In Search of the Thermal Eccentricity Distribution
Abstract
About a century ago, Jeans (1919) discovered that if binary stars reach a state approximating energy equipartition, for example, through many dynamical encounters that exchange energy, their eccentricity distribution can be described by {dN}/{de}=2e. This is referred to as the thermal eccentricity distribution, and has been widely used for initial conditions in theoretical investigations of binary stars. However, observations suggest that the eccentricity distributions of most observed binaries, and particularly those with masses ≲5 M _{⊙}, are flatter than thermal and follow more closely to a uniform distribution. Nonetheless, it is often argued that dynamical interactions in a star cluster would quickly thermalize the binaries, which could justify imposing a thermal eccentricity distribution at birth for all binaries. In this paper, we investigate the validity of this assumption. We develop our own rapid semianalytic model for binary evolution in star clusters, and also compare it with detailed Nbody and Monte Carlo star cluster models. We show that, for nearly all binaries, dynamical encounters fail to convert an initially uniform eccentricity distribution to thermal within a star cluster’s lifetime. Thus, if a thermal eccentricity distribution is observed, it is likely imprinted upon formation rather than through subsequent longterm dynamical processing. Theoretical investigations that initialize all binaries with a thermal distribution will make incorrect predictions for the evolution of the binary population. Such models may overpredict the merger rate for binaries with modest orbital separations by a factor of about two.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 2019
 DOI:
 10.3847/15384357/ab0214
 arXiv:
 arXiv:1902.00019
 Bibcode:
 2019ApJ...872..165G
 Keywords:

 binaries: general;
 globular clusters: general;
 methods: numerical;
 open clusters and associations: general;
 stars: black holes;
 stars: kinematics and dynamics;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 14 pages, 7 figures, accepted for publication in The Astrophysical Journal