Optical Identification of ROSAT sources in M 67: activity in an old cluster

Abstract

We present optical identification and high-resolution spectroscopy of ROSAT sources in the field of the old open cluster M 67. For the first time it is possible to analyze coronal and chromospheric activity of active stars in a solar-age cluster, and to compare it with field stars. ROSAT observed the high X-ray luminosity tail of the cluster sources. In agreement with what expected from studies of field stars, most of the detected X-ray sources are binaries, preferably with short periods and eccentric orbits. In addition, several of the M 67 ROSAT sources have peculiar locations in the cluster colour-magnitude diagram. This is most likely due to rather complex evolutionary histories, involving the presence of mass transfer or large mass losses. The X-ray luminosity of the sources does not scale with the stellar parameters in an obvious way. In particular, no relationship is found between coronal emission and stellar magnitude or binary period. The Ca II K chromospheric flux from most of the counterparts is in excess to that of single stars in the cluster by one order of magnitude. The X-ray luminosity of the sources in the old M 67 is one order of magnitude lower than the most active active binaries in the field, but comparable to that of the much younger binaries in the Hyades.