ROSAT HRI survey of the open cluster Stock 2

We have obtained two deep HRI observations: toward the open cluster Stock 2 in order to assess the X-ray luminosity level of the 112 high-probability cluster members falling in the area we have surveyed. Members have been sel ected on the basis of the result of a recent proper motion study based on t he same photographic material and the same reduction scheme as the GSC-II p roject. We have reached a limiting sensitivity of L-X similar to 3 10(29) e rg/sec for Stock 2 members and have detected 19 X-ray sources. The attained limiting sensitivity has prevented a complete sampling of the coronal emis sion level of Stock, and we have detected only three solar-type and one B9V members. We have derived the X-ray luminosity function for the Stock 2 sta rs in the 0.43 < (B-V)(o) < 0.8 color range and have compared it with the X -ray luminosity functions of the dG members of Praesepe, Hyades and Pleiade s open clusters. While the limiting Stock 2 and other cluster distances do not allow to derive firm conclusion, yet, our data indicate that the corona l emission level of Stock 2 solar-type stars is more similar to that of thr Hyades (Or the Praesepe) than to the Pleiades one. This in turn suggests that either Stock 2 is older than previously reported or that the current age-rotation-activity paradigm need to be revised. In this latter case Stock 2 would likely belong to the small group of open clu sters (Praesepe, NGC6333 and IC 4675) for which recent X-ray surveys indica te a more complex age-rotation-activity relation than currently thought.