Dynamical Friction on Star Clusters near the Galactic Center

Abstract

Numerical simulations of the dynamical friction suffered by a star cluster near the Galactic center have been performed with a parallelized tree code. Gerhard has suggested that dynamical friction, which causes a cluster to lose orbital energy and spiral in toward the Galactic center, may explain the presence of a cluster of very young stars in the central parsec, where star formation might be prohibitively difficult because of strong tidal forces. The clusters modeled in our simulations have an initial total mass of 105-106 M? and initial Galactocentric radii of 2.5-30 pc. We have identified a few simulations in which dynamical friction indeed brings a cluster to the central parsec, although this is only possible if the cluster either is very massive (~106 M?) or is formed near the central parsec (5 pc). In both cases, the cluster should have an initially very dense core (>106 M? pc-3). The initial core collapse and segregation of massive stars into the cluster core, which typically happens on a much shorter timescale than that characterizing the dynamical inspiral of the cluster toward the Galactic center, can provide the requisite high density. Furthermore, because it is the cluster core that is most likely to survive the cluster disintegration during its journey inward, this can help account for the observed distribution of presumably massive He I stars in the central parsec.