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Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2009

New X-ray detections of Herbig stars

B. Stelzer
  • Fonction : Auteur
J. Robrade
  • Fonction : Auteur
J. H. M. M. Schmitt
  • Fonction : Auteur

Résumé

Context: The interpretation of X-ray detections from Herbig Ae/Be stars is disputed as it is not clear whether these intermediate-mass pre-main sequence stars are able to drive a dynamo and ensuing phenomena of magnetic activity. Alternative X-ray production mechanisms, related to stellar winds, star-disk magnetospheres, or unresolved late-type T Tauri star companions have been proposed. Aims: The companion hypothesis can be tested by resolving Herbig stars in X-rays from their known visual secondaries. Furthermore, their global X-ray properties (such as detection rate, luminosity, temperature, variability) may give clues to 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: In a series of papers we have been investigating high-resolution X-ray Chandra images of Herbig Ae/Be and main-sequence B-type stars where known close visual companions are spatially separated from the primaries. Results: Here we report on six as yet unpublished Chandra exposures from our X-ray survey of Herbig stars. The target list comprises six Herbig stars with known cool companions, and three other A/B-type stars that are serendipitously in the Chandra field-of-view. In this sample we record a detection rate of 100%; i.e. all A/B-type stars display X-ray emission at levels of log(L_x/L_bol) ~ -5...-7. The analysis of hardness ratios confirms that HAeBes have hotter and/or more absorbed X-ray emitting plasma than more evolved B-type stars. Conclusions: Radiative winds are ruled out as an exclusive emission mechanism on the basis of the high X-ray temperatures. Confirming earlier results, 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). The diagnostics provided by the presently available data leave it open whether the hard X-ray emission of Herbig stars is due to young age or indicative of further coronally active low-mass companion stars. In the latter case, our detection statistics imply a high fraction of higher order multiple systems among Herbig stars.

Dates et versions

hal-00479887 , version 1 (03-05-2010)

Identifiants

Citer

B. Stelzer, J. Robrade, J. H. M. M. Schmitt, J. Bouvier. New X-ray detections of Herbig stars. Astronomy and Astrophysics - A&A, 2009, 493, pp.1109. ⟨10.1051/0004-6361:200810540⟩. ⟨hal-00479887⟩
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