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 timespan, which allows an overlap between spectroscopic binaries and those which can be found by high angular-resolution imaging. 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 $6 \pm 2$ out of 53, or $11^{+0.07}_{-0.04}$% 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 $26 \pm 10$% for the binary fraction of VLM 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 same one as found for GK stars, a compressed version of that, a version of the compressed distribution truncated at 15 AU, and a theoretical distribution which 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.Comment: 36 pages, accepted for publication in AJ, abstract shortened for arXiv.or

PPl 15: The First Brown Dwarf Spectroscopic Binary

PPl 15 is the first object to have been confirmed as a brown dwarf by the lithium test (in 1995), though its inferred mass was very close to the substellar limit. It is a member of the Pleiades open cluster. Its position in a cluster color-magnitude diagram suggested that it might be binary, and preliminary indications that it is a double-lined spectroscopic binary were reported by us in 1997. Here we report on the results of a consecutive week of Keck HIRES observations of this system, which yield its orbit. It has a period of about 5.8 days, and an eccentricity of 0.4+/-0.05. The rotation of the stars is slow for this class of objects. Because the system luminosity is divided between 2 objects with a mass ratio of 0.85, this renders each of them an incontrovertible brown dwarf, with masses between 60-70 jupiters. We show that component B is a little redder than A by studying their wavelength-dependent line ratios, and that this variation is compatible with the mass ratio. We confirm that the system has lithium, but cannot support the original conclusion that it is depleted (which would be surprising, given the new masses). This is a system of very close objects which, if they had combined, would have produced a low mass star. We discuss the implications of this discovery for the theories of binary formation and formation of very low mass objects.Comment: Latex, 18 pages, 4 figures, submitted to Astron.

A Survey for Low-mass Stars and Brown Dwarfs in the Upper-Scorpius OB Association

The Upper-Scorpius association is the OB association nearest to the Sun (145 pc). Its young age (5 Myr) makes it an ideal place to search for low-mass stars and brown dwarfs, as these objects should be relatively bright. We have performed a photometric search for the low-mass members of the association, using the R, I, and Z filters. The completeness limit is I=18.5 and the saturation limit is I=13. We obtain 138 candidate members, covering nearly the entire M spectral type range. We find an excess of brown dwarf candidates over the number predicted by a Miller-Scalo Initial Mass Function. In addition, we have performed infrared imaging and low resolution optical spectroscopy of selected candidates. We find that the infrared observations confirm the spectral types obtained with the optical photometry. Furthermore, we find H_alpha in emission in 20 of the 22 objects observed spectroscopically. As H_alpha is an indicator of youth, we believe that these 20 objects may belong to the association. One of them, UScoCTIO 128 has a very strong and constant H_alpha line (equivalent width: -130 A), and its position in the color magnitude diagram suggests that it is a brown dwarf with mass equal to 0.02 msun. Confirmation of this and the other candidates will have to wait for higher resolution observations that can reveal spectroscopic mass indicators like Li I and gravity indicators, such as K I and the subordinate lines of Na I.Comment: 20 pages, 7 figures, 3 tables, accepted in the Astronomical Journa

Multiplicity of Nearby Free-floating Ultra-cool Dwarfs: a HST-WFPC2 search for companions

We present HST/WFPC2 observations of a sample of 134 ultra-cool objects (spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys, with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool binary candidates are reported here. Eleven known binaries are confirmed and orbital motion is detected in some of them. We estimate that the closest binary systems in this sample have periods between 5 and 20 years, and thus dynamical masses will be derived in the near future. For the calculation of binary frequency we restrict ourselves to systems with distances less than 20 pc. After correction of the binaries bias, we find a ratio of visual binaries (at the HST limit of detection) of around 10%, and that ~15% of the 26 objects within 20 parsecs are binary systems with separations between 1 and 8 A.U. The observed frequency of ultra-cool binaries is similar than that of binaries with G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is also a clear deficit of ultra-cool binaries with separations greater than 15 A.U., and a possible tendency for the binaries to have mass ratios near unity. Most systems have indeed visual and near-infrared brightness ratios between 1 and 0.3. We discuss our results in the framework of current scenarios for the formation and evolution of free-floating brown dwarfs.Comment: 67 pages, 14 figures, Accepted for publication in AJ, September 2003. First submission to AJ: august 2002, 5 submission

Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga

We present high resolution optical spectra obtained with the HIRES spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs (LMTTs) in Taurus-Auriga. Of particular interest is the previously classified "continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass just below the stellar/substellar boundary. None of the LMTTs in Taurus are rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We demonstrate empirically that the full-width at 10% of the H-alpha emission profile peak is a more practical and possibly more accurate indicator of accretion than either the equivalent width of H-alpha or optical veiling: 10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent of stellar spectral type. Although LMTTs can have accretion rates comparable to that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass accretion rate appears to decrease with decreasing mass. The diminished frequency of accretion disks for LMTTs, in conjunction with their lower, on average, mass accretion rates, implies that they are formed with less massive disks than higher-mass T Tauri stars. The radial velocities, circumstellar properties and known binaries do not support the suggestion that many of the lowest mass members of Taurus have been ejected from higher stellar density regions within the cloud. Instead, LMTTs appear to have formed and are evolving in the same way as higher-mass T Tauri stars, but with smaller disks and shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap