THE X-RAY-EMISSION OF A-TYPE STARS

From X-ray images in the ROSAT public archives, we determine soft X-ray fluxes, or flux upper limits, for 74 A-type stars, which have been observed 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 sources. The X-ray luminosities we infer for these stars range from levels comparable to the Active Sun, at log L(x) similar to 27.6, to much brighter emission levels similar to those observed for active late-type binary systems, near log L(x) similar to 30.1. Another ten sources are identified with early-A stars (0.0<B-V<0.2). Five of these are confirmed 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 magnitude brighter than the X-ray upper limits for the nondetected stars. Additional study, including radial-velocity monitoring and/or optical interferometry, will be needed to determine whether the putatively single X-ray emitting stars are in fact single, or whether their emission is produced entirely or in part by unknown or unresolved binary companions. The level of X-ray emission associated with chemically normal, single A stars thus far appears to be uncorrelated with any obvious stellar property, including the rotation rate, which is known to greatly influence the dynamo activity and the X-ray emission levels of lower mass stars.