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**Session 3 - Clusters.**

*Display session, Monday, June 10*

*Tripp Commons, *

## [3.13] Resonant Relaxation in Stellar Systems

*K. P. Rauch, S. Tremaine (CITA)*
We demonstrate the existence of an enhanced rate of angular momentum
relaxation in nearly Keplerian star clusters, such as those found in the
centers of galactic nuclei containing massive black holes. The enhanced
relaxation arises because the radial and azimuthal orbital frequencies in a
Keplerian potential are equal, and hence may be termed resonant
relaxation\/. We explore the dynamics of resonant relaxation using both
numerical simulations and order-of-magnitude analytic calculations. We find
that the resonant angular momentum relaxation time is shorter than the
non-resonant relaxation time by of order M_\star/M, where M_\star is the
mass in stars and M is the mass of the central object. Resonance does not
enhance the energy relaxation rate. We examine the effect of resonant
relaxation on the rate of tidal disruption of stars by the central mass; we
find that the flux of stars into the loss cone is enhanced when the loss cone
is empty, but that the disruption rate averaged over the entire cluster is not
strongly affected. We show that relativistic precession can disable resonant
relaxation near the main-sequence loss cone for black hole masses comparable
to those in galactic nuclei. Resonant dynamical friction leads to growth or
decay of the eccentricity of the orbit of a massive body, depending on whether
the distribution function of the stars is predominantly radial or tangential.
The accelerated relaxation implies that there are regions in nuclear star
clusters that are relaxed in angular momentum but not in energy;
unfortunately, these regions are not well-resolved in nearby galaxies by the
Hubble Space Telescope.

**Program
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