HIPPARCOS distances of X-ray selected stars: implications on their nature as stellar population.

Abstract

We present the parallaxes, measured by Hipparcos, for a sample of X-ray selected stars. The stars belong to the stellar sample of the Einstein Extended Medium Sensitivity Survey. They are all at galactic latitude |b|>20deg, and are generally far away from known star forming regions. Several of these stars show lithium abundance and activity level typical of very young stars with ages comparable to that of the Pleiades. We show that the majority of our sample stars are on the main sequence, with only =~20% being giants. We do not find a significant presence of pre-main sequence stars in our sample, notwithstanding the fact that some of our stars have a considerable lithium abundance, showing that the stars observed are most likely young and active main-sequence objects.