1996A&A...305..785R

We present ROSAT PSPC pointed observations of the 50Myr old α Per open cluster. The X-ray observations, which were carried out as a raster scan, cover an area of about 10deg². In total, we detect about 160 X-ray sources, 88 of which have an optical counterpart (within 30arcsec) associated with α Per cluster candidates. Within the central region of our field of view, which is characterized by a limiting sensitivity L_X∼10^28.8–29erg/sec, we detect basically all late-F, G and K stars, while the detection rate among the M dwarfs is on the order of 60%. Given the sensitivity of our X-ray observations, the lower detection rate among the very low mass objects is consistent with the ROSAT results obtained for the Pleiades cluster. Although stars in each color range show a large spread in X-ray luminosity, the maximal X-ray luminosities appear to decrease from the range of late-F - G type stars to the M-type dwarfs. We interpret this as due to the fact that the maximum X-ray luminosity cannot exceed the saturation level L_X/L_bol∼10^–3, and is hence a function of the bolometric luminosity. The availability of rotational velocities for many of the X-ray detected objects permits us to study correlations between rotation and X-ray activity. For B-V_o>0.6, at a given mass, the weakest X-ray sources are slow rotators, while the strongest X-ray sources are rapid rotators. The relation between L_X/L_bol and rotation we find for the α Per low mass stars is the same as previously determined for low mass stars in the Pleiades, and a similar relation is found on correlating L_X/L_bol with rotation period. Using directly measured periods as well as periods estimated from rotational velocities, we derive Rossby numbers (R₀) for stars with B-V_o>0.3 finding that a well-defined relationship between L_X/L_bol and R₀ is present both for early (i.e., F) and late-type stars. A comparison of the X-ray luminosity distribution functions (XLDF) for our α Per sample and the Pleiades indicates that F and G-type stars in α Per are, as a whole, more X-ray luminous than their older counterparts in the Pleiades. On the other hand, no significant difference is found between the distributions of the K and M-type dwarfs in the two clusters. We argue that this finding is a consequence of the longer spin-down timescales of later-type objects, and hence of the fact that there are more rapid rotators among G stars in α Per than in the Pleiades, while this is not the case for K and M dwarfs.