On the origin of the X-ray emission from Herbig Ae/Be stars

Context. Herbig Ae/Be stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Alternative X-ray production mechanisms, related to stellar winds or star-disk magnetospheres have been proposed, but their X-ray emission has remained a mystery. Aims. A study of Herbig Ae/Be stars' global X-ray properties (such as detection rate, luminosity, temperature, variability), helps to constrain the emission mechanism by comparison to other types of stars, e. g. similar-age but lower-mass T Tauri stars, similar-mass but more evolved main-sequence A- and B-type stars, and with respect to model predictions. Methods. We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. The superior spatial resolution of this satellite with respect to previous X-ray instrumentation has allowed us to also examine the possible role of late-type companions in generating the observed X-rays. Results. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10. The detection fraction of Herbig Ae/Be stars is 76%, but decreases to 35% if all emission is attributed to further known and unresolved companions. The spectral analysis confirms the high X-ray temperatures (similar to 20MK) and large range of fractional X-ray luminosities (log L-x/L-*) of this class derived from earlier studies of individual objects. Conclusions. Radiative winds are ruled out as an emission mechanism on the basis of the high temperatures. The X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known), nor from other young late-type stars used for comparison. Therefore, either a similar kind of process takes place in both classes of objects, or there must be as yet undiscovered companion stars.