The hot stars in orbit around the M 31 central supermassive black hole: are they young or old?

Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA e-mail: [pierre.demarque;shanil.virani]@yale.edu

Received: 27 June 2006
Accepted: 2 October 2006

Abstract

Aims.The cluster of hot stars observed in orbit around the central black hole of M 31 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 M 31. 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.