A search for dormant binaries with degenerate components in ω Centauri and NGC 6397

¹ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18 00-716 Warszawa Poland
e-mail: mnr@camk.edu.pl
² Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Av. Vicuña MacKenna 4860, 782-0436 Macul, Santiago, Chile
³ Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA

Received: 29 June 2010
Accepted: 13 September 2010

Abstract

Aims. We report on the first spectroscopic search for quiescent degenerate binaries in globular clusters.

Methods. Our survey is based on a sample of short-period optical variables that are likely optical counterparts of the quiescent X-ray sources in ω Centauri (NGC 5139) and NGC 6397.

Results. The studied candidates have nearly sinusoidal light curves with amplitudes of 0.05–0.12 mag in V (0.35 mag in one case) and periods of 0.1–1.3 days. This type of variability, most probably from the ellipsoidal effect, has been observed in X-ray novae when they settled into quiescence after an outburst.

Conclusions. We find that two of the surveyed systems harbor dim components with masses in excess of 1 M_⊙, making them attractive targets for future investigations. We also suggest that there are two subpopulations of blue stragglers in ω Centauri, differing in either mass-transfer history or helium content, or even in both at once.