Abstract
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 magnetospher es have been proposed, but all in all 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 resolut ion of this satellite with respect to previous X-ray instrumentation h as allowed us to examine also 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 lowers to 35 % if all emission is attributed to further known and unresolved companions. The spectral analysis confirms the h igh X-ray temperatures (� 20 MK) and large range of fractional X-ray luminosities (log Lx/L∗) of this class derived from earlier studies of individual ob jects. Conclusions. Radiative winds are ruled out as emission mechanism on 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 on both classes of objects, or there must be as yet undiscovered companion stars.
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