Evaporation of Compact Young Clusters near the Galactic Center

Abstract

We investigate the dynamical evolution of compact young clusters (CYCs) near the Galactic center (GC) using Fokker-Planck models. CYCs are very young (<5 Myr), compact (<1 pc), and only a few tens of parsecs away from the GC, while they appear to be as massive as the smallest Galactic globular clusters (~104 M☉). A survey of cluster lifetimes for various initial mass functions, cluster masses, and Galactocentric radii is presented. Short relaxation times due to the compactness of CYCs and the strong tidal fields near the GC make clusters evaporate fairly quickly. Depending on cluster parameters, mass segregation may occur on a timescale shorter than the lifetimes of most massive stars, which accelerates the cluster's dynamical evolution even more. When the difference between the upper and lower mass boundaries of the initial mass function is large enough, strongly selective ejection of lighter stars makes massive stars dominate even in the outer regions of the cluster, so the dynamical evolution of those clusters is weakly dependent on the lower mass boundary. The mass bins for Fokker-Planck simulations were carefully chosen to properly account for a relatively small number of the most massive stars. We find that clusters with a mass ≲2 × 104 M☉ evaporate in ≲10 Myr. Two CYCs observed near the GC—the "Arches cluster" (G0.121+0.17) and the "Quintuplet cluster" (AFGL 2004)—are interpreted in terms of the models; their central densities and apparent ages are consistent with the hypothesis that they represent successive stages of cluster evolution along a common track, with both undergoing rapid evaporation. A simple calculation based on the total masses in observed CYCs and the lifetimes obtained here indicates that the massive CYCs make up only a fraction of the star formation rate (SFR) in the inner bulge estimated from Lyman continuum photons and far-IR observations. This is consistent with the observation that many stars in the inner bulge form outside the large clusters.