A ROSAT X-RAY STUDY OF THE PRAESEPE CLUSTER

We present the results of ROSAT PSPC observations of the Praesepe clus ter. 68 Praesepe candidates have been detected, above a threshold of a pproximate to 2 x 10(28) erg/sec, in the similar to 4 degrees x 4 degr ees area of the cluster covered by the observations. 56 out of the 68 detected objects are cataloged as high probability Praesepe members. P raesepe members of all spectral types have been detected with X-ray lu minosities ranging from the sensitivity limit to approximately 10(30) erg/sec in the ROSAT broad band. The highest X-ray luminosity has been measured for a very short period W UMa type SB2 binary. 2 out of the 4 Praesepe late-type giants have also been detected. X-ray luminosity distribution functions have been derived for late-type stars in the sa mple, taking into account both detections and upper limits. The main a nd most surprising finding are the low detection rates derived for Pra esepe low mass dwarfs. We detected about 30 % of the F and G stars, an d the detection rate among K and M dwarfs is even lower. Corresponding ly, the luminosity distribution functions for stars in selected color intervals are dominated by the contribution of upper limits, with the medians below the sensitivity threshold. The comparison with the Hyade s all-sky survey results shows an evident discrepancy between the aver age Xray properties of late-type dwarfs in the two apparently coeval c lusters; such a discrepancy must be an intrinsic one, since the observ ations are characterized by similar sensitivities.