Gas in Globular Clusters I: Gas Retention and Its Possible Consequences

Abstract: Globular clusters (GCs) host complex stellar populations whose chemical signatures imply early retention and reprocessing of stellar ejecta, yet direct evidence for intracluster gas is lacking. Here we present a unified theoretical framework for the accumulation, fate, and eventual removal of gas released by evolved stars in young GCs, and their implications for the production of multiple stellar populations. Using MIST stellar evolution tracks, we show that low-velocity (< 20 km/s) AGB winds, each released over 10^4 yr, are gravitationally retained in >10^5 MSun clusters. In addition, AGB winds in such clusters collide with each other and the previously retained winds, triggering a rapid switch' to efficient gas retention. Expected gas retention fractions, mapped across cluster initial mass and size, agree well with the observed second population fractions in Milky Way GCs. We then show that the accumulated gas cannot form new stars because protostellar cores are disrupted by encounters with pre-existing stars over 1 - 10 kyr. Instead, the gas is accreted onto pre-existing main-sequence stars and compact objects. Bondi-Hoyle accretion and time-dependent core-halo models indicate that both white dwarfs and neutron stars can grow and collapse within a few 100 Myr, and that lower-mass main-sequence stars can berejuvenated' into the 4 - 6 MSun range required to reproduce key abundance patterns. Therefore, in our model, the multiple populations will be found in sufficiently massive clusters, with the second-population stars being formed from the inner subset of first-population stars that accreted large fractions of their mass from the AGB-processed retained gas. Finally, we argue that a combination of feedback processes will clear the gas by 10^9 yr, thus reproducing the gas-poor conditions observed for present-day clusters.