Slow modes in stellar systems with nearly harmonic potentials: II. Gravitational loss-cone instability

Details Slow modes in stellar systems with nearly harmonic potentials: II. Gravitational loss-cone instability Slow modes in stellar systems with nearly harmonic potentials: II. Gravitational loss-cone instability

Mar 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010astl...36..175p&link_type=abstract

Astronomy Letters, Volume 36, Issue 3, pp.175-190

Astronomy and Astrophysics

Astronomy

Stellar Systems, Star Clusters And Associations, Stellar Dynamics

Scientific paper

Using a consistent perturbation theory for collisionless disk-like and spherical star clusters, we construct a theory of slow modes for systems having an extended central region with a nearly harmonic potential due to the presence of a fairly homogeneous (on the scales of the stellar system) heavy, dynamically passive halo. In such systems, the stellar orbits are slowly precessing, centrally symmetric ellipses (2: 1 orbits). We consider star clusters with monoenergetic distribution functions that monotonically increase with angular momentum in the entire range of angular momenta (from purely radial orbits to circular ones) or have a growing region only at low angular momenta. In these cases, there are orbits with a retrograde precession, i.e., in a direction opposite to the orbital rotation of the star. The presence of a gravitational loss-cone instability, which is also observed in systems of 1: 1 orbits in near-Keplerian potentials, is associated with such orbits. In contrast to 1: 1 systems, the loss-cone instability takes place even for distribution functions monotonically increasing with angular momentum, including those for systems with circular orbits. The regions of phase space with retrograde orbits do not disappear when the distribution function is smeared in energy. We investigate the influence of a weak inhomogeneity of a heavy halo with a density that decreases with distance from the center.

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