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A&A 457, 223-235 (2006)
DOI: 10.1051/0004-6361:20065006
On the origin of the X-ray emission from Herbig Ae/Be stars
B. Stelzer1, G. Micela1, K. Hamaguchi2 and J. H. M. M. Schmitt31 INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: stelzer@astropa.unipa.it
2 Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
3 Hamburger Sternwarte, Gojenbergsweg 12, 21029 Hamburg, Germany
(Received 10 February 2006 / Accepted 18 May 2006 )
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 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
.
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 (
MK) and large range of
fractional X-ray luminosities (
) 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.
Key words: X-rays: stars -- stars: early-type -- stars: pre-main sequence -- stars: activity -- binaries: general
© ESO 2006