The hot stars in orbit around the M31 central supermassive black hole: are they young or old?

Abstract

AIMS: The cluster of hot stars observed in orbit around the central black hole of M31 has been interpreted as a 200 Myr starburst. The formation of a population of young stars in close proximity to a massive black hole presents a difficult challenge to star formation theory. We point out that in a high stellar density environment, the course of stellar evolution is modified by frequent collisions and mergers. METHODS: Blue stragglers, which are the results of mergers in globular clusters, occupy the same position in the color-magnitude diagram as the observed hot stars in M31. For confirmation, the integrated spectrum of P3 is shown to be compatible with the spectral energy distribution of a blue horizontal branch field star. RESULTS: We suggest an old stellar population of evolved blue horizontal-branch stars and of merger products cannot be ruled out on the basis of the available data. Observations are suggested that would help distinguish between a ``young'' and ``old'' stellar population interpretation of the observations.