On the distribution of orbital eccentricities for very low-mass binaries

Abstract

We have compiled a sample of 16 orbits for very low-mass stellar (<0.1 M⊙) and brown dwarf binaries, including updated orbits for HD 130948BC and LP 415-20AB. This sample enables the first comprehensive study of the eccentricity distribution for such objects. We find that very low-mass binaries span a broad range of eccentricities from near-circular to highly eccentric (e ≈ 0.8), with a median eccentricity of 0.34. We have examined potential observational biases in this sample, and for visual binaries we show through Monte Carlo simulations that if we choose appropriate selection criteria then all eccentricities are equally represented (≲ 5% difference between input and output eccentricity distributions). The orbits of this sample of very low-mass binaries show some significant differences from their solar-type counterparts. They lack a correlation between orbital period and eccentricity, and display a much higher fraction of near-circular orbits (e < 0.1) than solar-type stars, which together may suggest a different formation mechanism or dynamical history for these two populations. Very low-mass binaries also do not follow the e 2 distribution of Ambartsumian, which would be expected if their orbits were distributed in phase space according to a function of energy alone (e.g., the Boltzmann distribution). We find that current numerical simulations of very low-mass star formation do not completely reproduce the observed properties of our binary sample. The cluster formation model of Bate agrees very well with the overall e distribution, but the lack of any high-e (>0.6) binaries at orbital periods comparable to our sample suggests that tidal damping due to gas disks may play too large of a role in the simulations. In contrast, the circumstellar disk fragmentation model of Stamatellos & Whitworth predicts only high-e binaries and thus is highly inconsistent with our sample. These discrepancies could be explained if multiple formation processes are responsible for producing the field population. © 2011. The American Astronomical Society. All rights reserved.