Dynamical evolution of star clusters around a rotating black hole with an accretion disc

Details Dynamical evolution of star clusters around a rotating black hole with an accretion disc Dynamical evolution of star clusters around a rotating black hole with an accretion disc

Aug 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.275..628r&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 275, Issue 3, pp. 628-640.

Physics

20

Accretion: Accretion Discs, Black Hole Physics, Celestial Mechanics, Stellar Dynamics, Stars: Kinematics, Galaxies: Nuclei, Galaxies: Star Clusters

Scientific paper

The dynamical evolution of relativistic star clusters around a massive Kerr black hole with an accretion disc is examined, in the regime where the black hole dominates the potential and star-disc interactions dominate the evolution. A set of diagrams exhibiting the time development of the energy-dependent distribution function, f(ZE), and the distributions of semimajor axes, a, eccentricities, e, and inclinations, i, of the model system are presented; plots of the latter three quantitities over time for a few illustrative orbits are also given. A simple approximation for the final radius of an orbit brought into the disc under star-disc interactions, namely a_f~=a_0(1-e^20) cos^4(i_0/2) (or, in terms of angular momentum, L_f~=(L_0+L_z,0)/2), is derived. It is found that the main effect of star-disc interactions on an isotropic cluster, besides the circularization and alignment of orbits, is to steepen an initial density profile rho_*~r^-n to the approximate `asymptotic' profile rho_*~r^-2.5 when n<=2.5, to leave the profile unchanged when n>=2.5, and in both cases to increase the central density (by several hundred very close to the black hole); initially anisotropic clusters are found to exhibit similar patterns. The numerical results can be explained in terms of a simple analytic model. Relativistic effects are found to affect the cluster properties significantly only at very small radii (<=10GM/c^2) in particular, the location of the last stable orbit limits the cluster's inner extent. By significantly increasing the central stellar density, star-disc interactions could be self-limiting by causing stellar collisions to become important; the future evolution of the cluster in this case will depend on the relative balance of the collisional, alignment and stellar evolutionary time-scales.

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