TY - JOUR
T1 - In Search of the Thermal Eccentricity Distribution
AU - Geller, Aaron M.
AU - Leigh, Nathan W.C.
AU - Giersz, Mirek
AU - Kremer, Kyle
AU - Rasio, Frederic A.
N1 - Funding Information:
We thank Douglas Heggie for his input and suggestions in constructing our semi-analytic model. This research was supported in part through the computational resources and staff contributions provided for the Quest high-performance computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology. K.K. acknowledges support by the National Science Foundation Graduate Research Fellowship Program under grant No. DGE-1324585. K.K. and F.A.R. acknowledge support from NASA ATP Grant NNX14AP92G and NSF Grant AST-1716762. M.G. was partially supported by NCN, Poland, through the grant UMO-2016/23/B/ST9/0273.
PY - 2019
Y1 - 2019
N2 - 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 . 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 semi-analytic model for binary evolution in star clusters, and also compare it with detailed N-body 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 long-term 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.
AB - 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 . 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 semi-analytic model for binary evolution in star clusters, and also compare it with detailed N-body 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 long-term 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.
KW - binaries: general
KW - globular clusters: general
KW - methods: numerical
KW - open clusters and associations: general
KW - stars: black holes
KW - stars: kinematics and dynamics
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U2 - 10.3847/1538-4357/ab0214
DO - 10.3847/1538-4357/ab0214
M3 - Article
AN - SCOPUS:85063522904
VL - 872
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 2
M1 - 165
ER -