Wolf‐Rayet and OB Star Self‐Enrichment of Globular Clusters?

Nitrogen abundance inhomogeneities are present among main-sequence stars in globular clusters of the Milky Way. Since N-rich cluster dwarfs are unlikely to have nucleosynthesized nitrogen within their own interiors, they presumably obtained their excess N from elsewhere. If the abundance inhomogeneities are due to the self-enrichment of a globular cluster, then the earliest source of nitrogen could have been massive cluster OB stars and Wolf-Rayet stars of type N. Several obstacles to WN stars' being a viable enrichment mode for globular clusters are discussed: (1) a W-R phase may have been inhibited for metal-poor OB stars, due to the low mass-loss rates expected of their radiatively driven winds, and (2) unless globular clusters had top-heavy stellar mass functions, their WN stars would have been too few in number to explain the amount of CNO-processed material in their nitrogen-rich, low-mass stars. In contrast, it is pointed out that OB stars in their main-sequence and supergiant phases of evolution might also eject CNO-processed material even if they do not evolve through a WN phase, while W-R stars have been discovered in some metal-poor dwarf galaxies and starburst galaxies. In addition, low metallicities could favor the production of WN stars over WC stars, and this could explain why the abundance inhomogeneities in Milky Way globular clusters generally involve excesses of CNO-processed material. One scenario is discussed in which globular clusters form at the interface between massive, colliding gas clouds. If such clouds have masses comparable to dwarf galaxies, with relative velocities characteristic of the Galactic halo, then the collision process could drive gas into an accreting protocluster at a rate that is sufficient to complete cluster formation within several million years. This timescale could allow nitrogen-rich wind ejecta from WN or OB stars to be incorporated into later cluster star formation. The ram pressure associated with a colliding-cloud environment might not only confine the stellar wind ejecta, but also funnel it into sites of active star formation within the protocluster. In this picture, globular clusters form as open systems with gas continuing to flow into them from a greater cloud-merger environment while star formation is occurring. In such a circumstance, not only may cluster WN and OB stars have promoted enrichment, but wind ejecta from massive stars that are external to the protocluster may also have been acquired.