1997A&A...320..865M

ROSAT X-ray (PSPC) and EUV (WFC) observations of a sample of 12 nearby Halo and Old-Disk low mass stars have been analyzed to determine their emission levels, properties of their coronal spectra and to characterize their temporal variations on time scales from hours to ten years. The light curves of the old-disk stars of our sample with the highest count statistics, show that variability of a factor two on a time scale of a few thousands of seconds is a common property of old-disk stars. The coronal emission of GJ 191, the only halo star with enough counts to make the time analysis feasible, shows significant variations on a time scale of six months. The time resolved spectral analysis of the most intense stars shows that their coronal emission is consistent with a two temperature model with (approximately) constant values of the temperatures along the entire observation, while the observed variability can be explained by changes in their emission measure distributions. We find that the emission measures of the two components change coherently during the observations. For all ten detected stars we have computed hardness ratio and X-ray luminosity. While our old-disk stars fit well in the body of the distribution outlined by a complete volume limited sample of dM and dK stars within 7pc (Schmitt et al. 1995ApJ...450..392S), the halo stars tend to occupy the softest and weakest envelope defined by population I low-mass stars.