Gas expulsion in massive star clusters? Constraints from observations of young and gas-free objects

Abstract

Gas expulsion is a central concept in some of the models for multiple populations and the light-element anticorrelations in globular clusters. If the star formation efficiency was around 30 per cent and the gas expulsion happened on the crossing timescale, this process could expel preferentially stars born with the chemical composition of the proto-cluster gas, while stars with special composition born in the centre would remain bound. Recently, a sample of extragalactic, gas-free, young massive clusters has been identified that has the potential to test the conditions for gas expulsion. We compute a large number of thin shell models, and calculate if the Rayleigh-Taylor instability is able to disrupt the shell before it reaches the escape speed. We show that the success of gas expulsion depends on the compactness index of a star cluster C5, proportionate to stellar mass over half-mass radius. For given C5, a certain critical, local star formation efficiency is required to remove the rest of the gas. Common stellar feedback processes may not lead to gas expulsion with significant loss of stars above C5 = 1. Considering pulsar winds and hypernovae, the limit increases to C5 = 30. If successful, gas expulsion generally takes place on the crossing timescale. Some observed young massive clusters have 1 < C5 < 10 and are gas-free at 10 Myr. This suggests that gas expulsion does not affect their stellar mass significantly, unless powerful pulsar winds and hypernovae are common in such objects. By comparison to observations, we show that C5 is a better predictor for the expression of multiple populations than stellar mass. The best separation between star clusters with and without multiple populations is achieved by a stellar winds-based gas expulsion model, where gas expulsion would occur exclusively in star clusters without multiple populations.