Global X-ray properties of the O and B stars in Carina
Y. Naze, P.S. Broos, L. Oskinova, L.K. Townsley, D. Cohen, M.F. Corcoran, N.R. Evans, M. Gagne, A.F.J. Moffat, J.M. Pittard, G. Rauw, A. ud-Doula, N.R. Walborn
The key empirical property of the X-ray emission from O stars is a
strong correlation between the bolometric and X-ray luminosities. In the
framework of the Chandra Carina Complex Project, 129 O and B stars have
been detected as X-ray sources; 78 of those, all with spectral type
earlier than B3, have enough counts for at least a rough X-ray spectral
characterization. This leads to an estimate of the Lx/Lbol ratio for an
exceptional number of 60 O stars belonging to the same region and
triples the number of Carina massive stars studied spectroscopically in
X-rays. The derived log(Lx/Lbol) is -7.26 for single objects, with a
dispersion of only 0.21dex. Using the properties of hot massive stars
listed in the literature, we compare the X-ray luminosities of different
types of objects. In the case of O stars, the Lx/Lbol ratios are similar
for bright and faint objects, as well as for stars of different
luminosity classes or spectral types. Binaries appear only slightly
harder and slightly more luminous in X-rays than single objects; the
differences are not formally significant (at the 1% level), except for
the Lx/Lbol ratio in the medium (1.0--2.5keV) energy band. Weak-wind
objects have similar X-ray luminosities but they display slightly softer
spectra compared to "normal" O stars with the same bolometric
luminosity. Discarding three overluminous objects, we find a very
shallow trend of harder emission in brighter objects. The properties of
the few B stars bright enough to yield some spectral information appear
to be different overall (constant X-ray luminosities, harder spectra),
hinting that another mechanism for producing X-rays, besides wind
shocks, might be at work. However, it must be stressed that the earliest
and X-ray brightest amongst these few detected objects are similar to
the latest O stars, suggesting a possibly smooth transition between the
two processes.
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