ROSAT SURVEY OF STELLAR X-RAY SOURCES IN THE YOUNG OPEN CLUSTER NGC-2516

The ROSAT PSPC detector was used to obtain a deep pointing with its ce nter on the young open cluster NGC 2516 and total integration time of 9284 sec. Altogether 64 X-ray sources were detected showing strong con centration towards the optical cluster centre. This suggests that most of the sources are actually associated with NGC 2516. 42 of these sou rces could be identified with optical stars in the field of the cluste r; after correction for a mean offset between optical and nominal ROSA T positions of about 13 '', optical star and X-ray source positions ar e found to agree, On the average, within about 9 '', while maximum obs erved positional differences amount to 27 ''. X-ray sources identified with optical cluster stars include HR 3147 (=HD 66 194; B2.5IVe), the visually brightest B star in the cluster, three close visual late B-t ype binaries (h 4027 A,B, h 4031 A,B, and I 1104 A,B) with projected s eparations ranging between about 4 10(16) cm and 6 10(16) cm, as well as several (six) chemically peculiar Bp/Ap(Si) and Ap(SrCrEu) stars. A ccording to their known spectral or photometric properties, optical co unterparts for most identified cluster X-ray sources are main-sequence stars between spectral types B9 and K0. No indication for a pulsed co mponent of the X-ray Aux pointing to a compact companion could be dete cted from X-ray photon arrival times for the prominent blue straggler star HR 3147, while one of the visual binary counterparts of X-ray clu ster sources (I 1104 A/B) possibly exhibits a pulsed component. The fa ct that about half of the chemically peculiar Bp/Ap stars known in the cluster, all of them of the (magnetic) CP2 variety, are found to be c ounterparts to ROSAT X-ray sources, can be taken to suggest that stell ar magnetic fields play a role in generating X-ray fluxes from early-t ype main-sequence stars. Luminosities of X-ray sources detected in the cluster are rather uniform, ranging between L(x)(0.1 - 2.5 keV) = 0.4 10(30) and 4.5 10(30) erg s(-1). 80 percent of the sources have relat ively hard spectra, X-ray photons being recorded only in PSPC high-ene rgy channels above 0.5 keV. For only very few (six) bright X-ray sourc es in the field of NGC 2516, at least 30% of observed X-ray quanta fal l into the low-energy channels between 0.16 and 0.5 keV; four of these soft-spectrum sources could be identified with optical late-type star s in front of the cluster, including the nearby solar-type star HR 313 8 (=HD 65 907, MK: G0V).