Why is the X-ray luminosity function different for the Hyades and Praesepe?

Abstract

We have studied dF-dK and dM stars belonging to Praesepe in order to determine membership via radial velocities and to compare several stellar properties, such as X-ray luminosities, H$α$ equivalent width, etc, between this cluster and Hyades, which have similar age. We show that all properties are analogous in both clusters except the distribution of Lx. This fact could be related to difficulties in the analysis of the Lx upper limits or to more fundamental reasons, such as binarity rates and differences in the properties of the binaries of each cluster.

4 pages, LATEX file. Mem.S.A.It style macro (Included). Contribution to "Cool Stars in Clusters and Associations: Magnetic Activity and Age Indicators", Palermo, May 1997