Measurements and Analysis of Helium-Like Triplet Ratios in the X-Ray Spectra of O-Type Stars Page: 3 of 52
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too close to the star to allow shocks of sufficient velocity to develop.
Other stars (01 Ori C and T Sco) have X-ray spectra that are unusually hard and have
relatively small line widths. While these stars might be considered prime candidates for
a coronal model of X-ray emission - especially after having magnetic fields detected via
Zeeman splitting (Donati et al. 2002, 2006) - their behavior is better understood in terms
of the magnetically channeled wind shock model, rather than a model of magnetic heating
(Schulz et al. 2000; Cohen et al. 2003; Schulz et al. 2003; Gagne et al. 2005; Donati et al.
Finally, we note that for all of the O giants and supergiants observed, the line profiles
are less asymmetric than predicted, given the high mass-loss rates measured for these stars
using radio free-free emission, H a emission, and UV absorption lines (Waldron & Cassinelli
2001; Kahn et al. 2001; Cassinelli et al. 2001; Miller et al. 2002; Kramer et al. 2003; Cohen
et al. 2006). This implies either a lower effective opacity to X-rays in their winds (e.g. due
to clumping or porosity effects (Feldmeier et al. 2003; Oskinova et al. 2004, 2006; Owocki &
Cohen 2006)), or lower mass-loss rates (Crowther et al. 2002; Massa et al. 2003; Hillier et al.
2003; Bouret et al. 2005; Fullerton et al. 2006).
One of the key diagnostic measurements available to us in understanding the nature of
X-ray emission in OB stars is the forbidden-to-intercombination line ratio in the emission
from ions that are isoelectronic with helium. This ratio is sensitive to the UV flux, and thus
to the proximity to the stellar surface. This allows us to constrain the location of the X-ray
emitting plasma independently of other spectral data, such as emission line profile shapes.
In this paper we discuss methods for using the f/i ratio to constrain the location of
X-ray emitting plasma in O star winds. In particular, we explore the effects of a spatially
distributed source motivated by the broad line profiles. We discuss the effects of photospheric
absorption lines, as well as the f /i ratio expected for a plasma emitted over a range of radii,
taking account of detailed line shapes when signal-to-noise allows. We find that accounting
in detail for photospheric absorption lines is not important, as long as the X-ray emission
originates over a range of radii.
These methods are then applied to He-like triplet emission in a set of archival Chandra
observations of O stars. Our primary result is that good fits can be acheived for most lines
with models having emission distributed over the wind, with minimum radii of about 1.5
stellar radii. We find that none of the data require the X-ray emitting plasma to be formed
very close to the photosphere.
This paper is organized as follows: In 2 we review the physics of line formation in
He-like species ( 2.1), explore the effects of spectral structure in the photoexciting UV field
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Leutenegger, Maurice A.; Paerels, Frits B.S.; /Columbia U., Astron. Astrophys.; Kahn, Steven M.; /SLAC; Cohen, David H. et al. Measurements and Analysis of Helium-Like Triplet Ratios in the X-Ray Spectra of O-Type Stars, article, June 19, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc892235/m1/3/: accessed February 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.