A Survey for Spectroscopic Binaries among Very Low Mass Stars

We report on the results of a survey for radial velocity variability in a heterogeneous sample of very low mass stars and brown dwarfs. One distinguishing characteristic of the survey is its time span, which allows an overlap between spectroscopic binaries and those that can be found by high angular resolution imaging. Despite our relatively low velocity precision, we are able to place a new constraint on the total binary fraction in these objects, which suggests that they are more likely the result of extending the same processes at work at higher masses into this mass range, rather than a distinct mode of formation. Our basic result is that there are 6 ± 2 out of 53, or 11%, spectroscopic binaries in the separation range 0-6 AU, nearly as many as resolved binaries. This leads to an estimate of an upper limit of 26% ± 10% for the binary fraction of very low mass objects (it is an upper limit because of the possible overlap between the spectroscopic and resolved populations). A reasonable estimate for the very low mass binary fraction is 20%-25%. We consider several possible separation and frequency distributions, including the one found for GK stars, a compressed version of that, a version of the compressed distribution truncated at 15 AU, and a theoretical distribution that considers the evaporation of small-N clusters. We conclude that the latter two bracket the observations, which may mean that these systems form with intrinsically smaller separations due to their smaller mass and then are truncated due to their smaller binding energy. We do not find support for the "ejection hypothesis" as their dominant mode of formation, particularly in view of the similarity in the total binary fraction compared with slightly more massive stars and the difficulty this mechanism has in producing numerous binary systems. Our conclusions must be viewed as tentative until studies with larger and better-posed samples and higher velocity precision are conducted.

[1]  R. Jayawardhana,et al.  Evidence for a T Tauri Phase in Young Brown Dwarfs , 2003, astro-ph/0303565.

[2]  Robert A. Donahue,et al.  Activity-Related Radial Velocity Variation in Cool Stars , 1997 .

[3]  Laird M. Close,et al.  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 , 2003, astro-ph/0301095.

[4]  S. Meibom,et al.  A Robust Measure of Tidal Circularization in Coeval Binary Populations: The Solar-Type Spectroscopic Binary Population in the Open Cluster M35 , 2004, astro-ph/0412147.

[5]  G. Chabrier.  Galactic Stellar and Substellar Initial Mass Function , 2003, astro-ph/0304382.

[6]  I. Neill Reid,et al.  High-Resolution Spectroscopy of Ultracool M Dwarfs , 2002, astro-ph/0204285.

[7]  The first wide ultracool binary dwarf in the field: DENIS-J055146.0-443412.2 (M8.5 + L0) , 2005, astro-ph/0508130.

[8]  Debra A. Fischer,et al.  Multiplicity among M Dwarfs , 1992 .

[9]  Eduardo L. Martin,et al.  Multiplicity of Nearby Free-floating Ultracool Dwarfs: A Hubble Space Telescope WFPC2 Search for Companions , 2003, astro-ph/0305484.

[10]  Companions of old brown dwarfs, and very low mass stars ? , 2003, astro-ph/0302029.

[11]  P.F.L. Maxted,et al.  On the frequency of close binary systems among very low-mass stars and brown dwarfs , 2005 .

[12]  Richard B. Larson,et al.  The Collapse of a Rotating Cloud , 1972 .

[13]  K. Luhman.  New Brown Dwarfs and an Updated Initial Mass Function in Taurus , 2004, astro-ph/0411447.

[14]  Gibor Basri,et al.  Observations of Brown Dwarfs , 2000 .

[15]  B. Reipurth,et al.  The Formation of Brown Dwarfs as Ejected Stellar Embryos , 2001, astro-ph/0103019.

[16]  Membership, binarity and accretion among very low-mass stars and brown dwarfs of the σ Orionis cluster , 2005 .

[17]  R. Jayawardhana,et al.  A Disk Census for Young Brown Dwarfs , 2003, astro-ph/0305412.

[18]  Multiplicity of very low-mass objects in the upper scorpius ob association: a possible wide binary population , 2005, astro-ph/0512258.

[19]  G. Basri,et al.  Rotation and Activity in Mid-M to L Field Dwarfs , 2002, astro-ph/0201455.

[20]  P. Padoan,et al.  The “Mysterious” Origin of Brown Dwarfs , 2002, astro-ph/0205019.

[21]  R. P. Butler,et al.  ATTAINING DOPPLER PRECISION OF 3 M S-1 , 1996 .

[22]  Eduardo Martin,et al.  PPl 15: The First Brown Dwarf Spectroscopic Binary , 1999, astro-ph/9908015.

[23]  T. Henning,et al.  The Decay of Accreting Triple Systems as Brown Dwarf Formation Scenario , 2005, astro-ph/0501075.

[24]  R. Fisher.  A Turbulent Interstellar Medium Origin of the Binary Period Distribution , 2003, astro-ph/0303280.

[25]  R. Durisen,et al.  Imprints of dynamical interactions on brown dwarf pairing statistics and kinematics , 2003 .

[26]  L. Testi,et al.  Accretion in brown dwarfs: An infrared view , 2004 .

[28]  Subhanjoy Mohanty,et al.  The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs , 2005, astro-ph/0502155.

[29]  G. Basri,et al.  Discovery of an M4 Spectroscopic Binary in Upper Scorpius: A Calibration Point for Young Low-Mass Evolutionary Models , 2005, astro-ph/0506501.

[30]  I. Bonnell,et al.  The formation mechanism of brown dwarfs , 2002, astro-ph/0206365.