The X-ray properties of the young open cluster around #alpha# Persei

We present ROSAT PSPC pointed observations of the 50 Myr old #alpha# Per open cluster. The X-ray observations, which were carried out as a raster scan, cover an area of about 10 deg"2. In total, we detect about 160 X-ray sources, 88 of which have an optical counterpart (with 30 arcsec) associated with #alpha# Per cluster candidates. Within the central region of our field of view, which is characterized by a limiting sensitivity L_X #approx# 10"2"8"."8"-"2"9 erg sec"-"1, 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. A comparison of the X-ray luminosity distribution functions (XLDF) for our #alpha# Per sample and the Pleiades indicates that F and G-type stars in #alpha# 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 #alpha# Per than in the Pleiades, while this is not the case for K and M dwarfs. (orig.)SIGLEAvailable from TIB Hannover: RN 9303(329) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

A X-ray study of the young open cluster IC 2602

We present the results of ROSAT PSPC observations of the 30 Myr old IC 2602 cluster; for the X-ray detected objects the results of a CCD photometric survey are also given. In X-rays, we detect a total of 110 objects within a 11 deg"2 area, above a threshold of typically 3-5 x 10"2"8erg/sec. 68 of the detected objects have been identified with at least one optical counterpart; 44 of these are new optical candidates for cluster membership provided by our CCD photometry. Stars of all spectral types have been detected, from the very early-types to the late-M dwarfs. Soft X-ray luminosities range between about 10"2"9 erg/sec to a few 10"3"0 erg/sec, with the maximum and average L_X decreasing with spectral type for B-V larger than #approx# 0.8. Many of the stars redder than B-V #approx# 0.8 show a L_X/L_b_o_l ratio at about the saturation level of 10"-"3. We construct X-ray luminosity distribution functions for objects in different color ranges and we compare them with those for the Pleiades. F, G, and early-K type candidates in IC 2602 appear to be more X-ray luminous than in the Pleiades, while no significant difference is seen among late-K and M dwarfs. Under the assumption that our IC 2602 sample is not severely affected by incompleteness, we argue that the above finding is related to the distribution of rotational velocities in the two clusters, with most of the late-type stars being fast rotators in both clusters, while, due to different spin-down timescales, the earlier type stars in IC 2602 are likely to rotate more rapidly than their counterparts in the Pleiades. (orig.)Available from TIB Hannover: RN 9303(307) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman