The evolution of the surface brightness of a star cluster as a result of residual star-forming gas expulsion

TL;DR

This study uses N-body simulations to analyze how residual gas expulsion affects the surface brightness evolution of young star clusters, revealing that re-virialisation does not significantly restore observable brightness.

Contribution

It provides the first detailed N-body analysis of surface brightness changes during gas expulsion, highlighting the limited observational reappearance of clusters after re-virialisation.

Findings

Surface mass density decreases significantly during gas expulsion.

Core radii grow and show re-virialisation effects.

Surface brightness within certain radii remains low post-expulsion.

Abstract

Direct N-body calculations are presented of the early evolution of exposed clusters to quantify the influence of gas expulsion on the time-varying surface brightness. By assuming that the embedded OB stars drive out most of the gas after a given time delay, the change of the surface brightness of expanding star clusters is studied. The influence of stellar dynamics and stellar evolution is discussed. The growth of the core radii of such models shows a remarkable core re-virialisation. The decrease of the surface mass density during gas expulsion is large and is only truncated by this re-virialisation process. However, the surface brightness within a certain radius does not increase noticeably. Thus, an embedded star cluster cannot reappear in observational surveys after re-virialisation. This finding has a bearing on the observed infant mortality fraction.