ROSAT OBSERVATIONS OF THE PLEIADES CLUSTER. I. X-RAY CHARACTERISTICS OF A COEVAL STELLAR POPULATION G. Micela and S. Sciortino

We report the results of a deep X-ray survey of the core region of the Pleiades cluster carried out with the ROSAT Observatory. In a single PSPC field ( 1 radius), we detect 99 of 214 Pleiades member stars, and compute upper limits for the remainder. We have detected all early dM stars which fell within 20 arcmin of the field center, where the instrument sensitivity is highest. We have computed maximum-likelihood integral X-ray luminosity functions in various B \Gamma V intervals, and have compared the results with previous studies of Pleiades cluster. Our results support the contention that B and dA star `detections' are likely due to X-ray emission from lower-mass companions; we confirm the existence of a stellar-age vs. X-ray--luminosity relation found with Einstein for dG stars, and extend it to dK and dM stars; and we construct an essentially statistically complete X-ray luminosity function for Pleiades dG stars and an almost complete X-ray luminosity function for dK stars. We have verified the stellar-rotation vs. X-ray--luminosity relation for Pleiades dG stars, and show that a similar relation does not hold for dK stars. Indeed, it appears that some stellar parameters other than age and/or rotation rate must affect the level of X-ray emission of Pleiades dG and dK stars. We demonstrate the lack of measurable variability in X-ray emission at small time scales of 6 months (except for flares), and the presence of variability by a factor two on timescales of 10 yr in at least 15% of Pleiades members. The magnitude of this variability (by a factor two) is too small to account for the spread of the X-ray luminosity functions. Subject headings: open cluster and associations: individual (Pleiades) - stars: late-type - stars: rotation - surveys - X-Rays: stars 1