AN X-RAY SURVEY OF THE YOUNG OPEN CLUSTER NGC-2516

We present an analysis of a 60-ks ROSAT X-ray observation of the Galac tic open cluster NGC 2516, which has an age of about 110 Myr and a les s than solar metallicity. 159 X-ray sources (0.5-2.0 keV) are found in the central portion of the Position Sensitive Proportional Counter (P SPC) together with six soft X-ray sources (0.1-0.4 keV). From the lite rature, we have constructed a uniform catalogue of photometrically sel ected cluster candidates. 65 of the X-ray sources are identified with photometric members of NGC 2516, and 25 X-ray sources are identified w ith probable cluster non-members or stars with no photometric measurem ents with which to assess cluster membership. The X-ray luminosity thr eshold is approximately 10(29) erg s(-1) and X-ray upper limits are de termined for a further 136 possible cluster members. X-ray emission is observed across the Hertzsprung-Russell diagram, from spectral types of B2Ve to the early K stars which define the faintness limit of our o ptical catalogue. At least 73 X-ray sources have no plausible counterp art brighter than V = 15. Some may be foreground or background stars, 10-15 are probably extragalactic, but the majority are likely to be lo wer mass stars in the cluster. Three of the soft X-ray sources are pro bable hot white dwarfs in binary systems with late-type stars. One of these may be in the cluster, the other two are foreground objects. X-r ay emission from the hottest star in the cluster is attributed to a sh ocked stellar wind. 20 per cent of late B- and A-type stars are detect ed, which is consistent with the X-ray emission being the result of un resolved late-type companions. At least four out of the six magnetic, chemically peculiar stars are X-ray sources. We conclude that this is probably intrinsic, although an explanation involving binary companion s cannot be entirely ruled out. Many F, G and K stars are detected, pr esumably as a result of dynamo-generated coronal activity. The peak le vel of Xray activity is reached among the late G stars, which have an X-ray to bolometric flux ratio of 10(-3). This suggests that these sta rs have not yet spun down below the empirical X-ray saturation rotatio n speed of 10-20 km s(-1). Interpretation of the late-type star X-ray luminosity functions is hampered by the fact that there are likely to be contaminating field stars among the X-ray upper limits. The U-B, B- V colour-colour diagram for X-ray-selected cluster members reveals an ultraviolet excess among the F and G stars of NGC 2516, which is best explained by a less than solar metallicity, [Fe/H] = -0.32 +/- 0.06. C omparison with younger and older clusters shows that the late-type sta rs in NGC 2516 do not simply obey the widely accepted rotation-activit y-age paradigm. We suggest a modification to the paradigm which explai ns the observations in terms of the different convection zone properti es that late-type stars of differing metallicities have at the same co lour or mass.