Measurements and Analysis of Helium-Like Triplet Ratios in the X-Ray Spectra of O-Type Stars Page: 2 of 52
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1. Introduction
Since the discovery of X-ray emission from OB stars by Einstein (Harnden et al. 1979;
Seward et al. 1979), the exact mechanism for X-ray production has been something of a
mystery. X-ray emission from OB stars had been predicted by Cassinelli & Olson (1979), who
proposed that an X-ray emitting corona could explain the observation of superionized O VI
through Auger ionization of O IV. However, subsequent observations showing less attenuation
of soft X-rays than would be expected from a corona lying below a dense stellar wind made
a purely coronal origin seem unlikely (Cassinelli & Swank 1983). Macfarlane et al. (1993)
also found that a distributed X-ray source was necessary to explain the observed O VI UV P
Cygni profile in ( Pup. Furthermore, with no expectation of a solar-type a - Q dynamo in
OB stars with radiative envelopes, the coronal model fell out of favor. Subsequently, several
scenarios in which magnetic field generation and dynamos could exist in OB stars have
been proposed (Charbonneau & MacGregor 2001; MacGregor & Cassinelli 2003; Mullan
& MacDonald 2005). Since these models have been proposed, the primary observational
evidence invoked by their proponents is anomalously low f/i ratios in the X-ray emission of
a few He-like ions in several stars. Re-examining these line ratios and determining whether
they require a coronal model to explain them is one of the main goals of this paper.
Shocks arising from instabilities in the star's radiatively driven wind have been consid-
ered to provide a more likely origin for the observed X-ray emission, as they are expected
to be present, given the line-driven nature of these winds (Lucy & White 1980; Lucy 1982;
Krolik & Raymond 1985; Owocki, Castor, & Rybicki 1988; Feldmeier 1995). However, there
have been difficulties in reproducing the observed X-ray properties of O stars, such as the
overall X-ray luminosity and the spectral energy distribution, from stellar wind instability
models (Hillier et al. 1993; Feldmeier 1995; Feldmeier et al. 1997a,b). Until recently, the
quality of the available spectral data provided little insight into these problems, since the
CCD and proportional counter spectra could not resolve individual spectral lines.
Recent high resolution X-ray spectroscopy of OB stars by the XMM-Newton Reflection
Grating Spectrometer (RGS) (Kahn et al. 2001; Mewe et al. 2003; Raassen et al. 2005)
and the Chandra High Energy Transmission Grating Spectrometer (HETGS) (Schulz et al.
2000; Waldron & Cassinelli 2001; Cassinelli et al. 2001; Miller et al. 2002; Cohen et al. 2003;
Kramer et al. 2003; Gagn6 et al. 2005; Cohen et al. 2006) have answered some questions
while raising new ones. Some stars have X-ray spectra that appear consistent with emission
from shocks in the wind, but the detailed comparisons to predicted spectral models are still
problematic. Both Waldron & Cassinelli (2001) and Cassinelli et al. (2001) have found low
forbidden-to-intercombination line ratios in one set of helium-like triplets each in the X-ray
spectra of ( Ori and ( Pup. They infer from this that some of the X-ray emitting plasma is
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Leutenegger, Maurice A.; Paerels, Frits B. S.; Kahn, Steven M. & Cohen, David H. Measurements and Analysis of Helium-Like Triplet Ratios in the X-Ray Spectra of O-Type Stars, article, June 19, 2006; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc892235/m1/2/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.