ROSAT observations of the Pleiades .1. X-ray characteristics of a coeval stellar population

We report the results of a deep X-ray survey of the core region of the Pleiades carried out with ROSAT. In a single PSPC field ( similar to 1 degrees radius), we detect 99 of 214 Pleiades stars, and we compute upper limits for the remainder. We have detected all early dM stars that fell within 20' of the field's center, where the instrument's sensitivity is highest. We have computed maximum-likelihood integral X-ray luminosity functions in various B - V intervals and compared the results with previous studies of the Pleiades. 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 versus X-ray luminosity relation found with the Einstein Observatory 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 versus X-ray luminosity relation for Pleiades dG stars and also 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 timescales of similar to 6 months (except for flares) and the presence of variability by a factor of 2 on timescales of similar to 10 yr in at least 15% of Pleiades members. The magnitude of this variability (by a factor of similar to 2) is too small to account for the spread of the X-ray luminosity functions.