Detection of Nine M8.0-L0.5 Binaries: The Very Low Mass Binary Population and Its Implications for Brown Dwarf and Very Low Mass Star Formation

By L. Close, N. Siegler, Melanie Freed, B. Biller. 2003

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Use of the highly sensitive Hokupa'a/Gemini curvature wave front sensor has allowed direct adaptive optics (AO) guiding on very low mass (VLM) stars with SpT = M8.0-L0.5. A survey of 39 such objects detected nine VLM binaries (seven of which were discovered for the first time to be binaries). Most of these systems are tight (separation <5 AU) and have similar masses (ΔKs < 0.8 mag; 0.85 < q < 1.0). However, two systems (LHS 2397a and 2M 2331016-040618) have large ΔKs > 2.4 mag and consist of a VLM star orbited by a much cooler L7-L8 brown dwarf companion. On the basis of this flux-limited (Ks < 12 mag) survey of 39 M8.0-L0.5 stars (mainly from the 2MASS sample of Gizis et al.), we find a sensitivity-corrected binary fraction in the range 15% ± 7% for M8.0-L0.5 stars with separations greater than 2.6 AU. This is less than the 32% ± 9% measured for more massive M0-M4 dwarfs over the same separation range. It appears M8.0-L0.5 binaries (as well as L and T dwarf binaries) have a much smaller semimajor axis distribution peak (~4 AU) compared to more massive M and G dwarfs, which have a broad peak at larger ~30 AU separations. We also find no VLM binary systems (defined here as systems with Mtot < 0.185 M☉) with separations greater than 15 AU. We briefly explore possible reasons why VLM binaries are slightly less common, nearly equal in mass, and much more tightly bound compared to more massive binaries. We investigate the hypothesis that the lack of wide (a > 20 AU) VLM/brown dwarf binaries may be explained if the binary components were given a significant differential velocity kick. Such a velocity kick is predicted by current "ejection" theories, where brown dwarfs are formed because they are ejected from their embryonic minicluster and therefore starved of accretion material. We find that a kick from a close triple or quadruple encounter (imparting a differential kick of ~3 km s-1 between the members of an escaping binary) could reproduce the observed cutoff in the semimajor axis distribution at ~20 AU. However, the estimated binarity (≲5%) produced by such ejection scenarios is below the 15% ± 7% observed. Similarly, VLM binaries could be the final hardened binaries produced when a minicluster decays. However, the models of Sterzik & Durisen and Durisen, Sterzik, & Pickett also could not produce a VLM binary fraction of 15% and a G star binary fraction of 57%. The observed VLM binary frequency could possibly be produced by cloud core fragmentation. However, our estimate of a fragmentation-produced VLM binary semimajor axis distribution contains a significant fraction of "wide" VLM binaries with a > 20 AU in contrast to observation. In summary, more detailed theoretical work will be needed to explain these interesting results that show VLM binaries to be a significantly different population from more massive M & G dwarf binaries.

Subfields: Astronomy and Astrophysics

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ε Indi Ba,Bb: The nearest binary brown dwarf (2003, background): “Adaptive optics imaging observations of low-mass stars and brown dwarfs generally use the source itself for self-guiding and correction (cf. observations of L dwarfs by Close et al. 2003).”
New companions to nearby low-mass stars (2013, background): “…of wide multiples (Poveda et al. E-mail: esther.jferrandez@upct.es 1994), lucky imaging (Baldwin et al. 2001; Law, Mackay & Baldwin 2006; Law, Hodgkin & Mackay 2008), adaptive optics (AO; Close et al. 2003) and open cluster colour–magnitude diagrams (Pinfield et al. 2003; Lodieu et al. 2007).”
Hubble Space Telescope Observations of Binary Very Low Mass Stars and Brown Dwarfs (2003, methodology): “We refer the reader to Reid et al. (2001) and the more recent Close et al. (2003) for a discussion of the similarities and differences of the M and L dwarf binaries relative to main-sequence stars.”
The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, background): “…the binary fractions and average separations decrease and the average mass ratios increase (q ≡ M2/M1) at lower primary masses (e.g, Basri & Reiners 2006, Bergfors et al. 2010, Bouy et al. 2003, Burgasser et al. 2003, Close et al. 2003, Duquennoy & Mayor 1991, Fischer & Marcy 1992, Figure 7).”
Adaptive Optics for Astronomy (2012, methodology): “One of the largest AO surveys of very low mass stars was carried out by Close et al. (2003) and Siegler et al. (2003), who used the Hokupa‘a AO system on Gemini North to survey 69 stars of spectral type M6.”
Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field (2006, background): “More than half of the sources in the 20-pc 2MU2 sample have either high spatial- resolution ground-based observations (Koerner et al. 1999; Close et al. 2003; Siegler et al. 2005) or imaging by the Hubble Space Telescope (Reid et al. 2001; Gizis et al. 2003; Reid et al. 2006b).”
THE HAWAII INFRARED PARALLAX PROGRAM. I. ULTRACOOL BINARIES AND THE L/T TRANSITION, (2012, unknown): “…(16) Burningham et al. (2009); (17) Chauvin et al. (2004); (18) Chauvin et al. (2005); (19) Chauvin et al. (2010); (20) Close et al. (2002); (21) Close et al. (2003); (22) Delorme et al. (2008); (23) Dupuy et al. (2009a); (24) Dupuy et al. (2009b); (25) Dupuy et al. (2009c); (26) Dupuy et al.…”
Stellar, brown dwarf and multiple star properties from hydrodynamical simulations of star cluster formation (2008, background): “…stellar mass (massive stars: Mason et al. 1998; Preibisch et al. 1999; intermediate-mass stars: Patience et al. 2002; solar-type stars: Duquennoy & Mayor 1991; M-dwarfs: Fischer & Marcy 1992; and very-low-mass stars and brown dwarfs: Close et al. 2003; Siegler et al. 2005; Basri & Reiners 2006).”
The stellar and sub-stellar IMF of simple and composite populations (2011, background): “BD–BD binary systems are known to exist (Basri 2000), in the field (Bouy et al. 2003; Close et al. 2003) and in clusters (Mart́ın et al. 2003).”
PLANETS AROUND LOW-MASS STARS (PALMS). IV. THE OUTER ARCHITECTURE OF M DWARF PLANETARY SYSTEMS (2014, unknown): “The 2MASS H-band spectrophotometric distance to the companion 2MASS J11240487+3808054 is 20.3 ± 1.3 pc using the relations from Dupuy & Liu (2012).”
Mapping the Shores of the Brown Dwarf Desert. I. Upper Scorpius (2007, background): “Several studies have targeted binaries in the field, finding evidence that these properties are massdependent (Duquennoy & Mayor 1991; Fischer & Marcy 1992; Close et al. 2003; Raghavan et al. 2010).”
Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf (2007, background): “…rule out the interaction of the magnetosphere with a stellar wind from a coronally active star, the means by which a large fraction of planetary maser emission is powered, as high resolution adaptive optics observations of TVLM 513 have found no evidence of a physical companion (Close et al. 2003).”
The Brown Dwarf Desert at 75-1200 AU (2004, background): “…include optical coronagraph (Nakajima et al. 1995), direct unocculted infrared images of main-sequence stars (Henry & McCarthy 1993; Hinz et al. 2002) and of white dwarf primaries (Farihi, Zuckerman, & Becklin 2003), adaptive optics (e.g., Close et al. 2003), and IR coronograph (Lowrance 2001).”
Giant Planet Companion to 2MASSW J1207334-393254 (2005, background): “2M1207A and b is very different from known brown dwarf binaries discovered in the field (Close et al. 2003, Gizis et al. 2003, Burgasser et al. 2003, Bouy et al. 2003) and within star forming regions (Neuhäuser et al. 2002), which are characte - ized by semi-major axes< 15 AU and a trend toward…”
THE ASTRALUX LARGE M-DWARF MULTIPLICITY SURVEY (2012, background): “For even later-type objects in the VLM range (very low mass stars and brown dwarfs), the multiplicity frequencies are as low as ∼10%–30% (e.g., Bouy et al. 2003; Close et al. 2003; Gizis et al. 2003; Joergens 2008).”
New Brown Dwarfs and an Updated Initial Mass Function in Taurus (2004, background): “For types later than M8, the observed binary fraction of 15% (Reid et al. 2001; Bouy et al. 2003; Burgasser et al. 2003; Close et al. 2003; Gizis et al. 2003) can be reasonably described as a natural extension of the decreasing binary fraction with primary mass evident among field stars,…”
MAPPING THE SHORES OF THE BROWN DWARF DESERT. II. MULTIPLE STAR FORMATION IN TAURUS–AURIGA (2011, background): “These surveys (e.g. Duquennoy & Mayor 1991, hereafter DM91; Fischer & Marcy 1992, hereafter FM92; Close et al. 2003; Bouy et al. 2003; Burgasser et al. 2003; Raghavan et al. 2010) have found that binary frequencies and properties are very strongly dependent on mass. Solar-mass stars have high binary”
Multiplicity of Nearby Free-floating Ultracool Dwarfs: A Hubble Space Telescope WFPC2 Search for Companions (2003, background): “…(HST) and/or adaptive optics led to the discovery of a significant number of binaries (Martı́n et al. 1997, 1999a, 2000a; Koerner et al. 1999; Reid et al. 2001; Neuhäuser et al. 2002; Close et al. 2002a, 2002b; Potter et al. 2002; Burgasser et al. 2003; Gizis et al. 2003; Close et al. 2003).”
Infrared Parallaxes for Methane T Dwarfs (2003, background): “(Recent programs targeting more L and T dwarfs (Martıǹ, Brander & Basri 1999; Koerner et al. 1999; Reid et al. 2001; Burgasser et al. 2003a; Close et al. 2003), indicate that ∼10-20% of objects observed in sufficient detail are found to be binary.)”
Binaries and the L Dwarf/T Dwarf Transition (2006, methodology): “…from high-resolution imaging studies using the Hubble Space Telescope (HST ) and ground-based adaptive optics instrumentation (Martı́n et al. 1999a; Reid et al. 2001b, 2006a; Bouy et al. 2003; Burgasser et al. 2003c, 2006c; Close et al. 2003; Gizis et al. 2003; Liu&Leggett 2005;Gelino et al. 2006).”