Detection of Nine M8.0-L0.5 Binaries The Very Low Mass Binary Population and its Implicatio

a r X i v :a s t r o -p h /0301095v 1 6 J a n 2003

Submitted 07/22/02,accepted 12/18/02,to appear in the April 10,2003issue of the Astrophysical Journal

Preprint typeset using L A T E X style emulateapj v.25/04/01

DETECTION OF NINE M8.0-L0.5BINARIES:THE VERY LOW MASS BINARY POPULATION AND ITS IMPLICATIONS FOR BROWN DWARF AND VLM STAR FORMATION

Laird M.Close 1,Nick Siegler 1,Melanie Freed 1,&Beth Biller 1

lclose@https://www.doczj.com/doc/07211645.html,

1Steward

Observatory,University of Arizona,Tucson,AZ 85721

Submitted 07/22/02,accepted 12/18/02,to appear in the April 10,2003issue of the Astrophysical Journal

ABSTRACT

Use of the highly sensitive Hokupa’a/Gemini curvature wavefront sensor has allowed direct adaptive optics (AO)guiding on very low mass (VLM)stars with SpT=M8.0-L0.5.A survey of 39such objects detected 9VLM binaries (7of which were discovered for the ?rst time to be binaries).Most of these systems are tight (separation <5AU)and have similar masses (?Ks <0.8mag;0.852.4mag and consist of a VLM star orbited by a much cooler L7-L8brown dwarf companion.Based on this ?ux limited (Ks <12mag)survey of 39M8.0-L0.5stars (mainly from the 2MASS sample of Gizis et al.(2000))we ?nd a sensitivity corrected binary fraction in the range 15±7%for M8.0-L0.5stars with separations >2.6AU.This is slightly less than the 32±9%measured for more massive M0-M4dwarfs over the same separation range (Fischer &Marcy 1992).It appears M8.0-L0.5binaries (as well as L and T dwarf binaries)have a much smaller semi-major axis distribution peak (～4AU)compared to more massive M and G dwarfs which have a broad peak at larger ～30AU separations.We also ?nd no VLM binary systems (de?ned here as systems with M tot <0.185M ⊙)with separations >15AU.

We brie?y explore possible reasons why VLM binaries are slightly less common,nearly equal mass,and much more tightly bound compared to more massive binaries.We investigate the hypothesis that the lack of wide (a >20AU)VLM/brown dwarf binaries may be explained if the binary components were given a signi?cant di?erential velocity kick.Such a velocity kick is predicted by current “ejection”theories,where brown dwarfs are formed because they are ejected from their embryonic mini-cluster and therefore starved of accretion material.We ?nd that a kick from a close triple or quadruple encounter (imparting a di?erential kick of ～3km/s between the members of an escaping binary)could reproduce the observed cut-o?in the semi-major axis distribution at ～20AU.However,the estimated binarity ( 5%;Bate et al.(2002))produced by such ejection scenarios is below the 15±7%observed.Similarly,VLM binaries could be the ?nal hardened binaries produced when a mini-cluster decays.However,the models of Sterzik &Durisen (1998);Durisen,Sterzik,&Pickett (2001)also cannot produce a VLM binary fraction above ～5%.The observed VLM binary frequency could possibly be produced by cloud core fragmentation.Although,our estimate of a fragmentation-produced VLM binary semi-major axis distribution contains a signi?cant fraction of “wide”VLM binaries with a >20AU in contrast to observation.In summary,more detailed theoretical work will be needed to explain these interesting results which show VLM binaries to be a signi?cantly di?erent population from more massive M &G dwarf binaries.

Subject headings:instrumentation:adaptive optics —binaries:general —stars:evolution —stars:

formation —stars:low-mass,brown dwarfs

1.introduction

Since the discovery of Gl 229B by Nakajima et al.(1995)there has been intense interest in the direct detection of brown dwarfs and very low mass (VLM)stars and their companions.According to the current models of Burrows et al.(2000)and Chabrier et al.(2000),stars with spectral types of M8.0-L0.5will be just above the stellar/substellar boundary.However,modestly fainter companions to such primaries could themselves be substellar.Therefore,a sur-vey of M8.0-L0.5stars should detect binary systems con-sisting of VLM primaries with VLM or brown dwarf sec-ondaries.

The binary frequency of M8.0-L0.5stars is interesting in its own right since little is known about how common M8.0-L0.5binary systems are.It is not clear currently if the M8.0-L0.5binary separation distribution is similar to that of M0-M4stars;in fact,there is emerging evidence that very low mass L &T dwarf binaries tend to have

smaller separations and possibly lower binary frequencies compared to more massive M and G stars (Mart′?n,Brand-ner,&Basri 1999;Reid et al.2001a;Burgasser et al.2003).Despite the strong interest in such very low mass (VLM)binaries (M tot <0.185M ⊙),only 24such systems are known (see Table 4for a complete list).A brief overview of these systems starts with the ?rst double L dwarf system which was imaged by HST/NICMOS by Mart′?n,Brand-ner,&Basri (1999).A young spectroscopic binary brown dwarf (PPL 15)was detected in the Pleiades (Basri &Mart′?n (1999))but this spectroscopic system is too tight to get separate luminosities for each component.A large HST/NICMOS imaging survey by Mart′?n et al.(2000)of VLM dwarfs in the Pleiades failed to detect any brown dwarf binaries with separations >0.2′′( 27AU).Detec-tions of nearby ?eld binary systems were more successful.The nearby object Gl 569B was resolved into a 0.1′′(1AU)binary brown dwarf at Keck and the 6.5m MMT (Mart′?n 1

2Close et al.

et al.(1999);Kenworthy et al.(2001);Lane et al.(2001)). Keck seeing-limited NIR imaging marginally resolved two more binary L stars(Koerner et al.(1999)).A survey with WFPC2detected four more(three newly discovered and one con?rmed from Koerner et al.(1999))tight equal mag-nitude binaries out of a sample of20L dwarfs(Reid et al. (2001a)).From the same survey Reid et al.(2002)found a M8binary(2M1047;later discovered independently by our survey).Guiding on HD130948with adaptive optics (AO),Potter et al.(2002a)discovered a companion binary brown dwarf system.Recently,Burgasser et al.(2003) have detected two T dwarf binaries with HST.Finally,12 more L dwarf binaries have been found by analyzing all the currently remaining HST/WFPC2data collected on L dwarfs(Bouy et al.2003).Hence,the total number of binary VLM stars and brown dwarfs currently known is just24.Of these,all but one have luminosities known for each component,since one is a spectroscopic binary. Here we have carried out our own high-spatial resolu-tion binary survey of VLM stars employing adaptive op-tics.In total,we have detected12VLM binaries(10of these are new discoveries)in our AO survey of69VLM stars.Three of these systems(LP415-20,LP475-855, &2MASSW J1750129+442404)have M7.0-M7.5spectral types and are discussed in detail elsewhere(Siegler et al. 2003).In this paper,we discuss the remaining9cooler binaries with M8.0-L0.5primaries detected in our survey (referred herein as M8.0-L0.5binaries even though they may contain L1-L7.5companions;see Table2for a com-plete list of these systems).

Two of these systems(2MASSW J0746425+200032 and2MASSW J1047127+402644)were in our sample but were previously imaged in the visible by HST and found to be binaries(Reid et al.(2001a,2002)). Here we present the?rst resolved IR observations of these two systems and new astrometry.The seven remaining systems were all discovered to be bina-ries during this survey.The?rst four systems dis-covered in our survey(2MASSW J1426316+155701, 2MASSW J2140293+162518,2MASSW J2206228-204705, and2MASSW J2331016-040618)have brief descriptions in Close et al.(2002b).However,we have re-analyzed the data from Close et al.(2002b)and include it here for completeness with slightly revised mass estimates. The very interesting M8/L7.5system LHS2397a discov-ered during this survey is discussed in detail elsewhere (Freed,Close,&Siegler2003)yet is included here for completeness.The newly discovered binaries2MASSW J1127534+741107and2MASSW J1311391+803222are presented here for the?rst time.

These nine M8.0-L0.5binaries are a signi?cant addi-tion to the other very low mass M8-T6binaries known to date listed in Table4(Basri&Mart′?n1999;Mart′?n, Brandner,&Basri1999;Koerner et al.1999;Reid et al. 2001a;Lane et al.2001;Potter et al.2002a;Burgasser et al.2003;Bouy et al.2003).With relatively short periods our new systems will likely play a signi?cant role in the mass-age-luminosity calibration for VLM stars and brown dwarfs.It is also noteworthy that we can start to charac-terize this new population of M8.0-L0.5binaries.We will outline how VLM binaries are di?erent from their more massive M and G counterparts.Since VLM binaries are so tightly bound we hypothesize that very little dynami-cal evolution of the distribution has occurred since their formation.We attempt to constrain the formation mech-anism of VLM stars and brown dwarfs from our observed semi-major axis distribution and binarity statistics.

2.an ao survey of nearby m8.0-l0.5field stars As outlined in detail in Close et al.(2002a),we utilized the University of Hawaii curvature adaptive optics system Hokupa’a(Graves et al.1998;Close et al.1998),which was a visitor AO instrument on the Gemini North Telescope. This highly sensitive curvature AO system is well suited to locking onto nearby,faint(V～20),red(V?I>3), M8.0-L0.5stars to produce～0.1′′images(which are close to the0.07′′di?raction-limit in the K′band).We can guide on such faint(I～17)targets with a curvature AO system(such as Hokupa’a)by utilizing its zero-read noise wavefront sensor(for a detailed explanation of how this is possible see Siegler,Close,&Freed(2002)).We utilized this unique capability to survey the nearest extreme M and L stars(M8.0-L0.5)to characterize the nearby VLM binary population.

Here we report the results of all our Gemini observing runs in2001and2002.We have observed all32M8.0-M9.5stars with Ks<12mag from the list of Gizis et al. (2000).It should be noted that the M8.0-M9.5list of Gizis et al.(2000)has some selection constraints:galactic lati-tudes are all>20degrees;and from0

Nine of our39targets were clearly tight binaries(sep< 0.5′′).We observed each of these objects by dithering over4di?erent positions on the QUIRC1024×1024NIR (1?2.5μm)detector(Hodapp et al.1996)which has a 0.0199′′/pixel plate scale at Gemini North.At each po-sition we took3x10s exposures at J,H,K′,and3x60s exposures at H,resulting in unsaturated120s exposures at J,H,and K′with a deep720s exposure at H band for each binary system.

3.reductions

We have developed an AO data reduction pipeline in the IRAF language which maximizes sensitivity and image res-olution.This pipeline is standard IR AO data reduction and is described in detail in Close et al.(2002a).

Unlike conventional NIR observing at an alt-az telescope (like Gemini),we disable the Cassegrain rotator so that the pupil image is?xed w.r.t.the detector.Hence the optics are not rotating w.r.t.the camera or detector.In this manner the residual static PSF aberrations are?xed in the AO images enabling quick identi?cation of real com-panions as compared to PSF artifacts.The pipeline cross-correlates and aligns each image,then rotates each image so north is up and east is to the left,then median combines

Very Low Mass Binaries3

the data with an average sigma clip rejection at the±2.5σlevel.By use of a cubic-spline interpolator the script pre-serves image resolution to the<0.02pixel level.Next the custom IRAF script produces two?nal output images, one that combines all the images taken and another where only the sharpest50%of the images are combined.

This pipeline produced?nal unsaturated120s expo-sures at J(F W HM～0.15′′),H(F W HM～0.14′′), and K′(F W HM～0.13′′)with a deep720s exposure (F W HM～0.14′′)at H band for each binary system.The dithering produces a?nal image of30x30′′with the most sensitive region(10×10′′)centered on the binary.Figures 1and2illustrates K′images of each of the systems.

In Table2we present the analysis of the images taken of the9new binaries from our Gemini observing runs. The photometry was based on DAOPHOT PSF?tting photometry(Stetson1987).The PSFs used were the re-duced12x10s unsaturated data from the next(and pre-vious)single VLM stars observed after(and before)each binary.The PSF stars always had a similar IR brightness, a late M spectral type,and were observed at a similar airmass.The resulting?magnitudes between the compo-nents are listed in Table2;their errors in?mag are the di?erences in the photometry between two similar PSF stars.The individual?uxes were calculated from the?ux ratio measured by DAOPHOT.We made the assumption ?K′～?Ks,which is correct to0.02mag according the models of Chabrier et al.(2000).Assuming?K′=?Ks allows us to use the2MASS integrated Ks?uxes of the blended binaries to solve for the individual Ks?uxes of each component(see Table3).

The platescale and orientation of QUIRC was deter-mined from a short exposure of the Trapezium cluster in Orion and compared to published positions as in Simon, Close,&Beck(1999).From these observations a platescale of0.0199±0.0002′′/pix and an orientation of the Y-axis (0.3±0.3degrees E of north)was determined.Astrom-etry for each binary was based on the PSF?tting.The astrometric errors were based on the range of the3values observed at J,H,and K′and the systematic errors in the calibration added in quadrature.

4.analysis

4.1.Are the companions physically related to primaries? Since Gizis et al.(2000)only selected objects>20de-grees above the galactic plane,we do not expect many background late M or L stars in our images.In the3.6x104 square arcsecs already surveyed,we have not detected a very red J?Ks>0.8mag background object in any of the?elds.Therefore,we estimate the probability of a chance projection of such a red object within<0.5′′of the primary to be<2x10?

5.Moreover,given the rather low space density(0.0057±0.0025pc?3)of L dwarfs(Gizis et al.2001),the probability of a background L dwarf within <0.5′′of any of our targets is<10?1

6.We conclude that all these very red,cool objects are physically related to their primaries.

4.2.What are the distances to the binaries? Unfortunately,there are no published trigonometric par-allaxes for six of the nine systems.The three systems with parallaxes are:2M0746(Dahn et al.2002);LHS2397a (van Altena,Lee,&Ho?eit1995);and2M2331(associ-ated with a Hipparcos star HD221356Gizis et al.(2000)). For the remaining six systems without parallaxes we can estimate the distance based on the trigonometric paral-laxes of other well studied M8.0-L0.5stars from Dahn et al.(2002).The distances of all the primaries were deter-mined from the absolute Ks magnitudes(using available 2MASS photometry for each star with trigonometric par-allaxes from Dahn et al.(2002)),which can be estimated by M Ks=7.71+2.14(J?Ks)for M8.0-L0.5stars(Siegler et al.2003).This relationship has a1σerror of0.33mag which has been added in quadrature to the J and Ks pho-tometric errors to yield the primary component’s M Ks val-ues in Table3and plotted as crosses in Figures3-11.As can be seen from Table3all but one of our systems are within29pc(the exception is2M1127at～33pc).

4.3.What are the spectral types of the components? We do not have spatially resolved spectra of both com-ponents in any of these systems;consequently we can only try to?t the M Ks values in Table3to the rela-tion SpT=3.97M Ks?31.67which is derived from the dataset of Dahn et al.(2002)by Siegler et al.(2003). Unfortunately,the exact relationship between M Ks and VLM/brown dwarf spectral types is still under study.It is important to note that these spectral types are only a guide since the conversion from M Ks to spectral type car-ries at least±1.5spectral subclasses of uncertainty.For-tunately,none of the following analysis is dependent on these spectral type estimates.

It is interesting to note that six of these secondaries are likely L dwarfs.In particular2M2331B is likely a L7and LHS2397aB is likely a L7.5.Both2M2331B and LHS 2397aB are very cool,late L companions.

4.4.What are ages of the systems? Estimating the exact age for any of these systems is dif-?cult since there are no Li measurements yet published (which could place an upper limit on the ages).An ex-ception to this is LHS2397a for which no Li was detected Mart′?n,Rebolo,&Magazzu(1994).For a detailed discus-sion on the age of LHS2397a see Freed,Close,&Siegler (2003).For each of the remaining systems we have conser-vatively assumed that the whole range of common ages in the solar neighborhood(0.6to7.5Gyr)may apply to each system(Caloi et al.1999).However,Gizis et al.(2000) observed very low proper motion(V tan<10km/s)for the 2M1127,2M2140,and2M2206systems.These three sys-tems are among the lowest velocity M8’s in the entire sur-vey of Gizis et al.(2000)suggesting a somewhat younger age since these systems have not yet developed a signi?-cant random velocity like the other older(～5Gyr)M8.0-L0.5stars in the survey.Therefore,we assign a slightly

younger age of3.0+4.5

?2.4

Gyr to these3systems,but leave large error bars allowing ages from0.6-7.5Gyr(～3Gyr is the maximum age for the kinematically young stars found by Caloi et al.(1999)).The other binary systems2M0746, 2M1047,2M1311,and2M2331appear to have normal V tan and are more likely to be older systems.Hence we assign

an age of5.0+2.5

?4.4

Gyr to these older systems(Caloi et al. 1999).It should be noted that there is little signi?cant di?erence between the evolutionary tracks for ages1?10

4Close et al.

Gyr when SpT

the exact age is not absolutely critical to estimating the

approximate masses for M8.0-L0.5stars(see Figure3).

4.5.The masses of the components

To estimate masses for these objects we will need to

rely on theoretical evolutionary tracks for VLM stars and

brown dwarfs.Calibrated theoretical evolutionary tracks

are required for objects in the temperature range1400-

2600K.Recently such a calibration has been performed

by two groups using dynamical measurements of the M8.5

Gl569B brown dwarf binary.From the dynamical mass

measurements of the Gl569B binary brown dwarf(Ken-

worthy et al.2001;Lane et al.2001)it was found that

the Chabrier et al.(2000)and Burrows et al.(2000)evo-

lutionary models were in reasonably good agreement with

observation.In Figures3to11we plot the latest DUSTY

models from Chabrier et al.(2000)which have been spe-

cially integrated across the Ks?lter so as to allow a direct

comparison to the2MASS Ks photometry(this avoids the

additional error of converting from Ks to K for very red

objects).We extrapolated the isochrones from0.10to0.11

M⊙to cover the extreme upper limits of some of the pri-

mary masses in the?gures.

We estimate the masses of the components based on the

age range of0.6-7.5Gyr and the range of M Ks values.The

maximum mass relates to the minimum M Ks and the max-

imum age of7.5Gyr.The minimum mass relates to the

maximum M Ks and the minimum age of0.6Gyr.These

masses are listed in Table3and illustrated in Figures3to

11as?lled polygons.

At the younger ages(<1Gyr),the primaries may be on

the stellar/substellar boundary,but they are most likely

VLM stars.The substellar nature of the companion is very

likely in the case of2M2331B and LHS2397aB,possible in

the cases of2M0746B,2M1426B,and2M2140B,and un-

likely in the cases of2M1047B,2M1127B,2M1311B,and

2M2206B which all appear to be VLM stars like their pri-

maries.Hence two of the companions are brown dwarfs,

three others may also be substellar,and four are likely

VLM stars.

5.discussion

5.1.The binary frequency of M8.0-L0.5stars

We have carried out the largest?ux limited(Ks<12)

high spatial resolution survey of M8.0-L0.5primaries.

Around these39M8.0-L0.5targets we have detected9

systems that have companions.Since our survey is?ux

limited we need to correct for our bias toward detect-

ing equal magnitude binaries that“leak”into our sample

from further distances.For example,an equal magnitude

M8binary could have an integrated2MASS magnitude

of Ks=12mag but be actually located at36pc whereas

a single M8star of Ks=12would be located just26pc

distant.Hence our selection of Ks<12leads to incom-

pleteness of single stars and low mass ratio(q 10f(ρ)dρ(2)

We consider two limiting cases for the f(ρ)distribution:

1)if all the systems are equal magnitude(q=ρ=1)

thenα=2(3/2)=2.8and the BF=12%;2)if there is

a?at f(ρ)distribution thenα=1.9and the BF=19%.

Consequently,the binary frequency range is12-19%which

Very Low Mass Binaries5 is identical to the range estimated https://www.doczj.com/doc/07211645.html,ter we will

see(Figure14)the true f(ρ)is indeed a compromise be-

tween?at and unity;hence we split the di?erence and

adopt a binary frequency of15±7%where the error is

the Poisson error(5%)added in quadrature to the(～4%)

uncertainty due to the possible range of the q distribution

(1.9<α<2.8).It appears that for systems with separa-

tions2.6

within the range15±7%.

Our M8.0-L0.5binary fraction range of15±7%is

marginally consistent with the28±9%measured for more

massive M0-M4dwarfs(Fischer&Marcy1992)over the

same separation/period range(2.6

in this study.However,Fischer&Marcy(1992)found a bi-

nary fraction of32±9%over the whole range of a>2.6AU.

If we assume that there are no missing low mass wide bi-

nary systems with a>300AU(this is a good assumption

since such wide sep 15′′systems would have been easily

detected in the2MASS point source catalog as is illus-

trated in Figure12),then our binary fraction of15±7%

would be valid for all a>2.6AU and would therefore be

slightly lower than32±9%observed for M0-M4dwarfs

with a>2.6AU by Fischer&Marcy(1992).Hence it ap-

pears VLM binaries(M tot<0.185M⊙)are less common

(signi?cant at the95%level)than M0-M4binaries over

the whole range a>2.6AU.

5.2.The separation distribution function for M8.0-L0.5

binaries

The M8.0-L0.5binaries are much tighter than M0-M4

dwarfs in the distribution of their semi-major axes.The

M8.0-L0.5binaries appear to peak at separations～4AU

which is signi?cantly tighter than the broad～30AU peak

of both the G and M star binary distributions(Duquennoy

&Mayor1991;Fischer&Marcy1992).This cannot be a

selection e?ect since we are highly sensitive to all M8.0-

L0.5binaries with sep>20?300AU(even those with

?H>10mag).Therefore,we conclude that M8.0-L0.5

stars likely have slightly lower binary fractions than G and

early M dwarfs,but have signi?cantly smaller semi-major

axes on average.

6.the vlm binary population in general

More observations of such systems will be required to

see if these trends for M8.0-L0.5binaries hold over bigger

samples.It is interesting to note that in Reid et al.(2001a)

an HST/WFPC2survey of20L stars found4binaries and

a similar binary frequency of10-20%.The widest L dwarf

binary in Koerner et al.(1999)had a separation of only9.2

AU.A smaller HST survey of10T dwarfs by Burgasser

et al.(2003)found two T binaries and a similar binary

frequency of9+15

?4%with no systems wider than5.2AU.

Therefore,it appears all M8.0-L0.5,L,and T binaries may have similar binary frequencies～9?15%(for a>3AU). In Table4we list all the currently known VLM bina-ries(de?ned in this paper as M tot<0.185M⊙)from the high-resolution studies of Basri&Mart′?n(1999);Mart′?n, Brandner,&Basri(1999);Koerner et al.(1999);Reid et al. (2001a);Lane et al.(2001);Potter et al.(2002a);Burgasser et al.(2003);Bouy et al.(2003).As can be seen from Fig-ure13VLM binaries have a～4AU peak in their separa-tion distribution function with no systems wider than15AU.

From Figure14we see that most VLM binaries have nearly equal mass companions,and no system has q<0.7. This VLM q distribution is di?erent from the nearly?at q distribution of M0-M4stars(Fischer&Marcy1992).Since the HST surveys of Mart′?n,Brandner,&Basri(1999); Reid et al.(2001a);Burgasser et al.(2003);Bouy et al. (2003)and our AO surveys were sensitive to1.0>q 0.5 for systems with a>4AU,the dearth of systems with 0.8>q>0.5in Figure14is likely a real characteristic of VLM binaries and not just a selection e?ect of insensi-tivity.However,these surveys become insensitive to tight (a<4AU)systems with q<0.5,hence the lack of detec-tion of such systems may be purely due to insensitivity.

6.1.Why are there no wide VLM binaries?

It is curious that we were able to detect8systems in the range0.1?0.25′′but no systems were detected past 0.5′′(～16AU).This is surprising since we(as well as the HST surveys of Mart′?n,Brandner,&Basri(1999);Reid et al.(2001a);Burgasser et al.(2003);Bouy et al.(2003)) are very sensitive to any binary system with separations >0.5′′and yet none were found.One may worry that this is just a selection e?ect in our target list from the spectroscopic surveys of Gizis et al.(2000)and Cruz et al.(2003),since they only selected objects in the2MASS point source catalog.There is a possibility that such a catalog would select against0.5′′?2.0′′binaries if they appeared extended in the2MASS images.However,we found that marginally extended PSFs due to unresolved binaries(separation 2′′)were not being classi?ed as ex-tended and therefore were not removed from the2MASS point source catalog.For example,Figure12illustrates that no known T-Tauri binary from list of White&Ghez (2001)was removed from the2MASS point source cat-alog.Although,due to the relatively poor resolution of 2MASS(FWHM～2?3′′),only systems with separations >3′′were classi?ed as binaries by2MASS,all the other T-Tauri binaries were unresolved and mis-classi?ed as single stars.In any case,we are satis?ed that no“wide”(0.5′′ separation 2′′)VLM candidate systems were tagged as extended and removed from the2MASS point source cata-log.Therefore,the lack of a detection of any system wider than0.5′′is not a selection e?ect of the initial use of the 2MASS point source catalog for targets.

Our observed dearth of wide systems is supported by the results of the HST surveys where out of16L and two T binaries,no system with a separation>13AU was detected.We?nd the widest M8.0-L0.5binary is16 AU while the widest L dwarf binary is13AU(Bouy et al.2003),and the widest T dwarf binary is5.2AU(Bur-gasser et al.2003).However,M dwarf binaries just slightly more massive( 0.2M⊙)in the?eld(Reid&Gizis1997a) and in the Hyades(Reid&Gizis1997b)have much larger separations from50-200AU.

In Figure15we plot the sum of primary and secondary component masses as a function of the binary separation for all currently known VLM binaries listed in Table4. It appears that all VLM and brown dwarf binaries(open symbols)are much tighter than the slightly more massive M0-M4binaries(solid symbols).

If we examine more massive(SpT=A0-M5)wide bi-

6Close et al.

nary systems from Close et al.(1990),we see such wide binaries have maximum separations best ?t by a max ～1000(M tot /0.185M ⊙)AU in Figure 15.Any system with such a separation (a =a max )will be disrupted by a di?er-ential velocity impulse (or “escape kick”)to one compo-nent of V esc >0.57km/s (see the solid upper line in Figure 15).However,if we try to ?t the VLM/brown dwarf bi-naries (de?ned here as systems with M tot <0.185M ⊙)we ?nd the maximum separation is better predicted by a max V LM ～23.2(M tot /0.185M ⊙)AU (lower dashed line in Figure 15).Since these are much smaller separa-tions we ?nd that any system with such a separation (a =a max V LM )will require a larger escape velocity kick of V esc >3.8km/s to become unbound.

To try and glean if VLM/brown dwarf binaries are re-ally more tightly bound,we compare their binding en-ergy to that of more massive binaries.Figure 16illus-trates how the minimum binding energy of binaries with M tot <0.185M ⊙is 16x harder than more massive ?eld M-G binaries.In other words the widest binaries with M tot <0.185M ⊙appear to be 16times “more bound”than the widest binaries with M tot >0.185M ⊙.

This hardening could be a relic from dissipative star/disk interactions with the accretion disks around each star (McDonald &Clarke 1995)and/or it could be from an ejection event,dynamical decay,or fragmentation.We will brie?y explore some of these possibilities in the next sections.

6.1.1.Can ejection explain the lack of wide VLM

binaries?Reipurth &Clarke (2001)suggest that VLM binary sys-tems may have been ejected from their “mini-cluster”stel-lar nurseries in close triple or quadruple encounters with more massive objects early in their lives.Consequently,these ejected VLM objects are starved of accretion mate-rial and their growth is truncated (see a cartoon of this scenario in Fig.17).Sterzik &Durisen (1998)estimate that around 5%of ejecta from pentuple systems are bi-naries.The typical ejection velocity estimated by Sterzik &Durisen (1998);Reipurth &Clarke (2001)is ～3km/s for single objects (on average these ejected single objects have masses >0.185M ⊙).Hence one might hypothesize tight VLM binaries of similar total mass may be ejected at similar velocities;however,we caution that more de-tailed simulations are required to estimate realistic ejec-tion velocities.In any case,only tightly bound binaries will survive the ejection process.This may explain the additional tightness of the VLM/brown dwarf binaries in Figure 15.More loosely bound VLM binaries were dis-solved (“kicked”past the V esc =3.8km/s line in Fig.15)when they were ejected from their mini-cluster.Hence,only relatively hard a <16AU (V esc >3.8km/s)low mass binaries have survived until today.

However,the ejection paradigm of Reipurth &Clarke (2001)as simulated in detail by Bate et al.(2002)only pre-dict a VLM/brown dwarf binary fraction of 5%which is below the 15±7%observed for M8.0-L0.5binaries and the 9+15?4observed for T dwarf binaries (Burgasser et al.2003).Therefore,further simulations will be required to see if a larger ( 9%)binary frequency can be produced when low mass binaries are ejected from the mini-cluster.

6.1.2.Is the dearth of wide low mass binaries due to

Galactic dynamical evolution?Over the lifetime of a binary there will be many stochas-tic encounters,which will increase the potential energy (and therefore its separation)of the binary slowly over time.Eventually,these encounters may also disrupt the binary.In fact,this disruption timescale is roughly pro-portional to M tot /a for separations <200AU according to the detailed models of Weinberg et al.(1987).As we can see from the solid line in Figure 15,a line of constant M tot /a or constant V esc (where V esc =0.57km/s)can ?t the widest A0-M0binaries,but the same line cannot ?t the widest of the much tighter VLM binaries (open sym-bols).This is quite puzzling since wide (>200AU)VLM binaries should get wider throughout their lifetime until they reach a =a max ～1000(M tot /0.185M ⊙)AU.By this point,they have reached the average minimum binding en-ergy (?E bind min ～3x1041erg)of ?eld wide binaries and are likely to be disrupted by a di?erential kick of only 0.6km/s.

So why do we not observe wider VLM binaries if they should dynamically evolve to a =a max ～1000(M tot /0.185M ⊙)AU ～1000AU over time?It is critical to note that only binaries that are wider than 1000(M tot /M ⊙)AU are wide enough to signi?cantly evolve in the galactic disk according the detailed models of Wein-berg et al.(1987).Therefore,only binaries of initial sepa-ration (a o )greater than a o ～185AU for M tot =0.185M ⊙will evolve to the minimum binding energy (which corre-sponds to separation of a max ～1000AU)over a 12Gyr lifetime.Any systems formed tighter than 1000(M tot /M ⊙)AU will not dynamically evolve to signi?cantly wider sep-arations over time.

So why don’t we detect any 185-1000AU VLM binaries?The answer may be very simple.If the formation process that forms VLM/brown dwarf binaries cannot produce bi-naries with a o >1000(M tot /M ⊙)AU then there will be no signi?cant evolution of a during the lifetime of the bi-naries.In other words the observed a distribution will be similar to the initial distribution (a o ).Since we currently observe no VLM systems with a >100AU,the observed separation distribution for VLM binaries is likely the same as their initial distribution.Figures 15and 16suggest that a o for VLM stars is strongly truncated at ～16AU.In addition,the observed values of V esc >3.8km/s and ?E bind >50×1041erg are therefore likely relics of the formation of these VLM binaries and need to be explained by any successful model of star/brown dwarf formation.6.1.3.Could VLM binaries be the decay product of a

dissolving mini-cluster?In the dynamical decay of these “mini-clusters”it is likely that the most massive components become a hard-ened binary as the lower mass objects are ejected by triple encounters (see bottom of Figure 17).Reipurth &Clarke (2001)have hypothesized that VLM binaries may also be ejected from these mini-clusters since they have very low masses and are tight enough to remain bound after ejec-tion.However,it is also possible that VLM binaries are not ejected but instead persevere until they are the ?nal most tightly bound binary remaining after the “mini-cluster”decays.

Very Low Mass Binaries7

The decay of3,4,and5-body“mini-clusters”has been studied by Sterzik&Durisen(1998).The?nal residual bi-naries produced by such decays have characteristics similar to those observed for VLM binaries.In particular,Sterzik &Durisen(1998)predict that binaries composed of stars with masses from0.1?0.2M⊙would have separations of ～10AU compared to～30AU for more massive stars. Furthermore,they predict the mass ratio q to be in the range0.6-1.0.Moreover,they predict a fairly sharp cut-o?at～10AU for“wide”low mass binaries.All these predictions are roughly consistent with our observations of the VLM binary population.

Since the?nal binary produced is typically biased to-wards the two or three most massive members of the mini-cluster(true of～99?98%cluster decays;Sterzik &Durisen(1998)),the likelihood of the most massive objects in the mini-cluster both having masses less than 0.090M⊙is rather small.Indeed,the binary fraction of 0.2

6.1.4.Can we explain the truncation at～16AU simply

by an initial semi-major axis distribution produced

by fragmentation?

One could avoid the problem of the unlikelihood of VLM binaries surviving the dynamics of an ejection or cluster decay if one simply produces VLM binaries in the same fashion that more massive binaries are thought to form. This is commonly thought to be through the fragmenta-tion of molecular cloud cores.A fragmenting very low mass molecular cloud core could directly produce a VLM binary without evoking any ejection or decay processes. It would be interesting to see if we can approximate a VLM binary semi-major axis distribution produced by fragmentation by scaling distributions of more massive bi-naries.If we assume M tot/a o is roughly constant for wide binaries(see Figure15),then we expect the initial a o dis-tribution to be somewhat tighter for VLM binaries com-pared to more massive binaries.We can estimate the a o for solar mass binaries from the well known young T-Tauri star a distributions.Young T-Tauri binaries are strongly suspected to have formed by fragmentation(White&Ghez 2001).We can utilize a HST sample of29T-Tauri binary systems in Taurus to produce an initial separation distri-bution.Such a distribution from the sample of White& Ghez(2001)has an a o peak at～30AU(as also found by Leinert et al.(1993);Ghez et al.(1993)).We can scale this to match our observed VLM binary peak of～4AU by simply scaling the distribution by a o

V LM

～0.13(a o

T T au

). Similarly we can scale the masses of the T Tauri binaries to also match our mean VLM binary mass of～0.15M⊙by multiplying by the same factor of0.13.In this manner we have created a plausible fragmentation-produced VLM a o distribution.However,we have assumed that the other properties of the distribution(such as the FWHM and tail distributions)can be scaled along with the mean a o and M tot.Assuming such a scaling is valid globally,we see that this fragmentation a o

V LM

distribution has～26%of systems wider than～40AU.Hence if our observed VLM distribution is to match this scaled fragmentation a o dis-tribution,we would expect～9of the34VLM/brown dwarf systems observed(see Table4)to have separations greater than40AU.However,from Figure15no VLM systems are observed to have separations greater than16 AU.Therefore,it appears very hard to explain the total lack of systems with separations greater than16AU by scaling the observed a o distribution of T-Tauri stars.In other words,if we scale the distribution to match the VLM ～4AU peak we over-produce wide systems(26%of sys-tems wider than40AU)compared to observations.It may be that our“toy model”of simply scaling the T Tauri a o distribution is too simple an approach,but,it is a?rst step which shows that such a sharp cut-o?(a o<16AU)is not easily produced by a fragmentation distribution.Detailed fragmentation simulations at the lowest masses will be re-quired to prove whether fragmentation can produce the VLM binary separation distribution observed.

6.1.5.What e?ect does an ejection“kick”have on the

binary distribution?

Assuming(as has been suggested by Reid et al.(2002); Bate et al.(2002))that eventually a close multiple en-counter ejects VLM(<0.185M⊙)binaries,it would be interesting to see if such an“ejected”VLM binary dis-tribution would have a maximum separation of～16AU as observed.The full dynamical modeling of the e?ect of a binary’s ejection from a mini-cluster is beyond the scope of this paper.However,we can simply explore,as a “toy-model”,the possibility that the ejection process pro-vides a hard di?erential velocity kick of order～3km/s to the two components of an ejected VLM binary.We take the value of～3km/s since this is the value calculated for the ejection velocity of on average more massive single objects from a dissolving mini-cluster(Sterzik&Durisen 1998;Reipurth&Clarke2001).We assume,this may be of the right order of magnitude for the di?erential velocity kick between the VLM members of the ejected binary.We caution that this is only a simple toy-model,the true dif-ferential velocity applied during the ejection can only be calculated by detailed simulations of each ejection event. In our simple toy-model ejection only binaries with V esc 3.0km/s will survive the ejection process.More-over,systems that are more tightly bound than V esc=3.0 will hardly be e?ected by such a kick in the tidal limit of Weinberg et al.(1987).To survive in the general galactic

8Close et al.

?eld we see (from Figure 15)V esc 0.6km/s.Hence one may expect today’s population of VLM binaries to all have V esc V LM (3.0+0.6)km/s.

It is very interesting to note that we “observe”(from ?gure 15)V esc 3.8km/s for wide VLM binaries which is similar to the “predicted”value of V esc V LM 3.6km/s.Therefore,the observed cut-o?at 16AU could be pro-duced by a population of VLM binaries where each system has been subjected to a di?erential kick of ～3km/s in the ejection process.We conclude that it is possible to produce the observed distribution of semi-major axes for VLM bi-naries by applying a strong di?erential velocity kick to each binary in the distribution.However,it is still not clear if a VLM binary frequency of 9?15%could be produced by ejection,since ejection of a VLM tight binary may be a very rare event.

7.future observations

Future observations of all of these binaries should deter-mine if there is still Li present in their spectrum.The most useful Li observation would spatially resolve both compo-nents in the visible spectrum.Currently there is no visible wavelength AO systems capable of guiding on a V ～20source and obtaining resolutions better than 0.1′′in the op-tical (Close 2000,2002).Therefore,we will have to carry out such observations from space with HST/STIS perhaps.Trigonometric parallax measurements should also be ob-tained in the near future.Within the next few years one should also be able to measure the masses of both compo-nents of several of these systems as they complete a signif-icant fraction of their orbit.Hence,further observations of these systems are a high priority for the calibration of the brown dwarf mass-age-luminosity relation.Continued searches for new VLM binaries will help de?ne this very interesting and important binary population with greater precision.

8.conclusions about the vlm population in

general

Based on all 34VLM (M tot <0.185M ⊙)systems cur-rently known from this work and that of Basri &Mart′?n (1999);Mart′?n,Brandner,&Basri (1999);Koerner et al.(1999);Reid et al.(2001a);Lane et al.(2001);Potter et al.(2002a);Burgasser et al.(2003);Bouy et al.(2003),we ?nd the following general characteristics:

?VLM binaries are tight (peak separation ～4AU)with no systems wider than 16AU (this is ～10times tighter than the slightly more massive M0-M4binaries);?they tend to have nearly equal mass companions (q ～0.9)with no detected companions below ～70%of the primary’s mass;?they have a corrected binary frequency of 15±7%for spectral types M8-L0.5,and 9+15?4%for T dwarf bi-naries (Burgasser et al.2003)for separations a >2.6AU.This is less than the 32±9%binary frequency of M0-M4binaries (Fischer &Marcy 1992)and the ～50%binarity of G dwarf binaries (Duquennoy &Mayor 1991)for similar separations.

We ?nd the formation of such a VLM distribution to be problematic with current simulations of binary star forma-tion.

?Ejecting VLM binaries from their formation mini-clusters (Reipurth &Clarke 2001)can likely pro-duce the observed cut-o?in separation at 16AU,but ejection simulations (Bate et al.2002)produce a binarity of 5%which is smaller than the ～9?15%binarity observed.?If we ignore ejection and assume that these VLM binaries are instead the residual (most massive)sys-tem remaining from the dynamical decay of the mini-cluster one can likely also reproduce the tight cut-o?at 16AU.But the strong bias to higher masses in the ?nal binary predicts that too few VLM binaries would be made in this fashion –assuming each component is picked randomly from the IMF (Sterzik &Durisen 1998;Durisen,Sterzik,&Pickett 2001).?We brie?y look at a “toy-model”of a VLM binary population produced by fragmentation.It appears that simply scaling an observed fragmentation pro-duced T-Tauri distribution to force a peak at 4AU produces systems with separations much greater than the 16AU cut-o?observed.Hence,it is not obvious that the characteristics of the VLM binary distribution are accurately predicted by any of these methods.Future detailed modeling (taking into account circumstellar gas disks and their e?ects on dy-namical interactions/ejections)will be required to see if ejection or dynamical decay can be common enough to ex-plain the ～9?15%binarity of VLM systems.Future detailed fragmentation simulations of the smallest cores will be needed to discern if some type of truncation occurs at low masses that limits VLM binaries to separations of less than 16AU.

The Hokupa’a AO observations were supported by the University of Hawaii AO group.(D.Potter,O.Guyon,&P.Baudoz).Support for Hokupa’a comes from the Na-tional Science Foundation.We thank the anonymous ref-eree for many detailed comments that led to an all round better paper.LMC acknowledges support by the AFOSR under F49620-00-1-0294and from NASA Origins NAG5-12086.We would also like to send a big mahalo nui to the Gemini operations sta?.These results were based on observations obtained at the Gemini Observatory,which is operated by the Association of Universities for Research in Astronomy,Inc.,under a cooperative agreement with the NSF on behalf of the Gemini partnership:the Na-tional Science Foundation (United States),the Particle Physics and Astronomy Research Council (United King-dom),the National Research Council (Canada),CONI-CYT (Chile),the Australian Research Council (Australia),CNPq (Brazil)and CONICET (Argentina).

Very Low Mass Binaries9 REFERENCES

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Burrows,A.,Hubbard,W.B.,Lunine,J.I.,Marley,M.S.,Saumon, D.2000,Protostars and Planets IV(Tucson:University of Arizona Press,eds Mannings,V.,Boss,A.P.,Russell,S.S.),p.1339 Basri,G.,&Mart′?n,E.L.1999,AJ,118,2460

Bouy,H.,Brandner W.,Mart′?n,E.,Delfosse,X.,Allard,F.,&Basri, G.2003,AJ,submitted

Caloi,V.,Cardini,D.,D’Antona,F.,Badiali,M.,Emanuele,A., Mazzitelli,I.1999,A&A,351,925.

Chabrier,G.,Bara?e,I.,Allard,F.,&Hauschildt,P.2000,ApJ,542, 464

Close,L.M.,Richer,H.B.,&Crabtree,D.R.1990,AJ,100,1968 Close,L.M.,Roddier,F.J.,Roddier,C.A.,Graves,J.E.,Northcott, M.J.,Potter,D.1998,Proc.SPIE Vol.3353,p.406-416.Adaptive Optical System Technologies,D.Bonaccini,R.K.Tyson,Eds Close,L.M.2000,Proc.SPIE Vol.4007,p758-772.Adaptive Optical Systems Technology,P.L.Wizinowich,Ed.

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Durisen,R.H.,Sterzik,M.F.,&Pickett,B.K.2001,A&A,371,952 Duquennoy,A.,Mayor,M.1991,A&A,248,485

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Freed,M.,Close,L.M.,&Siegler,N.2003,ApJ,in press(Feb10 issue)

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Ghez,A.M.,Neugebauer,G.&Matthews,K.1993,AJ,106,2005 Gizis,J.E.,Kirkpatrick,J.D.,Burgasser,A.,Reid,I.N.,Monet,D.G., Liebert,J.,Wilson,J.C.2001,ApJ,551,L163

Gizis,J.E.et al.,Monet,D.G.,Reid,I.N.,Kirkpatrick,J.D.,Liebert, J.,Williams,R.J.2000,AJ,120,1085

Hodapp,K.-W.,Hora,J.L.,Hall,D.N.B.,Cowie,L.L.,Metzger, M.,Irwin,E.,Vural,K.,Kozlowski,L.J.,Cabelli,S.A.,Chen,C. Y.,Cooper,D.E.,Bostrup,G.L.,Bailey,R.B.,Kleinhans,W.E. 1996,New Astronomy,1,177

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Siegler,N.,Close,L.M.,Mamajek,E.,Freed,M.2003,ApJ,in prep Simon,M.,Close,L.M.,&Beck,T.1999,AJ,117,1375

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Tinney,C.G.,Mould,J.R.,&Reid,I.N.1993,AJ,105,1045 Wainscoat R.J.,&Cowie,L.L.1992,AJ,103,332.

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10Close et al.

Table1

M8.0-L0.5Stars Observed with No Likely Physical Companions Between0.1′′-15′′

2MASS other name K s SpT Ref.

References.—(1)Gizis et al.(2000);(2)Kirkpatrick et al.(1995);(3)Cruz et

al.(2003);(4)Kirkpatrick et al.(1997);(5)Tinney et al.(1993)

Very Low Mass Binaries11

Table2

The binary systems observed

System?J?H?K′Sep.(′′)PA Age(Gyr)

a discovery paper Reid et al.(2001a)

b discovery paper Reid et al.(2002)

c observations made on Feb7,2002UT

d observations mad

e on April25,2002UT

e observations made on June20,2001UT

f observations made on Sept20,2001UT

g discovery paper Freed,Close,&Siegler(2003)

12Close et al.

Table3

Summary of the new binaries’A&B components

Name J H Ks M K s c SpT a Est.Mass b Est.D(pc)c Sep.(AU)P(yr)d

a Spectral types estimated by3.97x M

?31.67(Dahn et al.2002;Siegler et al.2003)with±1.5spectral subclasses of error in these estimates

Ks

(note a value of SpT=10is de?ned as L0in the above equation).

b Masses(in solar units)from the models of Chabrier et al.(2000)–see Figure3.

c Photometric distances estimate

d for th

e primaries by M

Ks=7.71+2.14(J?Ks)which is valid for M7

d Periods estimated assuming face-on circular orbits.In th

e cases of2M0746and LHS2397a,the larger o

f the2observed separations(2.7AU

&3.86AU from Reid et al.(2001a)and Freed,Close,&Siegler(2003),respectively)have been used to more accurately estimate the period.

e These systems have trigonometric parallaxes.

Very Low Mass Binaries13

Table4

All Known Resolved VLM Binaries e

Name Sep.Est.Est.M A Est.M B Est.Period b Ref.c

AU SpT A/SpT B M⊙M⊙yr

a PPL15is a spectroscopic binary(Basri&Mart′?n1999)

b This“period”is simply an estimate assuming a face-on circular orbit

c REFERENCES–(0)Basri&Mart′?n(1999);(1)Kenworthy et al.(2001);(2)Lane et al.(2001);(3)Potter et al.

(2002a);(4)Reid et al.(2001a);(5)Mart′?n,Brandner,&Basri(1999);(6)Koerner et al.(1999);(7)Freed,Close,

&Siegler(2003);(8)Reid et al.(2002);(9)Burgasser et al.(2003);(10)Mart′?n,Brandner,&Basri(1999);(11)

Siegler et al.(2003);(12)Bouy et al.(2003)

d Gl569B and HD130948B ar

e binary brown dwarfs that orbit normal stars,for AO observations these bright

primary stars were guided on,not the brown dwarfs

e We de?ne VLM binaries as2star systems where M

tot<0.185M⊙.Very young evolving systems(like GG

TauBaBb(White&Ghez2001))are not included,nor are over-luminous systems which are not resolved into

binaries.

14Close et al.

Fig. 1.—The12x10s K′image of the L0.52MASSW J0746425+200032binary(which is the one overlap object between our study and that of Reid et al.(2001a)).Note the very di?erent position(P A=85.7?,sep=0.121′′=1.49AU on2002/2/7)of our image compared to the PA=168.8?sep2.7AU published by Bouy et al.(2003)from HST observations of Reid et al.(2001a)at2000/4/15.This suggests that the orbit could have a period of～4yr.In the near future this system could have an orbital solution.At a resolution of<0.1′′both components are clearly visible.We also show K′images of the new binaries2MASSW J1047127+402644,2MASSW J1127534+741107,& 2MASSW J1311391+803222.The pixels are0.199′′pix?1.The contours are linear at the90,75,60,45,30,15,and1%levels.North is up and east left in each image.

Very Low Mass Binaries15

Fig.2.—In?gure(a)we see the12x10s K′image of the2MASSJ1426316+155701binary discussed in Close et al.(2002b)at a resolution of0.131′′.In Figures(b-d)we show K′images of the binaries2MASSW J2140293+162518,2MASSWJ2206228-204705,and2MASSWJ 2331016-040618,respectively.The pixels are0.0199′′pix?1.The contours are linear at the90,75,60,45,30,15,and1%levels.North is up and east left in each image.

16Close et al.

Fig.3.—The latest Chabrier et al.(2000)DUSTY evolutionary models are shown custom integrated over the Ks bandpass.The locations of the2components of2M0746are indicated by the crosses.The polygon encloses the error in the M Ks and age range0.6?7.5Gyr of the system.The M Ks of each secondary is determined by the addition of?Ks plus the M Ks of the primary.The errors for the primary are enclosed by the upper polygon(outlined by a solid thick line),the secondary is bounded by the lower polygon(outlined by a thick dashed line).The models suggest a primary mass of0.082M⊙with a range0.078?0.083M⊙with temperatures of2375K(2323-2409K).For the secondary the models suggest a mass of0.078M⊙with a range0.068?0.079M⊙with temperatures of2173K(2034-2252K).The isochrones plotted are0.3,0.6,0.65,0.7,0.85,1.2,1.7,3.0,5.0,7.5,&10.0Gyr.The isotherms run in equal intervals from2600K to1400K in steps of 200K.

Very Low Mass Binaries17

Fig.4.—The same as?gure3except for2M1047.Note the larger errors in M Ks compared to2M0746since we have to currently rely on a photometric parallax for this system.The models suggest a primary mass of0.092M⊙with a range0.079?0.102M⊙with temperatures of 2660K(2460-2830K).For the secondary the models suggest a mass of0.084M⊙with a range0.068?0.090M⊙with temperatures of2430K (2164-2625K).

18Close et al.

Fig.5.—Figure3from Freed,Close,&Siegler(2003).Note that the isochrones are di?erent for this plot compared to all the other plots in this paper(but the isotherms are the same).More detail about LHS2397a can be found in Freed,Close,&Siegler(2003).LHS2397aB (mass0.068M⊙)is one of the tightest brown dwarf companions ever to be imaged around a star.

Very Low Mass Binaries19

Fig.6.—The same as for?gure3except for2M1127.Note the larger errors in M Ks compared to2M0746since we have to currently rely on a photometric parallax for this system.The models suggest a primary mass of0.092M⊙with a range0.080?0.101M⊙with temperatures of2650K(2460-2810K).For the secondary the models suggest a mass of0.087M⊙with a range0.074?0.095M⊙with temperatures of2540K (2330-2710K).

20Close et al.

Fig.7.—The same as for?gure3except for2M1311.Note the larger errors in M Ks compared to2M0746since we have to currently rely on a photometric parallax for this system.The models suggest a primary mass of0.089M⊙with a range0.078?0.098M⊙with temperatures of2610K(2400-2760K).For the secondary the models suggest a mass of0.088M⊙with a range0.074?0.095M⊙with temperatures of2540K (2331-2710K).

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奔跑吧兄弟的策划内部编号：（YUUT-TBBY-MMUT-URRUY-UOOY-DBUYI-0128）

奔跑吧 K303 活动时间 2015年4月26日8：30-12:00（暂定） 集合地点 橘子洲公园游客服务中心旁边的大草坪（下桥后往南边走） 裁判长：陈老师 游戏方案： 约十人一组，一共5组（由所到人数再定） 队伍产生方式：随机抽签 队伍产生后选定队长，设计好队名，队旗，口号，展示造型。并贴好名牌，调试对讲机。 10min后集体展示，由裁判长评选出第一名，可获得撕名牌游戏中“复活卡”一张（被OUT后获得重生机会）；第二名，可获得撕名牌游戏中“女生特权卡”一张（限女生被抓时求放过）；第三名，可获得撕名牌游戏中“延时卡”一张；（即将被抓住时可准许5秒逃跑时间）。 热身游戏一：十人九足 每队十人,男女交错排成一横排，相邻的人把腿系在一起，一起跑向终点（50m），用时最短的胜出。第一名将获得30秒优先奔跑权利，第二名将获得20秒优先奔跑权利，第三名将获得10秒优先奔跑权利。 热身游戏二：盲人足球?

组内两两搭档，每对搭档中只有一个人戴蒙眼布，另一个人不戴。只有被蒙上眼睛的队员才可以踢球，他的搭档负责告诉他向什么方向走、做什么，并踢进5m远的球门。两人不能有肢体接触。在规定的时间3min内，看哪一组进的球最多，哪一组就获胜。 第一名将获得3个智慧锦囊，第二名将获得2个智慧锦囊，第三名将获得1个智慧锦囊。 撕名牌游戏 两轮比赛综合排名最后的组将成为铃铛人，铃铛人将铃铛绑在小腿上，携带任务卡率先出发，并按照提示将任务卡隐藏在指定位置。（铃铛必须一直挂在小腿上，直至OUT） 10min后，各组出发，寻找任务卡，或者撕下铃铛人的名牌，铃铛人OUT后，需告诉对手任务卡隐藏位置。任务卡一共十一张，需全部集中在某一队手中，该队获胜。若任务卡分散在不同组，则需要相互撕名牌，被撕掉名牌者OUT，任务卡给对手，最终某队获得全部任务卡，赢得终极大奖！撕掉对方名牌后，请及时通过对讲机告诉裁判，“某某OUT”,被OUT的人严禁继续参与撕名牌游戏，且不能透露信息给还在游戏中的人。 剧情还可能反转哦！一个半小时后，若铃铛组还有任务卡没被其他组找到，铃铛组获胜！ 终点集合 预计11:40到达观景台，集体吟诵《沁园春。长沙》 颁奖，班级合影 自由活动 注意事项：