Surviving infant mortality in the hierarchical merging scenario

TL;DR

This paper investigates how initial conditions of star clusters influence their survival after gas expulsion, revealing that initial stellar distribution and velocity dispersion are more critical than star formation efficiency.

Contribution

It demonstrates through simulations that initial stellar distribution and velocity dispersion are key factors in cluster survivability, challenging the traditional emphasis on star formation efficiency.

Findings

Initial star formation efficiency is not a reliable predictor of cluster survival.

Clusters with low star formation efficiencies can still survive gas expulsion.

Variations in initial conditions primarily determine cluster fate.

Abstract

We examine the effects of gas expulsion on initially sub-structured and out-of-equilibrium star clusters. We perform $N$-body simulations of the evolution of star clusters in a static background potential before removing that potential to model gas expulsion. We find that the initial star formation efficiency is not a good measure of the survivability of star clusters. This is because the stellar distribution can change significantly, causing a large change in the relative importance of the stellar and gas potentials. We find that the initial stellar distribution and velocity dispersion are far more important parameters than the initial star formation efficiency, and that clusters with very low star formation efficiencies can survive gas expulsion. We suggest that it is variations in cluster initial conditions rather than in their star formation efficiencies that cause some clusters to be destroyed while a few survive.