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Spectroscopy of Very Low Mass Stars and Brown Dwarfs in IC2391 Lithium depletion and Halpha

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a r X i v :a s t r o -p h /0406436v 1 19 J u n 2004To appear in ApJSpectroscopy of Very Low Mass Stars and Brown Dwarfs inIC 2391:Lithium depletion and H αemissionDavid Barrado y Navascu´e s Laboratorio de Astrof´ısica Espacial y F´ısica Fundamental,INTA,P.O.Box 50727,E-2808Madrid,SPAIN barrado@laeff.esa.es John R.Stauffer IPAC,California Institute of Technology,Pasadena,CA 91125,USA stauffer@ and Ray Jayawardhana Department of Astronomy,University of Michigan,830Dennison Building,Ann Arbor,MI 48109,USA rayjay@ABSTRACTWe have obtained intermediate-resolution optical spectroscopy of 44candi-date very low mass members of the nearby young open cluster IC 2391.Of these,26spectra are totally new,14were already analyzed in a previous paper andanother four are in common.These spectra,taken at the Cerro Tololo 4-meterand Magellan I and II telescopes,allow us to confirm 33of them as likely clustermembers,based on their spectral types,presence of Li,and H αemission.Amongthese new cluster members is CTIO-160(M7),the first IC 2391candidate to sat-isfy all criteria for being a substellar member of the cluster,including detection ofthe Li 6708A doublet.With the enlarged membership,we are able to locate thelithium depletion boundary of the cluster more reliably than in the past.Basedon comparison to several theoretical models,we derive an age of50±5Myr forIC2391.We also estimate new ages for the Alpha Per and Pleiades clusters;ourages are85±10Myr and130±20Myr,respectively.We derive an estimate of theinitial mass function of IC2391that extends to below the substellar limit,andcompare it to those of other well-studied young open clusters.The index of thepower law mass function for IC2391isα=0.96±0.12,valid in the range0.5to0.072M⊙.Subject headings:stars:low mass,brown dwarfs–stars:pre-main-sequence–stars:luminosity,mass functions–open clusters and associations:individual:IC23911.IntroductionHundreds of open clusters1are known in the Galaxy.However,few among them are well characterized(size,distance and reddening)and fewer still are close enough to allow their stellar population to be investigated in detail.Only the Pleiades,the Hyades,Alpha Per and a few other clusters have been systematically studied.Our goal is to add other clusters to this list,and IC2391has been the focus of some of our efforts.IC2391is a young cluster with an estimated age,based on Main Sequence isochrone fitting,of35Myr(Mermilliod1981).It is one of the nearest clusters,with a Hipparcos distance modulus of(m−M)o=5.82±0.07(Robichon et al.1999).The interstellar reddening in its direction is very low,E(B−V)=0.04or0.06,as estimated by Becker&Fenkart (1974)and Patten&Simon(1996),respectively.This last work also estimated a distance of (m−M)o=5.95±0.10,which is the value we will use here.In Barrado y Navascu´e s,Stauffer and Patten(1999,hereafter Paper I),we presented some spectra of very low mass members of the cluster and a preliminary age estimate,53Myr, based on the lithium depletion technique(see Stauffer et al.1998;Basri,Marcy&Graham 1996).Subsequently,in Barrado y Navascu´e s et al.(2001a,hereafter Paper II),we conducted an extensive photometric survey in the optical which yielded a substantial population of candidate members,both very low mass stars and brown dwarfs.The combination of this database with infrared photometry from2MASS(Skrutskie et al.1997,see also Cutri et al.2003)allowed us to extract from the initial sample those objects which might beinterlopers,based on the analysis of different color-magnitude and color-color diagrams.In the introduction to Paper II,we described the results achieved by previous studies,both those papers that looked for new cluster members,more massive than those presented in Paper II,and those papers that studied the properties of true cluster members(X–ray emission,rotation,lithium abundance and so on).Since then,the only other works that have been published dealing with properties of members of this cluster are Randich et al. (2001)–spectra–and Allen et al.(2003)–the luminosity function and the age.Here,we present medium resolution optical spectra of a large number of candidate members discovered in Paper II and consider the membership of those candidates by studying their spectral types,Hαemission,sodium and lithium content;derive again,in a more accurate way thanks to the larger number of objects,the location of the lithium depletion boundary of the cluster and hence its age;and study the Initial Mass Function(IMF).2.Observations2.1.Sample.The spectra presented in this paper correspond to IC2391candidate members discovered by two different groups.Thefirst(generally brighter)set was selected from Patten&Simon (1996)and Patten&Pavlovsky(1999).The second group was extracted from Paper II, where we presented a large sample of low mass stars and brown dwarf candidates in the range12<I C<21,discovered in a deep optical and infrared photometric search.An initial subset of this survey was previously published in Paper I,and it includes medium resolution spectroscopy,allowing the confirmation of the membership of most of them.These spectra were obtained in January1999with the Ritchey-Chr´e tien spectrograph at the Cerro Tololo Interamerican Observatory(CTIO)4m telescope and have a spectral resolution equivalent to2.7˚A(see Paper I for details).In the present study,we have carried out spectroscopic observations at medium resolu-tion for a total of44candidate members.Of these,26have been observed for thefirst time, while18were already analyzed in Paper I.Four out of these18have been reobserved.We especially selected those objects located around the lithium depletion boundary in a color-magnitude diagram(generally very cool stars with masses slightly larger than the substellar limit at0.072M⊙).Figure1displays the I magnitude versus the(R−I)color in the Cousins system.Proposed IC2391members are included as crosses.Open circles represent those candidates with spectra(in some cases at quite low signal-to-noise)from Paper I,whereas candidates with new spectroscopic data are illustrated with solid circles in the diagram.Anempirical ZAMS from Barrado y Navascu´e s et al.(2001b)is displayed as a solid line,as well as several50Myr isochrones(Baraffe et al.1998;Siess et al.2000;D’Antona&Mazzitelli 1997;long-dashed,dotted and short dashed,respectively).The location of the lithium de-pletion boundary(LDB)estimated in Paper I is also included.Due to the lack of space and since the main goal of Paper I was to establish the location of the LDB of the cluster,we did not publish all the spetra of that sample.We now present them in Figure2.Table1lists all the targets observed both in Paper I–except the three non-members analyzed there–and in this paper,and includes accurate position(from the2MASS survey, Cutri et al.2003),optical as well as near infrared photometry,and information regarding how the spectra were collected(telescope,instrument and date).2.2.Multifiber spectroscopy from CTIO.On1999March10th and13th,we collected multifiber medium resolution spectroscopy with the Hydra II bench spectrograph at the CTIO4m telescope under a shared risk program. The grating KPGL-D was used with the OG-570filter.The achieved resolution was2.7˚A as measured in ThAr comparison spectra observed with the same set-up,and covering the spectral range6300-8500˚A.The total exposure time was11.5hours,but we divided this time into12individual observations of1hour each(except the last one,which lasted only30minutes)during these two nights.Each exposure was processed individually with the IRAF2package“hydra”.We used domeflats.After the extraction of each1-D spectrum and its calibration in wavelength, we combined all the data corresponding to the same target into afinal spectrum using a median algorithm in order to remove the hits by cosmic rays.No correction for telluric lines or instrumental profile was carried out.Thefinal spectra can be seen in Figure3.2.3.Spectroscopy from Magellan I&II.During two different observing runs mainly devoted to other goals,we were able to complement the previous data with additional spectra collected at the Magellan6.5m twin telescopes,located at Las Campanas Observatory.Thefirst campaign took place on2002 December11-14th.We used Magellan I and the MIKE echelle spectrograph.MIKE providesspectral resolution that is actually too high for our purposes.For this reason and in order to improve thefinal signal-to-noise ratio,we degraded the resolution by rebinning the original data during the read-out to2and8pixels in the spatial and spectral directions,respectively, thus obtaining spectra with a resolution of0.55˚A and complete spectral coverage of4500-7250˚A.Four IC2391candidate member were observed with MIKE.We show the Hαorder from these spectra in Figure4a.The second run took place on2003March11th,andfive additional candidates were observed,in this case with Magellan II and the B&C spectrograph, using the1200l/mm grating,yielding a2.3˚A resolution.These last spectra can be seen in Figure4b(probable non-members are not included in the diagram).In most cases,we took three individual exposures of1200seconds each,fully reduced each exposure separately,and then added together the three resultant1D spectra at the end.Additional details can be found in Barrado y Navascu´e s et al.(2004).3.AnalysisTable2lists our results,including the derived spectral type,the measured Hα,sodium and lithium equivalent widths at6563,8200and6708˚A,respectively,and the estimated ef-fective temperature and lithium abundance.We used calibrations by Bessell(1979),Leggett (1992)and Basri et al.(2000),and the(R−I)C color and the spectral types for the T eff determination,and curves of growth by Zapatero Osorio et al.(2002)for the lithium abun-dance.In the subsequent section we analyze and discuss these results.3.1.Membership.3.1.1.Spectral types.For most of our targets,spectral types were derived by comparison with spectral stan-dards and/or other cluster members whose spectral types were known,in a similar manner to Kirkpatrick et al.(1999)and Mart´ın et al.(1999),using several spectral indices defined in the red side of the optical spectrum.These standards,whose spectral types go from K7to M8,were observed with the same spectral set-up.In the case of the echelle spec-tra collected with the MIKE spectrograph,we estimated the spectral types using the TiO band-head which starts at7053˚A.The depth of this feature is very sensitive to the effective temperature and it is an excellent spectral type indicator.In this case,in addition to our IC2391candidates,we observed standards of almost every spectral sub-class in the vicinity of M5,with half a sub-class steps.A visual inspection,comparing IC2391members withthe standard stars and verifying the spectral classification by direct comparison among the IC2391objects themselves,was also carried out.We believe that the uncertainty of the IC2391spectral types we have derived is of order half a subclass.Five of our fainter targets(I C∼20mag),observed with HYDRAII,have spectra with a very low signal-to-noise and we have not attempted spectral classification.The spectral type of several of our brighter candidates is in strong disagreement with membership in the cluster,since they do not correspond to the value expected from the optical and infrared colors.This is the case of two stars–CTIO-002and CTIO-067–of K spectral type,probable background giants.These two stars,as well as the fainter objects, have no detectable Hαin emission or the line is seen in absorption(see next subsection).3.1.2.Hαemission.The strength of the Hαemission can be used as a criterion to establish the membership of a cluster candidate.As stated before,two objects of K spectral type,warmer than the expected values from their colors,lack Hαemission and,therefore,they can be classified as likely non-members.The same argument is valid for three out of thefive faint objects located at the end of the cluster sequence(I C∼20),since we would have expected,at least,some emission.Note,however,that this criterion is a statistical one,and membership cannot be completely ruled out for these three objects or confirmed for the other two.In any case,the data suggest that there is a strong pollution rate for this range,about60%.The comparison of the Hαdistribution between clusters of different ages is displayed in Figure5.For clarity,we have only included the cluster associated to the multiple starσOrionis(Wolk1996)and the Alpha Per cluster.The data come from B´e jar et al.(1999); Barrado y Navascu´e s et al.(2001c,2002,2003);Zapatero Osorio et al.(2002),Prosser(1992, 1994);Prosser&Randich(1998);Stauffer et al.(1999).We note that theσOrionis cluster, with an age close to5Myr,has a significant number of stellar and substellar members with Hαfar beyond the limits of thefigure and might contain about20%of classical TTauri stars and substellar analogs(Barrado y Navascu´e s et al.2003;Barrado y Navascu´e s&Mart´ın 2003;Jayawardhana et al.2003),which are characterized,among other things,by strong, asymmetric and broad Hαemission lines.Wefind that the distribution of Hαemission in IC2391is very similar to that for the somewhat older Alpha Persei cluster.Only one star belonging to IC2391stands out in thefigure,CTIO-059.This object is shown with the average value corresponding to the two observation we carried out in January1999at CTIO,but in fact it was observed in two consecutive nights,and showedvery different values(49.5and18.8˚A,see Paper I).Therefore,it seems that we detected a flare in that cluster member at that time.Figure6displays the Hαequivalent width versus time.There might be some variability on short time scales,within the same night.This variability could be related with rotation (see,for instance,possible modulation due to rotation in CTIO-038or CTIO-074).Variabil-ity on a longer time scale might be present too,but the dataset is too sparse and we cannot confirm this at the present time.In any case,it would be very interesting to compare this information with light curves derived for the same objects.3.1.3.Sodium doublet at8200˚A.We have also measured the equivalent widths of the sodium doublet at8200˚A.The strength of this alkali feature is sensitive to gravity(see,for instance,Mart´ın et al.1996). Since IC2391members are much younger thanfield objects of similar spectral types,and should have larger radii,it is possible to use this characteristic as a youth indicator.All our targets but one(CTIO-046)have W(Na i)in agreement with a young age and,therefore, with membership.The large equivalent width measured in CTIO-046indicates that it is a more evolved object,which confirms our other indications that this star is probably not a member of the cluster.3.1.4.Lithium at6707˚A.Figure7and8display the area around the Li i6708˚A for all our spectra.The vertical dashed line indicate the location of this feature.In some cases,the signal-to-noise is indeed good,and lithium is unambiguously detected;see,for example,the cases of CTIO-145and 038(January1999),CTIO-081(March1999),CTIO-195,192and026(December2002)and CTIO-160(March2003).Other cases,such as the two spectra of CTIO-077(January and March1999),are less certain.In any case,the new data allow us a significant improvement in the determination of the location of the LDB(Section3.2).3.1.5.Confirmed members at the substellar limit.So far,including the data published in Paper I,we have collected medium resolution spectroscopy for47candidate members(out of the206identified in Paper II and a handful of brighter objects).The membership of another three(CTIO-040,CTIO-094and VXR27)was rejected in Paper I.Our targets were selected from those classified as probable and possible members in Paper II based on their optical and infrared photometry(132objectsin total).In that study,we estimated the pollution rate for that subsample–candidates identified for thefirst time in Paper II–as25%.Out of the47spectroscopically observed candidates(including some VXRP and PP objects),nine have been classified as non-members (the membership of three out of this nine,namely CTIO-040,CTIO-094and VXR27,was rejected in Paper I)and anotherfive as possible members(four from the CTIO sample).Of them,38objects(the CTIO-xxx)were discovered and presented in Paper II and another six objects–brighter–come from previous studies(from Patten&Simon1996and Patten& Pavlovsky1999).Therefore,taking into account these38CTIO objects–including4possible members in the CTIO sample-,and CTIO-040and CTIO-094–the non-members discussedin Paper I,the pollution rate is in the range20-30%for the CTIO sample,depending on how the possible members are counted.3.2.Lithium depletion boundary and cluster age.3.2.1.Lithium equivalent width versus magnitudes and colors.The initial lithium depletion boundary for IC2391was located at I C=16.2±0.2and (R−I)C=1.91in Paper I.Figures7and8contain the area around the Li i6708˚A doublet for the bonafide members.The initial estimate of the LDB location is confirmed,but there are two stars–PP07and CTIO-206–that show lithium in their spectra despite the fact they are brighter than the LDB.Figures9a and9b display color-magnitude diagrams using optical and infrared data.We have used I C from Paper II in the y-axis for thefirst case and Ks from2MASS in the second.In both cases these two stars are clearly above the LDB.Actually,CTIO-206,the fainter of these two,might be a binary composed of two very low mass stars of almost the same mass,which would solve the puzzle.However,the other star,PP07,is well above the LDB,by about2magnitudes.In both cases,the Na i(8200) equivalent width is in agreement with a young object.A similar situation has been discovered in the Pleiades cluster by Oppenheimer et al. (1997),and their interpretation was that the two supposed Pleiades members(HHJ339=HCG332 and HHJ409=HCG509)are,in fact,young interlopers in the line of sight.Recently,Deacon&Hambly(2003)have derived membership probabilities for them.Although they are very low(P=0.155and P=0.284,respectively),these values are not conclusive,and membership cannot be completely excluded.The alternative would be that there is a mechanism which can prevent lithium depletionduring the pre-main sequence phase in M dwarfs.Note that some warmer Alpha Per and Pleiades members(K spectral type)may inhibit lithium depletion using a mechanism related to rotation or magneticfield strength(see,for instance,Soderblom et al.1993;Garc´ıa L´o pez, Rebolo&Mart´ın1994;Randich et al.1998;D’Antona et al.1998).However,our medium resolution spectrum of PP07does not seem to be broader than the rest(although it would have been very difficult to detect anything with a projected rotational velocity of less than ∼100km/s),and its activity in Hαis average compared with other cluster members of the same color or spectral type.Another,more speculative,possibility is that these two objects are,in fact,bona-fide very low mass members of the cluster,which have recently swallowed a companion(a brown dwarf).The sudden additional mass accretion could explain their location in these color-magnitudes diagrams and the strong lithium feature in the spectrum.This mechanism has been invoked to explain the tendency among planet-harboring stars to be metal rich(Santos et al.2001;Gonzalez et al.2001).Possible evidence of planetary engulfment has been presented by Israelian et al.(2001;2003),although it has been called into question by others(Reddy et al.2002).In any event,Figure9clearly shows the lithium chasm(i.e.,the lack of lithium for late K and early M in the cluster).Figure10helps to determine with greater degree of accuracy the location of the chasm and,therefore,its cool border,the LDB.Objects with lithium are displayed with solid symbols(circles for our data,squares for data from Randich et al.2001), whereas open symbols represent objects without it(triangles for our upper limits).The LDB∼1.9.Ourfirst detection of lithium is located at I C=16.286, keeps its location at(R−I)LDBCwhereas the last star without it has a magnitude of I C=16.144(Ks=13.394and13.587, respectively,if2MASS data are used instead).Since the adopted distance and reddening are (m−M)0=5.95±0.10and E(B−V)=0.06,equivalent to A I=0.112and A K=0.021(Rieke& Lebofsky1985),these values yield M(I C)LDB=10.15and M(Ks)LDB=7.52.The distance yielded by Hipparcos would locate the LDB0.13mag fainter than these values.3.2.2.Lithium depletion and a new lithium depletion boundary age estimate.The evolution of the lithium depletion boundary and the lithium chasm with age,from the empirical point of view,is illustrated in Figure11,where we show a comparison with the three clusters where this type of data are available(IC2391,Alpha Per and the Pleiades). Additional information can be found in Rebolo et al.(1996);Stauffer et al.(1998,1999); Mart´ın et al.(1998);Basri&Mart´ın(1999).Note that a detection of the LDB has been attempted in a fourth cluster,NGC2547,by Oliveira et al.(2003,see also Jeffries et al.2003),but they were not able to detect it unambiguously.We have already determined the absolute magnitudes in the I C and Ksfilters where the LDB appears in ing bolometric corrections of BC I=0.070(Bessell1991; Comer´o n et al.2000)and BC K=2.756(Tinney et al.1993;Leggett et al.1996),the LDB is located at M(bol,Ks)LDB=10.229and M(bol,Ic)LDB=10.251,or M(bol)LDB=10.24mag.We have analyzed in the same way all the data available in the literature for Alpha Per and the Pleiades clusters:-In the case of Alpha Per,taking into account a distance modulus of(m−M)0=6.23 and E(B−V)=0.096,and assuming that the LDB is defined by AP310,AP322and AP300 (AP325might be a binary based on its location in the color-magnitude diagram,CMD),we derive M(I C)LDB=11.42and M(Ks)LDB=8.31.In the same way as for IC2391,M(bol)LDB= 11.31mag.-For the Pleiades cluster,the LDB is defined by CFHT-Pl-09,CFHT-Pl-10,Roque16, and Teide2(CFHT-Pl-13).Assuming(m−M)0=5.60and E(B−V)=0.04(Pinsonneault et al.1998),we derive M(I C)LDB=12.18,M(Ks)LDB=8.94,and M(bol)LDB=12.14mag.In all cases,an error of0.15mag has been estimated for the location of the LDB boundary,taking into account distances,reddening,and the gap between Li detection and non-detection.With these values and the predictions of theoretical models,it is possible to estimate the age of each cluster.Figure12displays the absolute magnitudes of an object whose lithium has been depleted almost completely(one percent of the original lithium abundance)versus the time.The models correspond to Baraffe(m)–M(I C)and M(Ks)–and D’Antona &Mazzitelli(1997)–M(bol).Similar diagrams can be created using models by Burrows et al.(1997).The ages derived are52,51and46Myr for IC2391,79,89and79Myr for Alpha Per,and122,124and153Myr for the Pleiades.That is,we estimate the plausible ages for these three clusters as50±5,85±10and130±25Myr,respectively.Note,however, that Jeffries&Naylor(2001)have reevaluated the error budget for them,both experimental and systematic,and estimated that the errors can be significantly larger.As an example, a different I-band bolometric correction(Monet et al.1992,using a relationship between (V−I)and(R−I)colors)for the LBD in the Pleiades yields an age of140Myr instead153 Myr.If we had used Baraffe’s models instead of those from D’Antona&Mazzitelli(1997), the ages derived from the M(bol)would have been135or125Myr for these two different bolometric corrections.Moreover,the use of Hipparcos distance would have added about3 Myr to the age derived for IC2391.Burke,Pinsonneault&Sills(2004)have also revaluated the the ages for these three clusters plus NGC2547,and examined the errors in the analysis.Their values are55±6, 101±12and148±19Myr for IC2391,Alpha Per and the Pleiades(48,87and126Myr when introducing an ad hoc offset to the I-band bolometric correction).Within the error bars,all these results agree with each other.As stated in Stauffer et al.(1998;1999),Barrado y Navascu´e s,Stauffer&Bouvier (1998),and Paper I–see a summary in Barrado y Navascu´e s,Stauffer&Bouvier(1999)–these ages are about a50%older than the values obtained byfitting isochrones to the upper part of the Main Sequence.Recently,Allen et al.(2003)have derived an age for IC2391 based on the luminosity function(LF)of the cluster.They obtained a value,35Myr,lower than the LDB age and identical to the main-sequence turn-offage(Mermilliod1981).By fitting the data published in Paper II with models of the LF,they argue that LF has a peak at M(I)=14-15mag,which should be produced by the deuterium burning,and that the age cannot be∼50Myr.Their result supports a recent claim by Song,Bessell and Zuckerman (2002),who state that the LDB age might overestimate the real age for young clusters. However,the analysis by Allen et al.(2003)was carried out prior to our new spectroscopic data,which indicate that the pollution rate in that range(I C∼20)is very large(∼60%). The alleged LF peak is not obvious at all now.Additionally,Dobbie et al.(2002)have pointed out that a drop in the LF exists around M7-M8(Teff∼2500K),which might be due to dust formation in the atmospheres of these objects.For IC2391,this happens at about I C∼19.5mag.For these three clusters,the lithium depletion boundaries are located at the spectral types of M5,M6.5and M6.5or,in effective temperatures,3050,2800and2650K,for IC2391,Alpha Per and the Pleiades,respectively.When expressed in mass,using models from Baraffe et al.(1998),they take place at0.12,0.085and0.075M⊙,repectively.All the LDB data are summarized in Table3.3.2.3.Thefirst confirmed brown dwarf in the cluster.So far,although a large number of candidate brown dwarfs were presented in Paper II,none of these candidates were established via spectroscopy.This confirmation implies that:(a)it is a member of the cluster based on all available criteria;(b)it has a bolometric magnitude that would place it below the substellar mass limit if it were a member of the cluster;and(c)there is a detection of the lithium6708A doublet.Using our new age estimate and models by Baraffe et al.(2002),the interstellar red-dening and the cluster distance,the substellar limit is located at I C=17.06.Several of the targets in this sample are fainter than this value,and their spectroscopic properties agree with membership.However,their spectra are not good enough to have a lithium detection beyond a doubt,which would confirm the substellar nature.Only in one case,CTIO-160 (whose spectral type is M7),is lithium clearly identified and its naturefirmly established, making this object thefirst brown dwarf unambiguously identified in the IC2391cluster. Note,however,that errorbars in the object photometry and the uncertainties in the models are large enough to change the classification of this object.3.3.Mass Function.We have derived a mass function for IC2391using non-dusty models from Baraffe et al.(1998)and the Ic magnitudes.Figure13depicts our results.We have assumed different ages,ranging from25to50Myr(these values are close to the turn-offand the LDB ages, respectively).In any case,when expressed as a power law,the index(α=0.96±0.12)does not depend on the age in this interval(i.e.,the derived value is very similar when using these three ages).The MF is valid between a mass of0.5M⊙and the substellar limit. Below0.072M⊙,there is a sudden drop,which might be partially explained by the lack of survey completeness beyond I C∼18.5for cluster members(0.050M⊙for50Myr isochrone from Baraffe et al.1998).However,we have detected a significant number of candidate members with magnitudes around I C∼20.Our medium resolution spectroscopy indicates that,despite the strong pollution in this range,about60%,some seem to be members of the cluster.Therefore,the gap at I C∼19,mass∼0.05M⊙might be real.Dobbie et al. (2002)and Jameson et al.(2003)have pointed out that several young clusters show a lack of substellar members of M7-M8spectral type,more or less in the same location as in the case of IC2391(see the case of Alpha Per in the same diagram).They explain this fact as an effect of dust formation at this spectral type and its effect in the luminosity function as a new source of opacity,which would decrease the overall luminosity for the cooler objects. In any event,the number of objects discovered so far in the IC2391cluster at the low end of the cluster sequence is too low to confirm this possibility.Figure13also contains a comparison with several Mass Functions corresponding to young clusters of different ages,such as Alpha Per,the Pleiades,and M35all of them derived in the same manner(see Bouvier et al.1998;Barrado y Navascu´e s et al.2001b;Barrado y Navascu´e s et al.2002;Barrado y Navascu´e s&Stauffer2003;Barrado y Navascu´e s2003). The index of the Mass Function power law is very similar in all cases,except in the case of the low mass stellar members of M35,a very rich cluster,where some mass segregation。

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天体英语知识点总结高中

天体英语知识点总结高中

天体英语知识点总结高中一、IntroductionThe study of celestial bodies, or heavenly bodies, is known as astronomy. Astronomy is a natural science that involves the observation and analysis of celestial phenomena. It has been a subject of human fascination for millennia, with civilizations around the world creating myths, legends, and astronomical calendars associated with the movements of the sun, moon, and stars.In recent centuries, astronomy has developed into a field of scientific inquiry, using advanced tools and techniques to study the universe and its contents. This has led to many groundbreaking discoveries and a better understanding of the cosmos.In this article, we will explore some key concepts and terms related to astronomy and celestial bodies, providing a comprehensive overview of this fascinating field of study.二、The Solar SystemThe solar system is the collection of celestial bodies that orbit the sun, including planets, moons, asteroids, comets, and other objects. The sun is the central star of the solar system, providing light and heat to the planets and other bodies that orbit it.1. Planets: There are eight recognized planets in the solar system, including Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These planets vary in size, composition, and distance from the sun, and each has its own unique characteristics.2. Moons: Moons are natural satellites that orbit planets. The most well-known example is Earth's moon, but many other planets in the solar system also have moons of their own. Some planets, such as Jupiter and Saturn, have a large number of moons, while others have only a few.3. Asteroids: Asteroids are small, rocky bodies that orbit the sun. They are found primarily in the asteroid belt, which lies between the orbits of Mars and Jupiter. Some asteroids have elliptical or irregular orbits that bring them close to the inner solar system, posing a potential threat to Earth and other planets.4. Comets: Comets are icy bodies that orbit the sun in highly elliptical orbits. When a comet approaches the sun, its ice begins to vaporize, creating a bright glowing tail that can be seen from Earth. Comets are thought to be remnants from the early solar system and provide valuable insights into its formation and evolution.5. Dwarf Planets: In addition to the eight recognized planets, there are also a number of dwarf planets in the solar system. These are celestial bodies that are similar to planets in some ways but do not meet all the criteria for being classified as planets. The best-known example is Pluto, which was reclassified as a dwarf planet in 2006.三、Stars and GalaxiesStars are massive, luminous spheres of plasma that emit light and heat through nuclear fusion. They are the building blocks of galaxies, which are vast collections of stars, gas, and dust held together by gravity.1. Star Formation: Stars form from the gravitational collapse of dense regions within interstellar clouds of gas and dust. As the cloud contracts, it heats up and eventually reaches a temperature and density at which nuclear fusion reactions can occur, turning hydrogen into helium and releasing large amounts of energy in the process.2. Stellar Evolution: Stars go through a life cycle that depends on their initial mass. Low-mass stars, such as our sun, spend most of their lives in a stable state, fusing hydrogen into helium in their cores. Eventually, they exhaust their nuclear fuel and expand into red giants before shedding their outer layers and becoming white dwarfs. High-mass stars, on the other hand, undergo more dramatic evolutionary stages, including supernova explosions and the formation of neutron stars or black holes.3. Galaxies: Galaxies come in a variety of shapes and sizes, from small irregular galaxies to giant elliptical and spiral galaxies. The Milky Way, the galaxy in which our solar system resides, is a spiral galaxy with several arms of stars and gas. Galaxies are thought to have formed from the gravitational collapse of large clouds of gas and dust in the early universe.4. The Universe: The universe is the vast expanse of space and time that contains all known matter and energy. It is believed to have originated from a single point in an event known as the Big Bang, which occurred approximately 13.8 billion years ago. The study of the universe, its origin, and its evolution is a fundamental aspect of astronomy, leading to many important discoveries and insights into the nature of the cosmos.四、Observational AstronomyObservational astronomy is the study of celestial objects and phenomena through direct observation using telescopes, cameras, and other instruments. This branch of astronomy plays a crucial role in advancing our understanding of the universe.1. Telescopes: Telescopes are the primary tools used by astronomers to observe celestial objects. They collect and magnify light from distant objects, allowing astronomers to study stars, planets, galaxies, and other phenomena in great detail. There are several types of telescopes, including optical telescopes that work by collecting visible light, as well as radio telescopes, X-ray telescopes, and other specialized instruments that capture non-visible forms of radiation.2. Astronomical Imaging: Astronomical imaging refers to the process of capturing and analyzing images of celestial objects. Modern digital cameras and imaging sensors have revolutionized this field, allowing astronomers to produce high-resolution images of distantgalaxies, planetary surfaces, and other astronomical features. Imaging data is often used to study the composition, structure, and evolution of celestial bodies.3. Spectroscopy: Spectroscopy is the study of the interaction between light and matter. By analyzing the spectrum of light emitted or absorbed by an object, astronomers can learn about its composition, temperature, and other important properties. Spectroscopy has been instrumental in advancing our knowledge of stars, galaxies, and the interstellar medium.4. Astronomical Surveys: Astronomical surveys are large-scale projects that aim to systematically observe and catalog objects in the sky. These surveys cover a wide range of wavelengths and are used to study the distribution of galaxies, the structure of the universe, and the properties of individual celestial objects. The data collected from these surveys is crucial for advancing our understanding of the cosmos.五、Astrophysics and CosmologyAstrophysics is the branch of astronomy that seeks to understand the physical properties and behavior of celestial objects and phenomena. It involves the application of principles from physics and other sciences to study the universe and its contents.1. Stellar Physics: Stellar physics focuses on the study of stars, including their structure, evolution, and energy production. It seeks to explain the processes that govern the behavior of stars, such as nuclear fusion in their cores, the generation of magnetic fields, and the formation of stellar remnants.2. Galactic and Extragalactic Astrophysics: This subfield of astrophysics deals with the study of galaxies and the large-scale structures of the universe. It explores the distribution of matter and energy, the formation of galaxies, and the dynamics of galactic clusters. It also addresses the properties of objects outside our galaxy, such as quasars, pulsars, and active galactic nuclei.3. Cosmology: Cosmology is the study of the origin, evolution, and ultimate fate of the universe. It seeks to understand the large-scale structure of the cosmos, the nature of dark matter and dark energy, and the cosmic microwave background radiation left over from the Big Bang. Cosmologists use principles from general relativity and particle physics to develop theoretical models of the universe and test them against observational data.4. Black Holes and Neutron Stars: Black holes and neutron stars are extreme objects that result from the gravitational collapse of massive stars. They have unique properties, such as intense gravitational fields and the emission of powerful radiation. The study of these objects is an important area of research in astrophysics, as they provide insights into the behavior of matter under extreme conditions.六、ConclusionAstronomy is a fascinating and dynamic field of study that encompasses a wide range of topics, from the study of stars and planets to the exploration of the universe on the largest scales. It offers a unique perspective on the nature of our existence and our place in the cosmos.The knowledge and insights gained from astronomy have practical applications in many areas, including space exploration, the development of new technologies, and the search for extraterrestrial life. As our understanding of the universe continues to expand, so too will the impact of astronomy on our lives and our understanding of the world around us.In conclusion, astronomy is a cornerstone of human curiosity and scientific endeavor, revealing the wonders of the universe and the beauty of the celestial realm. The study of celestial bodies continues to capture the imagination of people around the world and drive us to explore and understand the cosmos in ever-greater detail.。

219401791_玉米醇溶蛋白纳米颗粒的体外消化特性

219401791_玉米醇溶蛋白纳米颗粒的体外消化特性

张子豪，徐硕，逄格雨，等. 玉米醇溶蛋白纳米颗粒的体外消化特性[J]. 食品工业科技，2023，44（13）：1−8. doi: 10.13386/j.issn1002-0306.2022110187ZHANG Zihao, XU Shuo, PANG Geyu, et al. In Vitro Digestive Properties of Zein Nanoparticles[J]. Science and Technology of Food Industry, 2023, 44(13): 1−8. (in Chinese with English abstract). doi: 10.13386/j.issn1002-0306.2022110187· 青年编委专栏—食品营养素包埋与递送（客座主编：黄强、蔡杰、陈帅） ·玉米醇溶蛋白纳米颗粒的体外消化特性张子豪，徐　硕，逄格雨，张雨晴，于少轩*，宋元达（山东理工大学农业工程与食品科学学院，山东淄博 255000）摘　要：为了研究基于天然生物大分子的纳米递送体系在消化过程中物理化学性质的变化，本研究以姜黄素（Curcumin ）为芯材，玉米醇溶蛋白（Zein ）为壁材，通过反溶剂沉淀法制备了包埋姜黄素的玉米醇溶蛋白纳米颗粒（CZNPs ），通过光谱学方法和电子显微镜对CZNPs 的物化性质进行了表征，并在体外模拟消化模型中对CZNPs 的消化特性进行了研究。

结果表明，当姜黄素与Zein 的质量比为1:40时，姜黄素的包埋率最高，为99%±1%，制得的CZNPs 为球形纳米颗粒，平均粒径为118.6±0.7 nm 、Zeta 电位为19.9±3.79 mV ，且颗粒之间出现轻微粘连。

在体外模拟胃消化过程中，随着消化时间的延长，CZNPs 出现明显聚集，其平均粒径增至8000 nm ；且部分Zein 发生降解，生成小分子量的氨基酸，同时缓慢释放出姜黄素。

X-ray spectroscopy of clusters of galaxies

X-ray spectroscopy of clusters of galaxies

X-ray spectroscopy of clusters of galaxiesNaomi Ota【期刊名称】《天文和天体物理学研究》【年(卷),期】2012(012)008【摘要】Clusters of galaxies are the most massive objects in the Universe and precise knowledge of their mass structure is important to understand the history of structure formation and constrain still unknown types of dark contents of the Universe.X-ray spectroscopy of galaxy clusters provides rich information about the physical state of hot intracluster gas and the underlying potential structure.In this paper,starting from the basic description of clusters under equilibrium conditions,we review properties of clusters revealed primarily through X-ray observations considering their thermal and dynamical evolutions.The future prospects of cluster studies using upcoming X-ray missions are also mentioned.【总页数】22页(P973-994)【作者】Naomi Ota【作者单位】Department of Physics, Nara Women's University, Kitauoyanishimachi, Nara, Nara 630-8506, Japan【正文语种】中文【相关文献】1.Testing X-ray measurements of galaxy cluster gas mass fraction using the cosmic distance-duality relation [J], Xin Wang;Xiao-Lei Meng;Yong-Feng Huang;Tong-Jie Zhangbining optical and X-ray observations of galaxy clusters to constrain cosmological parameters [J], Heng Yu; Zong-Hong Zhubining optical and X-ray observations of galaxy clusters to constrain cosmological parameters [J], Heng Yu; Zong-Hong Zhu4.Surface Characterization of Ni_(64)P_(20)Fe_(16)Amorphous Alloy by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy [J], 姜洪刚;王景唐;丁炳哲5.X-Ray Photoelectron Spectroscopy and Raman Spectroscopy Studies on Thin Carbon Nitride Films Deposited by Reactive RF Magnetron Sputtering [J], Masao Matsuoka;Sadao Isotani;Ronaldo DMansano;Wilmer Sucasaire;Ricardo ACPinto;Juan CRMittani;Kiyoshi Ogata;Naoto Kuratani因版权原因，仅展示原文概要，查看原文内容请购买。

汞原子荧光检测记忆效应消除措施

汞原子荧光检测记忆效应消除措施

AbstractThe measurement of mercury in aqueous solutions by ICP-AES is adversely affected by the memory effect wherein mercury accumulates within the sample introduction system and is slowly released over time to give increasing response signals at the same initial mercury concentration. The memory effect is obviated by the addition of Hg(II) complexants: thiourea and gold(III) chloride are both effective in preventing mercury sorption and vapor buildup with the latter being preferred because the memory effect vanishes more rapidly. Conditions are described wherein it is possible to quantify low levels of mercury(II) in aqueous solutions by ICP-AES under routine operating conditions that can be applied to other metal ions by adding 1 mg of gold(III) chloride per 3 mg of mercury(II) to those solutions.1. IntroductionThe toxicity of mercury at very low levels has led to its stringent control with a maximum contaminant level of 2 μg/L being set by the US Environmental Protection Agency [1]. This has led to the development of techniques for accurate monitoring of mercury levels [2] via cold vapor atomic absorption spectrometry [3,4], plasma atomic emission spectrometry [5], atomic fluorescence spectrometry [6,7], inductively coupled plasma (ICP) optical emission spectrometry [8] and ICP-mass spectrometry [9-11]. Despite methods with good sensitivity and selectivity, few can directly measure Hg(II) in aqueous solutions under routine operating conditions as can be done with other ions [12]. Mercury accumulates in instruments irrespective of its initial there is a gradual increase of the mercury signal strength with time, a non-linear calibration, and a long wash-out time [13]. Mercury contamination of the instruments may be attributed to its adsorption on the transfer tubing, spray chamber and nebulizer [14].Mercury in solution exists as an equilibrium of Hg(0), Hg(I), and Hg(II).Since only Hg(0) is volatile, one method of eliminating the memory effect entails complete reduction of the mercury to Hg(0) with the reducing agents stannous chloride and sodium borohydride [15]. The mercury vapor is then purged from solution into an absorption cell. This cold vapor technique [16] is frequently used for mercury determination in solution when coupled with atomic absorption or atomic fluorescence spectrometry. The standard EPA mercury determination method is based on this technique [17]. The generation of mercury vapor does, however, require additional equipment for its generation and measurement.An alternative technique involves the stabilization of mercury in solution as Hg(II). Gold(III) chloride and potassium dichromate are typical stabilizing agents that give complete conversion to Hg(II) thus obviating volatilization [18]. The technique has been applied to the measurement of mercury via ICP-mass spectrometry (MS) [19]. The addition of chelants that bind Hg(II) also stabilizes it within aqueous solutions and limits loss through volatilization. Thus the addition of Triton X-100, ammonia and EDTA allows for the accurate measurement of mercury in aqueous solutions by ICP-MS [20]. The use of 2-mercaptoethanol reduces the memory effect to the point that ICP-MS gives a detection limit for mercury of 5.1 μg/L [21].Techniques have been developed to allow the use of cold vapor coupled atomic absorption spectrometry (CV AAS) [22] and ICP-MS for mercury determination. The detection limits are 10μg/L for the former and 3 μg/L for the latter [23]. While ICP is thus an excellent technique for mercury determination, its extension to the more commonly available ICP-atomic emission spectroscopy (AES) remains problematic due to the memory effect [24]. It is reasonable to expect that mercury-stabilizing agents added to solutions would eliminate the memory effect in ICP-AES as they have with ICP-MS. The use of a 2% L-cysteine solution containing Hg(II), but only at a very low level (1 μg/L), obviates the memory effect in ICP-AES [25]. We now report a rapid and accurate method for the determination of total mercury in aqueous solutions at the mg/L level using ICP-AES through a comparative study of gold(III) chloride, triethylamine, ethylenediaminetetraacetic acid (EDTA), disodium ethylenediaminetetraacetate (Na2EDTA) and thiourea.2. Experimental section2.1. ReagentsMercury solutions are prepared by diluting a 1000 mg/L mercury(II) nitrate reference solution containing 1.8% nitric acid. Au(III) solutions are prepared from a 1000 mg/L gold atomic absorption standard solution in 0.5 M HCl. Triethylamine, ethylenediamine tetraacetic acid (EDTA), disodium EDTA (Na2EDTA), and thiourea (analytical grade) were purchased from Acros and used directly without purification. Nanopure water with resistivity N 18 MΩwas used from a Barnstead ultrapure water system.2.2. InstrumentationA Spectroflame M120E (Spectroanalytical Instruments) inductively coupled atomic emission spectrometer (ICP-AES) controlled by Spectro Smart Analyzer Vision software was used for analysis of the mercury in solution. Sample solutions were introduced by peristaltic pump. Ultra-pure argon from Airgas was used. The wavelength of analysis was set at 184.956 nm. The operating conditions (also appropriate for the analysis of other transition metal ions) are detailed in Table 1. Measurement was initiated immediately after the preflush.2.3 Analytical procedureMercury(II) solutions with 2.0 mg Hg(II)/L were prepared in50 mL volumetric flasks also containing one of the following: Au(III) at 1.0 mg/L, 0.1% triethylamine, EDTA (saturated solution), 0.1% Na2EDTA, or 0.1% thiourea (following reports in the literature with ICP-MS, 0.1% Triton X-100 was added to the amine and thiourea solutions as a wetting agent). One set of solutions was allowed to stand for 1 h before analysis and a second set was allowed to stand for 17 h. Au(III) solution at1.0 mg/L was made by successive dilutions of a 1000 mg/L standard reference solution. Triethylamine, Na2EDTA and thiourea solutions were prepared by dilution of their 1% stock solutions. Due to its low solubility (0.5 g/L), the EDTA solution was prepared by adding 0.05 g directly to the Hg(II) solution in a 50 mL volumetric flask and then using only the supernatant for the ICP measurement. Mercury standard solutions at levels up to 10.0 mg/L containing up to 10.0 mg Au(III)/L were prepared by successive dilution of 1000 mg/L Hg(II) and Au(III) reference solutions.After the plasma was ignited, nanopure water acidified with 1% nitric acid was used to wash thesample introduction system and the instrument allowed to stabilize for 15-20 min. The optics were then reprofiled, the sample solution aspirated into the ICP-AES, and measurement initiated immediately after a 20 s preflush. Three measurements were taken as the solution was continuously aspirated and averaged. Nitric acid solutions (1 M) containing 1.0 mg Au(III)/L were used to rinse the sample introduction system between samples for 1-2 min.3. ResultsThe memory effect increases the mercury count and leads to erroneous results for the concentration of mercury in successive samples. Fig. 1 shows the changes in mercury signal intensity measured at different times when Hg(II) standard solution at 0.2 mg/L is aspirated into the ICP-AES. The same result is seen with a Hg(II) solution at 0.005 mg/L, the lowest level in this study. Continuously introducing the same mercury solution results in a gradual increase of signal response, with equilibrium not being achieved. The intensity of the mercury signal in Fig. 1 at the fourth measurement has increased about twice compared to that of the first measurement. The mercury signal does not correlate with the sample concentration and those concentrations are not reproducible. Mercury in the sample solution must thus be adsorbed on the walls of the transfer tubing, spray chamber and/or nebulizer during its transport before atomization and released over time. No maximum mercury sorption within the sample introduction system is observed. Further-more, signal response only slowly returns to the baseline level after aspiration is changed from the mercury solution to distilled water. Washing the system with water for 10 min does not remove all of the adsorbed mercury: when 2.0 mg/L Hg(II) solution is used, a mercury signal with intensity of 166410 cps is still observed (Fig. 2). This significant memory effect obviates the routine analysis of Hg(II) by ICP-AES without the addition of mercury-stabilizing agents.TEA, EDTA, Na2EDTA, thiourea and gold(III) chloride are compared to determine which can most effectively stabilize Hg (II) thus eliminating the memory effect in the ICP-AES. Since the mercury standard concentrations are low (2.0 mg/L) and high background concentrations can affect plasma temperature and nebulization, the matrix composition is kept as dilute as possible. The concentrations of TEA, Na2EDTA and thiourea in the mercury sample solutions were thus set at 0.1% and the Au(III) set at 1.0 mg/L; the saturated EDTA solution had the same Au (III) level.Table 2 shows the change in mercury signal intensities over three measurements when TEA, EDTA, Na2EDTA, thiourea, or gold(III) chloride is added to the mercury solutions and the solutions allowed to stand for 1 h before measurement. Within each set of three measurements, the second and third measurements were taken immediately after the one that preceded it. A significant memory effect remains with all but the gold-stabilized solution. With TEA, the third measurement gives a mercury signal that is 2.5 times greater than the first measurement while, with EDTA and Na2EDTA, it is nearly twice as great. Thiourea shows a variation of less than 20% in the maximum mercury signal intensity though the initial mercury response is still much greater than that of the Au(III)-stabilized solution. The latter shows no memory effect.While complexants with nitrogen-and sulfur-based ligands are known to form stable Hg complexes (e.g., immobilized tertiary amines display a high affinity for Hg(II) from dilute solutions after a contact time of less than 30 min [26] and soluble thiourea can complex Hg2+ and CH3Hg+ from aqueous solutions [27]), it is possible that attaining equilibrium requires a timelonger than 1 h. The respective solutions were thus allowed to stand for 17 h after which point the measurements were made (Table 2). Though the situation improves compared to the results with a 1 h contact time, the increase in mercury signal response remains evident with TEA, EDTA, and Na2EDTA. (The memory effect is evident with Na2EDTA even after a 5-day contact time.) The lengthy contact time, however, clearly diminishes, and may eliminate, the memory effect when thiourea is used as the complexant.Table 3 shows the effect of adding varying amounts of Au (III) to solutions of mercury at 2.0 mg/L. The addition of a level as low as 1.0 mg/L eliminates the memory effect: reproducible signal intensities are observed over three measurements after a 1 h contact time. A change in Au(III) level from 1.0 to 10.0 mg/L does not affect the signal intensity at a Hg(II) level of 2.0 mg/L. Each signal intensity varies from the average by b 1.5%. Au(III) at the low level of 1.0 mg/L is sufficient to keep Hg(II) at low concentrations stable in aqueous solutions. Given that the contact time required to achieve this reproducibility is less than that required by thiourea, gold(III) chloride is the agent of choice for stabilizing the mercury signal.A linear calibration curve was obtained (brN = 0.999-1.000) with solutions of 1.0 mg Au(III)/L and increasing levels of Hg (II) (0.1, 0.2, 0.4, 0.6, 1.0, and 2.0 mg/L). No matrix effect was found upon the addition of 0.04 N sodium nitrate, chloride, or sulfate. Mercury at the μg/L level can be measured with the gold technique. For example, contacting a solution containing 10 mg Hg(II) /L with crosslinked polystyrene beads bearing immobilized dimethylamine ligands for 17 h gave ICP-AES spectra that were nearly identical to the baseline of a blank solution, indicating a Hg(II) level of less than 10 μg/L.In order to determine the lower limit of the Au:Hg ratio, six solutions of 10 mg Hg(II)/L were prepared with 0.5, 1.0, 2.0, 3.0, 6.0, and 10.0 mg Au(III)/L (Table 4). Three measurements were recorded for each solution; 1 M HNO3 containing 1.0 mg Au (III)/L was used to wash the sample introduction system between Hg(II)-containing solutions. The solution with 10 mg of gold was read first in the ICP, followed by the solutions with lower gold levels, as given in Table 4. The steady Hg signals from the last three solutions were as expected from the earlier runs. However, the first solution with 0.5 mg Au showed a decrease in signal intensity as each measurement was taken whereas an increase in intensity would have been expected due to the memory effect when the level of Au got too low. The solutions with 1.0 and 2.0 mg Au also suggested a decrease but one much less pronounced than the first solution. The first four solutions (i.e., 10.0 mg Hg/L with 0.5, 1.0, 2.0, and 3.0 mg Au/L) were then prepared again for re-investigation and to study the effect of Au(III) in the wash solution by changing that solution to 2 M HCl since the chloride ion is known to bind both mercury and gold ions. The order of reading in the ICP was as given in Table 5, from left to right; the solutions with 0.5, 1.0, and 2.0 mg Au show a decrease in the Hg signal over the four measurements taken (though the decrease is not pronounced with the 2.0 mg solution) and a steady Hg(II) signal is obtained with the 3.0 mg Au solution. These results suggest that there is an effect on the Hg(II) signal if the intensities are read in a sequence from more concentrated to less concentrated Au solutions (Table 4) or less concentrated to more concentrated solutions (Table 5); this is discussed in the following section.4. DiscussionThe results clearly indicate that the addition of a low level of gold(III) chloride to mercury-containing solutions yields a reproducibly stable signal. The mercury may be stabilized by binding to gold(III), though the nature of that binding is unclear. In optimizing the gold level, it became evident from the lowest Au level in Table 4 that there is another effect operative since the Hg signal intensity decreases with time (rather than in-creases from the Hg memory effect). It was also noted that the 0.5 mg Au solution was measured last in the sequence shown in Table 4 and that the sample introduction system had been washed with a solution of 1 M HNO3 containing 1 mg Au/L after measurement of the preceding 1 mg Au solution. The decrease in mercury signal intensity may thus be due to the fact that Au(III) has its own memory effect:[25] since Au(III) is retained in the sample introduction system, its release into subsequent Hg-containing solutions causes the decrease in Hg signal strength as the Au memory effect diminishes the Hg memory effect.The influence of Au(III) in the wash solution was removed by changing the wash solution to 2 M HCl (since the chloride ion complexes both Hg(II) and Au(III) to form HgCl2? and AuCl2?). Table 5 shows the signal from the 0.5 and 1.0 mg Au solutions decreases over time because of the memory effect from the Au in solution; at 2 mg Au, the Au in the solution is almost sufficient to overcome the Hg memory effect from the outset; at 3 mg Au, it is now completely sufficient to overcome the Hg memory effect and is thus the recommended ratio in routine studies. Use of 1 M HNO3 with 1 mg AuCl3 /L as the wash solution is recommended for more rapid removal of the mercury. Therefore the addition of gold(III) chloride or thiourea to Hg-containing solutions can obviate the memory effect through their ability to complex Hg(II). However, Au(III) has an additional advantage in having faster kinetics of complexation than thiourea thus allowing for greater analytical throughput.。