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 (2001) has suggested that dynamical friction, which causes a cluster to lose orbital energy and spiral in towards 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 owing to strong tidal forces. The clusters modeled in our simulations have an initial total mass of 10^5-10^6 Msun 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 is either very massive (~10^6 Msun), or is formed near the central parsec (<~ 5 pc). In both cases, the cluster should have an initially very dense core (> 10^6 Msun pc-3). The initial core collapse and segregation of massive stars into the cluster core, which typically happens on a much shorter time scale 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 which is most likely to survive the cluster disintegration during its journey inwards, this can help account for the observed distribution of presumably massive HeI stars in the central parsec.