arXiv subjects

W. L. Waldron

Publications and source records attributed to W. L. Waldron.

22 records · Page 2Linked to original sources

arXiv2007

X-ray Emission from Magnetically Torqued Disks of Oe/Be Stars

We focus attention on the Oe/Be stars to test the concept that the disks of these stars form by magnetic channeling of wind material toward the equator. Calculations are made of the X-rays expected from the Magnetically Torqued Disk (MTD) model for Be stars discussed by Cassinelli et al. (2002), by Maheswaran (2003), and by Brown et al. (2004). The dominant parameters in the model are the $β$ value of the velocity law, the rotation rate of the star, $S_o$, and the ratio of the magnetic field energy density to the disk gravitational energy density, $γ$. The model predictions are compared with the $ROSAT$ observations obtained for an O9.5 star $ζ$ Oph from \Berghofer et al. (1996) and for 7 Be stars from Cohen et al. (1997). Extra considerations are also given here to the well studied Oe star $ζ$ Oph for which we have $Chandra$ observations of the X-ray line profiles of the triad of He-like lines from the ion Mg XI.

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arXiv2004

High Resolution X-ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

The Cygnus OB2 Association contains some of the most luminous OB stars in our Galaxy, the brightest of which are also among the most luminous in X-rays. We have obtained a Chandra High Energy Transmission Grating Spectrometer (HETGS) observation centered on Cyg OB2 No. 8a, the most luminous X-ray source in the Association. Although our analysis will focus on the X-ray properties of Cyg OB2 No. 8a, we also present limited analyses of three other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous studies of early-type stars, we find that the X-ray properties of Cyg OB2 No. 8a are very similar to those of other OB stars that have been observed using high-resolution X-ray spectroscopy. From analyses of the He-like ion "fir" emission lines, we derive radial distances of the He-like line emission sources and find these fir-inferred radii are consistent with their corresponding X-ray continuum optical depth unity radii. Contrary to other O-star results, the emission lines of Cyg OB2 No. 8a show a large range in line centroid shifts (roughly -800 to +250 km/s). We discuss the implications of our results in light of the fact that Cyg OB2 No. 8a is a member of a rather tight stellar cluster, and shocks could arise at interfaces with the winds of these other stars.

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arXiv2004

Quasi-simultaneous XMM-Newton and VLA observation of the non-thermal radio emitter HD\168112 (O5.5III(f^+))

We report the results of a multiwavelength study of the non-thermal radio emitter HD168112 (O5.5III(f^+)). The detailed analysis of two quasi-simultaneous XMM-Newton and VLA observations reveals strong variability of this star both in the X-ray and radio ranges. The X-ray observations separated by five months reveal a decrease of the X-ray flux of ~30%. The radio emission on the other hand increases by a factor 5-7 between the two observations obtained roughly simultaneously with the XMM-Newton pointings. The X-ray data reveal a hard emission that is most likely produced by a thermal plasma at kT ~2-3 keV while the VLA data confirm the non-thermal status of this star in the radio waveband. Comparison with archive X-ray and radio data confirms the variability of this source in both wavelength ranges over a yet ill defined time scale. The properties of HD168112 in the X-ray and radio domain point towards a binary system with a significant eccentricity and an orbital period of a few years. However, our optical spectra reveal no significant changes of the star's radial velocity suggesting that if HD168112 is indeed a binary, it must be seen under a fairly low inclination.

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arXiv2001

Detection of Doppler Shifted X-ray Line Profiles from the Wind of Zeta Puppis (O4f)

We report on a 67 ks HETG observation of the optically brightest early O-star, Zeta Pup (O4 f). Many resolved X-ray lines are seen in the spectra over a wavelength range of 5 to 25 A. Chandra has sufficient spectral resolution to study the velocity structure of isolated X-ray line profiles, and to distinguish the individual forbidden, intercombination, and resonance (fir) emission lines in several He-like ions even where the individual components are strongly Doppler broadened. In contrast with X-ray line profiles in other hot stars, Zeta Pup shows blue-shifted and skewed line profiles, providing the clearest and most direct evidence that the X-ray sources are embedded in the stellar wind. The broader the line, the greater the blueward centroid shift tends to be. The N VII line at 24.78 A is a special case, showing a flat-topped profile. This indicates it is formed in regions beyond most of the wind attenuation. The sensitivity of the He-like ion fir lines to a strong UV radiation field is used to derive the radial distances at which lines of S XV, Si XIII, Mg XI, Ne IX, and O VII originate. The formation radii correspond well with continuum optical depth unity at the wavelength of each line complex, indicating that the X-ray line emission is distributed throughout the stellar wind. However, the S XV emission lines form deeper in the wind than expected from standard wind shock models.

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