1997A&A...322..785O

This study represents the first X-ray observations of an extended sample of Pop II field binaries, aimed at investigating the properties of old, metal-poor stellar coronae. Analysing X-ray observations from the ROSAT all-sky survey, we detected only 13 (out of 86) Pop II systems (15% detection rate). The X-ray luminosity function, taking into account both detections and upper limits, has its median at logL_x≤28.1erg/s, indicating a low average X-ray luminosity, with a high-luminosity tail at logL_x∼29-31erg/s. The only extreme metal-poor system detected is HD 89499. Thus, the detection rate of extreme Pop II systems is lower than of intermediate Pop II, possibly indicating extreme Pop II to be typically less luminous. The X-ray luminosity is not very well correlated with orbital period; long-period Pop II binaries may have high X-ray luminosities and, surprisingly, short-period systems are not per se strong X-ray emitters. For a subsample of emission-line Pop II binaries, i.e. the halo component analogs to the RS CVn binaries, the median X-ray luminosity is at least one order of magnitude lower than for the RS CVns. The lower activity levels of the Pop II systems may be caused in part by the presence of fewer evolved stars in the sample and lower metallicity. The extremely old age of Pop II binaries may also give rise to the unexpectedly low X-ray luminosities of some systems (e.g., CD-481741, BD+53080).