3.Three of the nonmembers,2MASS J11085651−7645154,2MASS J11105185−7642259,and2MASS J11105353−7726389, were identified as candidates because they exhibited ex-cesses in preliminary photometric measurements that were used for selecting candidates for spectroscopy.How-ever,upon further data analysis,we found that those ap-parent excesses were caused by contamination from cos-mic rays,and that the sources do not exhibit excesses in ourfinal photometry.For this reason,these three objects are excluded from Figure3.3.2.Spectroscopy of Candidates3.2.1.ObservationsWe performed optical and near-IR spectroscopy on nine of the candidate members of Chamaeleon I that were selected in§3.1using the Low Dispersion Survey Spec-trograph(LDSS-3)on the Magellan II Telescope and the Gemini Near-Infrared Spectrograph(GNIRS)at Gem-ini South Observatory.The GNIRS observations were conducted through program GS-2005A-C-13.Table3 summarizes the observing runs and instrument config-urations for the spectra.In Table4,we indicate the night on which each new member was observed.We also observed known late-type members of Chamaeleon I for comparison to the candidates,obtaining LDSS-3spec-tra of OTS44and Cha J11091363−7734446and GNIRS spectra of T50,Hn12W,Cha Hα11,and CHSM17173. The procedures for the collection and reduction of the LDSS-3and GNIRS spectra were similar to those de-scribed by Luhman(2004c)and Luhman et al.(2004), respectively.3.2.2.NonmembersOur spectroscopic targets are fainter than most of the known members of Chamaeleon I in the IRAC bands,which indicates that they should be low-mass stars or brown dwarfs if they are members of the clus-ter.Among the17candidates observed spectroscopi-cally by Luhman(2007)and in this work,13sources do not exhibit the molecular absorption bands that char-acterize cool,low-mass objects.They also lack the hy-drogen emission lines that are often seen in young stars with disks.Therefore,we classify these candidates as nonmembers.The spectral classifications of eight of these nonmembers were presented by Luhman(2004a). The remainingfive sources(Cha J11081724−7631107, Cha J11083029−7733067,Cha J11084006−7627085,Cha J11090876−7626234,Cha J11120546−7631376)are clas-sified as<M0based on the absence of molecular absorp-tion bands.3.2.3.New Member in Optical SampleThe remaining four candidates do show late-type spectral features and evidence of membership in Chamaeleon I.One of these candidates is in the LDSS-3sample and the other three objects are in the GNIRS sample.Wefirst discuss the source observed by LDSS-3,Cha J11070768−7626326(hereafter Cha1107−7626). The optical spectrum of this candidate is shown in Fig-ure4.The most obvious feature of this spectrum is the strong Hαemission line.The equivalent width of the line is uncertain because the surrounding contin-uum is weak and poorly measured,but it appears to have a value of several hundred angstroms.The line is clearly stronger than the emission observed in active late-typefield dwarfs(Gizis et al.2002)and indicates the presence of accretion.The Hαemission and the mid-IR excess emission together demonstrate the youth of Cha1107−7626,and thus we conclude that it is a mem-ber of Chamaeleon I.To measure its spectral type,we have compared Cha1107−7626to optically-classified late-type mem-bers of Chamaeleon I and other star-forming regions (Luhman2004a,2007)as well as standardfield dwarfs (Kirkpatrick et al.1999).The results of our classifi-cation are illustrated in Figure4,where we compare Cha1107−7626to the young late-type objects KPNO4, OTS44,and Cha J11091363−7734446and the L0field dwarf2MASS J03454316+2540233(Kirkpatrick et al. 1999).KPNO4and OTS44have optical spectral types of M9.5(Brice˜n o et al.2002;Luhman2007)and Cha J11091363−7734446was classified as≥M9with IR spectra(Luhman2007).We now classify the lat-ter as M9.5±0.5based on the similarity to KPNO4 and OTS44in the optical data in pared to the three young M9.5objects,Cha1107−7626ex-hibits weaker TiO and VO absorption,which suggests a later spectral type.Indeed,its spectrum agrees bet-ter with the L0field dwarf,as shown in Figure4.Be-cause the transition from M to L types is defined by the disappearance of TiO absorption at7000-7200˚A (Kirkpatrick et al.1999),we need a spectrum with a higher signal-to-noise ratio at those wavelengths to definitively classify Cha1107−7626as L-type.However, given that its spectrum agrees significantly better with L0than M9.5,we tentatively classify it as L0.The comparison of Cha1107−7626to thefield dwarf in Figure4indicates that it has negligible extinction (A J 0.3).By combining its J-band magnitude with a bolometric correction for L0(Dahn et al.2002)and a distance modulus of6.05(Luhman2008),we estimate a bolometric luminosity of3.3×10−4L⊙.At this luminos-ity,Cha1107−7626is tied with Cha J11083040−7731387 (Luhman2007)as the least luminous known member of Chamaeleon I.To estimate the mass of Cha1107−7626,we begin by examining its position on the Hertzsprung-Russell(H-R)diagram.To do this,we must convert the spectral type of Cha1107−7626to an effective temperature.In our previous studies of young late-type objects,we have performed conversions of this kind with the temperature scales from Luhman(1999)and Luhman et al.(2003), which spanned M1to M9.The former scale was designed to produce coevality for the components of the young quadruple system GG Tau(K7-M7.5)on the model isochrones from Baraffe et al.(1998).Luhman et al. (2003)adjusted that scale at M8and M9to improve the coevality between the coolest members of Taurus and IC348and the members at earlier types.The resulting scale was then extrapolated to M9.25and M9.5as cooler objects were discovered in Taurus and Chamaeleon I (Brice˜n o et al.2002;Luhman et al.2004;Luhman2007). For the purposes of this study,we continue this extrap-olation to L0for Cha1107−7626,arriving at a value ing this temperature and our luminosityDisk Population of Chamaeleon I5estimate,we have placed Cha1107−7626on the H-R di-agram in Figure5.We have included all other known low-mass members of Chamaeleon I(Luhman2007)as well as the three new members that we have found with GNIRS(§3.2.4).Four of the previously known mem-bers have formal spectral types of≥M9because they were classified through H2O absorption bands,and the variation of these bands with optical spectral type is un-known for young objects later than M9.5.However,the strengths of the H2O bands in these objects do agree with those of OTS44and KPNO4,which have optical types of M9.5.Thus,it is likely that they have spectral types near M9.5.The cluster sequence for Chamaeleon I in Figure5 is parallel to the model isochrones for members earlier than M8,which is expected since the adopted temper-ature scale was designed to produce coevality for the GG Tau system.However,the sequence does not remain parallel to the isochrones at later types.Most mem-bers earlier than M8appear between the isochrones for 1and10Myr,while the coolest brown dwarfs,includ-ing Cha1107−7626,are bracketed by10and100Myr. It is unlikely that these old ages are valid.For in-stance,the lifetimes of molecular clouds are too short (τ<10Myr,Hartmann,Ballesteros-Paredes,&Bergin 2001)for the Chamaeleon I clouds to have undergone star formation across such an extended period of time. In addition,objects with ages of10-100Myr would have dispersed from the cloud long ago.Instead,the old isochronal ages are probably a reflection of errors in the adopted temperature scale and/or the evolution-ary models.If the models are valid,then the temper-ature scale for young objects at M9-L0must be much cooler than the adopted one,which is already cooler than the scale measured forfield dwarfs(Kirkpatrick 2005).Significant errors are probably present in both the temperature scale,which is unconstrained at the latest types,and the predicted effective temperatures,which are sensitive to various details of the models,such as the treatment of convection(Baraffe et al.2002).Com-pared to effective temperature,bolometric luminosity appears to be less sensitive to model uncertainties,at least for ages ofτ 1Myr(Baraffe et al.2002),and is easier to measure.Therefore,we have chosen to esti-mate the mass of Cha1107−7626from its luminosity. If we assume that it has an age in the range of val-ues exhibited by the stellar members of Chamaeleon I (τ=1-6Myr,Luhman2007),then we arrive at a mass of0.004-0.01M⊙for Cha1107−7626based on the luminosities predicted by Burrows et al.(1997)and Chabrier et al.(2000).Zapatero Osorio et al.(2007) and Scholz&Jayawardhana(2007)have reported detec-tions of disks for brown dwarfs at comparable masses in theσOri cluster using IRAC data.However,because of the very large uncertainties in the5.8and8.0µm photometry in those studies(0.3-0.5mag),their detec-tions of excess emission have low significance.Thus, Cha1107−7626is the least massive brown dwarf that has definitive evidence for a circumstellar disk.3.2.4.New Members in Infrared SampleIn the GNIRS sample,the three candidates that show evidence of membership in Chamaeleon I are2MASS J11084296−7743500,2MASS J11062942−7724586,and 2MASS J11070369−7724307(hereafter2M1108−7743, 2M1106−7724,2M1107−7724).The near-IR spectra of these objects exhibit strong H2O absorption bands, which indicates that they are low-mass stars or brown dwarfs rather than galaxies.High levels of reddening in their spectra demonstrate that they are notfield stars in the foreground of the cluster and they are too bright to be backgroundfield dwarfs.We also detect emission in H21-0S(1)at2.12µm in2M1108−7743,which is a signature of very young stars.Finally,the spectral features that are sensitive to surface gravity in our data,such as the shape of the H-and K-band continua(Lucas et al.2001; Luhman et al.2004;Kirkpatrick et al.2006),are incon-sistent withfield dwarfs and giants.Instead,the spectra of2M1108−7743,2M1106−7724,and2M1107−7724 agree well with known cluster members observed with GNIRS in terms of the gravity-sensitive features.Based on these properties and the presence of mid-IR ex-cess emission,we classify these sources as members of Chamaeleon I.To measure the spectral types of2M1108−7743, 2M1106−7724,and2M1107−7724,we have compared their H2O and atomic absorption features to those of the optically-classified members of Chamaeleon I that we observed with GNIRS.To facilitate this comparison, we dereddened the spectra of the new members to give them the same slopes as the known members.We also smoothed the spectra to a low resolution when compar-ing the broad H2O absorption bands.The dereddened, smoothed versions of the spectra are shown in Figure6. Each object is compared to the young standards that pro-vide the closest match to the strength of the H2O absorp-tion.Through this comparison and a similar analysis of the atomic absorption lines,we classify2M1108−7743, 2M1106−7724,and2M1107−7724as M4±1,M6±1, and M7.5±1,respectively.These spectral types corre-spond to masses of∼0.3,0.1,and0.04M⊙,respec-tively,according to the theoretical evolution models of Baraffe et al.(1998)and Chabrier et al.(2000)and the temperature scale of Luhman et al.(2003)for ages of a few Myr.The process of dereddening the near-IR spectra of 2M1108−7743,2M1106−7724,and2M1107−7724to match the young standards,which have little if any ex-tinction(Luhman2007),produces extinction estimates of A J=5.6,5.6,and4.7,respectively.It is not surpris-ing that these objects are highly obscured given their locations.2M1108−7743is only14′′from the protostar IRN while the other two sources are within the group of embedded stars near Cederblad110.Spitzer and2MASS images of2M1106−7724,2M1107−7724,and the proto-stars in Cederblad110are shown in Figure7.Based on their high levels of extinction and their close proximity to known protostars,2M1108−7743,2M1106−7724,and 2M1107−7724may be the youngest known low-mass members of the cluster.By combining our extinction estimates for these sources with their H and K s mag-nitudes,the appropriate bolometric corrections,and the distance of Chamaeleon I,we arrive at luminosities of 0.053,0.035,and0.020L⊙,respectively.Because the H-and K-band spectra for these sources do not show any evidence of continuum veiling,these luminosity estimates should be uncontaminated by disk emission.The spectral types,extinctions,luminosities,member-6Luhman et al.ship evidence,and near-IR photometry for the four new members of Chamaeleon I are provided in Table4.The IRAC and MIPS photometry for these sources are pre-sented in the tabulation of Spitzer data for all known members of the cluster in§4.1.The IRAC and MIPS measurements for the13nonmembers are listed in Ta-ble5.3.3.Remaining CandidatesIn the previous section,we obtained spectra for a small sample of the candidate disk-bearing members of Chamaeleon I that were identified in§3.1with Figure3. To investigate the nature of the remaining candidates,we compare them to the known cluster members in Spitzer color-magnitude and color-color diagrams in Figure8. Many of the candidates are as red as the reddest known members,indicating that they are protostellar if they are members.In addition,bonafide members are likely to be brown dwarfs given that nearly all of the candidates are fainter than the stellar members of the cluster,although stars with edge-on disks can also appear very faint.For instance,Cha J11081938−7731522has a stellar mass ac-cording to its spectral type,but it is the faintest and reddest known member in[3.6]and[3.6]−[24],respec-tively,which is explained by the presence of an edge-on disk(Luhman2007).However,rather than brown dwarfs or stars with edge-on disks,the vast majority of the can-didates are probably galaxies.We can refine our sample of candidate members and remove some of the galaxy contaminants by apply-ing additional criteria.The initial sample of candi-dates was defined by colors of[3.6]−[4.5]>0.15 and[5.8]−[8.0]>0.3.However,the sources with [3.6]−[4.5] 0.7and[5.8]−[8.0] 1.2within that sample have colors that differ significantly from those of all known members of Chamaeleon I,as shown in Fig-ure3.These objects can be rejected as likely galax-ies.Because galaxy contamination increases with fainter magnitudes,brighter candidates are less likely to be galaxies.The probability of membership is also higher for candidates that are close to known members of Chamaeleon I.By applying these criteria,we have iden-tified the most promising candidate disk-bearing mem-bers,which are listed in Table6.The brightest candi-date,2MASS J11085367−7521359,is probably a solar-mass star if it is associated with Chamaeleon I.The magnitudes of the remaining candidates are indicative of brown dwarfs.As noted in Table6,some of the candi-dates may be class I sources or companions to known members.The new member2M1106−7724and the candidate Cha J11062788−7724543could comprise the youngest low-mass binary system discovered to date.In addition to revealing new candidate members of Chamaeleon I,our mid-IR measurements provide con-straints on the membership of sources that have been previously cited as possible members.In a census of Chamaeleon I,Luhman(2004a)assigned membership to a few objects that lacked spectroscopic classifica-tion but that exhibited mid-IR excess emission.One of these objects is ISO13(Nordh et al.1996;Persi et al. 2000).The near-IR counterpart for ISO13was not pos-itively identified in previous studies because of uncer-tainties in the coordinates measured with ISO.Based on a comparison of the photometry from ISO to mea-surements from IRAC for sources in the vicinity of the ISO coordinates,we conclude that ISO13corresponds to 2MASS J11025579−7724304.For this source,we mea-sure magnitudes of13.87,13.68,12.68,9.87,and6.54at 3.6,4.5,5.8,8.0,and24µm,respectively.When we com-bine these data with photometry from the Two-Micron All-Sky Survey(2MASS,Skrutskie et al.2006),wefind that the colors of ISO13are relatively blue at1-5µm but become very red at8and24µm.These colors are incon-sistent with those of known members of Chamaeleon I,as illustrated by the position of ISO13in the IRAC color-color diagram in Figure3.Therefore,we remove ISO13 from our list of members of Chamaeleon I because it is probably a galaxy.Luhman(2004a)included ISO206in a list of candidate members based on an apparent excess at6.7µm in photometry from ISO.However,mid-IR ex-cess emission is not present in the IRAC data for this source.As a result,there is no longer any reason to con-sider ISO206as a possible member.Finally,Persi et al. (2001)identified three candidate class I brown dwarfs in the vicinity of Cederblad110through the presence of near-IR excess emission.In our IRAC data,wefind that one of these candidates,NIR72,is a patch of nebulosity rather than a stellar source while the other two objects, NIR84and NIR89,do not exhibit excess emission,and thus are not class I sources.pleteness for Members with DisksWe now use our mid-IR data to examine the com-pleteness of the current census of disk-bearing members of Chamaeleon I.We consider only the areas that have been imaged in all four bands of IRAC,which encompass 170of the229known members of the cluster.Within these areas,156members have photometric errors less than0.1mag in all IRAC bands,while the remaining14 known members lack accurate photometry in at least one band because they are saturated,extended,too close to a brighter star,or below the detection limit of all four bands(Cha-MMS1).Thus,the photometric criteria that we used for selecting candidate members with disks in §3.1recover all known members with disks in the areas with4-band coverage with the exception of the brightest stars,faint companions,and the youngest protostars.As a result,our sample of candidates should be complete for disk-bearing members in the same range of magnitudes and extinctions exhibited by the known stellar popula-tion.To quantify the completeness limits of this candi-date sample,we have plotted the photometric complete-ness limits at8µm for the shortest and longest expo-sures in the color-magnitude diagram in Figure8(§2.2). Meanwhile,to evaluate the completeness of the current census of confirmed members with disks,we consider the remaining candidates that lack spectroscopy.As shown in Figure8,only one candidate is present in the magni-tude range of young stars in Chamaeleon I([3.6] 12), which is in Table6.Thus,the current census is nearly complete for stars with disks in thefields imaged by all four bands of IRAC,which encompass most of the Chamaeleon I cloud(Figure1).Our survey is not com-plete for brown dwarfs with disks,as indicated by the large number of candidates lacking spectroscopy in Fig-ure8.However,based on optical and near-IR surveys, the census of substellar cluster members(both with and without disks)is complete for specificfields and ranges。