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S. Skinner (Univ. of Colorado), D. Cohen (Swarthmore College), M. Gagne (West Chester Univ.), S. Owocki, R. Townsend (Bartol / Univ. of Delaware)
The sigma Orionis cluster is an excellent target for studying the X-ray properties of a rich population of young stars with ages of a few million years viewed through low intervening extinction. The cluster population spans the entire stellar mass spectrum from massive OB stars to low-mass brown dwarfs. We present results of a sensitive Chandra observation of the sigma Orionis cluster acquired with ACIS-S/HETG. We use the newly acquired spectra to test hot star X-ray emission models based on the wind shock paradigm.
The grating spectrum of sigma Ori AB (O9.5V + B) is dominated by low-temperature emission lines originating in cool plasma (kT < 1 keV). No significant asymmetries or Doppler shifts are seen, but the brightest lines do reveal slight excess broadening (albeit well below that expected for lines formed far out in the wind at the terminal speed of 1100 km/s). These properties, along with weak forbidden lines, are more suggestive of line formation closer to the star where the wind has not yet reached terminal speed. In sharp contrast, the ACIS-S CCD spectrum of the magnetic B star sigma Ori E (B2Vp) shows multi-temperature structure with a hot component at kT > 2 keV and its emission is variable. The hotter plasma is inconsistent with classical (non-magnetic) wind shock models. It could originate in an as yet unseen late-type companion, but we discuss an alternate interpretation in terms of magnetically confined wind shocks and summarize related MHD model development for hot stars.
This work was supported by NASA / SAO grant GO3-4007A.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.