Multiplicity of Nearby Free-floating Ultra-cool Dwarfs a HST

a r X i v :a s t r o -p h /0305484v 2 10 J u n 2003accepted for publication in AJ (September 2003),First sub-

mission:August 2002

Multiplicity of Nearby Free-?oating Ultra-cool Dwarfs:

a HST-WFPC2search for companions

Herv′e Bouy E.S.O,Karl Schwarzschildstra?e 2,D-85748Garching,Germany hbouy@6f4ce10c76c66137ee0619c7 Wolfgang Brandner Max-Planck Institut f¨u r Astronomie,K¨o nigstuhl 17,D-69117Heidelberg,Germany brandner@mpia.de Eduardo L.Mart′?n Institute for Astronomy,University of Hawai‘i,2680Woodlawn Drive,Honolulu,HI 96822,USA ege@6f4ce10c76c66137ee0619c7 Xavier Delfosse Laboratoire d’Astrophysique de l’Observatoire de Grenoble,414rue de la piscine,F-38400Saint Martin d’H`e re,France Xavier.Delfosse@obs.ujf-grenoble.fr

France Allard

Centre de Recherche Astronomique de Lyon (UML 5574),Ecole Normale Sup′e rieure,

69364Lyon Cedex 07,France

fallard@ens-lyon.fr

Gibor Basri

University of California at Berkeley,Astronomy Department,MC 3411Berkeley,CA

94720,USA

basri@6f4ce10c76c66137ee0619c7

–2–

ABSTRACT

We present HST/WFPC2observations of a sample of134ultra-cool objects (spectral types later than M7)coming from the DENIS,2MASS and SDSS sur-

veys,with distances estimated to range from7pc to105pc.Fifteen new ultra-

cool binary candidates are reported here.Eleven known binaries are con?rmed

and orbital motion is detected in some of them.We estimate that the closest

binary systems in this sample have periods between5and20years,and thus dy-

namical masses will be derived in the near future.For the calculation of binary

frequency we restrict ourselves to systems with distances less than20pc.After

correction of the binaries bias,we?nd a ratio of visual binaries(at the HST

limit of detection)of around10%,and that～15%of the26objects within20

parsecs are binary systems with separations between1and8A.U.The observed

frequency of ultra-cool binaries is similar than that of binaries with G-type pri-

maries in the separation range from2.1A.U.to140A.U.There is also a clear

de?cit of ultra-cool binaries with separations greater than15A.U.,and a possi-

ble tendency for the binaries to have mass ratios near unity.Most systems have

indeed visual and near-infrared brightness ratios between1and0.3.We discuss

our results in the framework of current scenarios for the formation and evolution

of free-?oating brown dwarfs.

Subject headings:stars:very low mass,ultra-cool dwarfs,brown dwarfs,binary

1.Introduction

E?ective strategies to detect brown dwarfs are proper motion surveys(e.g Ruiz et al. (1997)),wide?eld CCD surveys in star forming regions,open clusters like the Pleiades(e.g Stau?er et al.(1994);Zapatero Osorio et al.(1997);Bouvier et al.(1998)),or the new generation of optical and near-infrared all sky surveys like the Sloan Digital Sky Survey (York et al2000),the DEep Near Infrared Survey(Epchtein et al.1997),and the2Micron All Sky Survey(Skrutskie et al.1997).So far,DENIS has produced a list of～300nearby very low-mass objects(Delfosse et al.1997a,1999,2003;Mart′?n et al.1999a),and a similar number has been detected by2MASS(Kirkpatrick et al.1997,1999,2001;Burgasser et al.1999;Gizis et al.2000;Kirkpatrick et al.2003).To date,several hundreds of nearby free-?oating ultra-cool and brown dwarfs have been identi?ed,and many more candidates are waiting for con?rmation.This nearby sample is ideal for resolving ultra-cool and brown dwarfs binaries,and is large enough for statistical studies.

–3–

While systematic surveys of their physical properties are just starting,we already have indications that binary brown dwarfs are not rare.PPL15,the?rst brown dwarf in the Pleiades with con?rmed lithium(Basri et al.1996),turned out to be a spectroscopic binary (Basri&Mart′?n1999).Several direct imaging surveys in the?eld or in open clusters using the high spatial resolution provided by both HST and/or adaptive optics led to the discovery of a signi?cant number of binaries(Mart′?n et al.1997,1999a,2000a;Koerner et al.1999; Reid et al.2001;Neuh¨a user et al.2002;Close et al.2002a,b;Potter et al.2002;Burgasser et al.2003;Gizis et al.2003;Close et al.2003).

There are several key questions which need to be answered and for which brown dwarf binaries are an important piece of the puzzle:where do free-?oating brown dwarfs originate? Are they ejected stellar embryos(Reipurth&Clarke2001;Bate et al.2002;Delgado-Donate et al.2003),or do they form more isolated like ordinary stars due to fragmentation of collapsing molecular cloud clumps(Bodenheimer et al.1999)?

One can then ask,how the binary properties of brown dwarfs,such as the frequency, distribution of separations,distribution of mass-ratios(is there a lower mass limit for com-panions to brown dwarfs?),or the relation between orbital period and eccentricity,compare to the properties of stellar binaries.These properties hold important clues on the origin of (binary)brown dwarfs.An agreement of brown dwarf and stellar binary properties would suggest the same formation mechanism for both types of objects.

Resolved brown dwarf binaries provide a very valuable opportunity to measure sub-stellar dynamical masses.The?rst such measurement has been reported by Lane et al. (2001)in the brown dwarf binary Gliese569B,discovered by Mart′?n et al.(2000a).Dynam-ical masses for brown dwarfs of di?erent ages are very much needed,as the mass estimates currently depend on untested theoretical evolutionary tracks,model atmospheres,and as-sumptions on the internal structure of brown dwarfs.In particular,a brown dwarf with a given age and mass might have very similar colours and luminosity as a younger brown dwarf with a lower mass.This degeneracy in the mass-luminosity relation for sub-stellar objects makes …… 此处隐藏：34152字，全部文档内容请下载后查看。喜欢就下载吧 ……