Residual gas expulsion from young globular clusters

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Abstract
The results of N-body simulations of the effects of the expulsion of residual gas (that gas not used in star formation) from very young globular clusters is presented. Globular clusters of a variety of initial masses, Galactocentric radii, concentration and initial mass function slope with star formation efficiencies of ≲50 per cent were simulated. The residual gas was expelled by the action of massive stars in one of three idealized ways: gradually by their UV flux and stellar winds; gradually by the input of energy by supernovae; and in a ‘supershell’ expanding from the cluster centre. The clusters were compared shortly after the gas expulsion with the results of Chernoff & Shapiro to estimate whether they would survive for a Hubble time. It is found that the expulsion of ≲50 per cent of a globular cluster's mass in a short period of time considerably affects the structure of the cluster. However, many clusters are estimated to be able to survive with reasonable initial conditions, even if their star formation efficiencies are possibly as low as 20 per cent. It is found that the central density required within a proto-globular cluster at star formation in order for it to survive at a given Galactocentric radius is independent of the mass of stars in the cluster. For globular clusters in the inner few kpc of the Galaxy this value is found to be around 103 M⊙ pc−3, falling as Galactocentric radius increases. This value is similar to the central densities found in giant molecular clouds in the Galaxy today. It is suggested that a globular cluster could reasonably form with that central density with a star formation efficiency of ≈40 per cent and an initial mass function slope α ≈ 3.
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