THE X-RAY-EMISSION OF A-TYPE STARS

From X-ray images in the ROSAT public archives, we determine soft X-ra y fluxes, or flux upper limits, for 74 A-type stars, which have been o bserved during deep integrations with the PSPC. Nine supposedly single , late-A stars (0.20<B-V<0.35) are found to coincide with X-ray source s. The X-ray luminosities we infer for these stars range from levels c omparable to the Active Sun, at log L(x) similar to 27.6, to much brig hter emission levels similar to those observed for active late-type bi nary systems, near log L(x) similar to 30.1. Another ten sources are i dentified with early-A stars (0.0<B-V<0.2). Five of these are confirme d double stars, the rest are ostensibly single. The maximum luminosity we detect in the early-A stars, log L(x)=30.1, is 3.5 orders of magni tude brighter than the X-ray upper limits for the nondetected stars. A dditional study, including radial-velocity monitoring and/or optical i nterferometry, will be needed to determine whether the putatively sing le X-ray emitting stars are in fact single, or whether their emission is produced entirely or in part by unknown or unresolved binary compan ions. The level of X-ray emission associated with chemically normal, s ingle A stars thus far appears to be uncorrelated with any obvious ste llar property, including the rotation rate, which is known to greatly influence the dynamo activity and the X-ray emission levels of lower m ass stars.