Computer Science
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........35m&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF COLORADO AT BOULDER, 1995.Source: Dissertation Abstracts International, Volume: 57-02, Section: B,
Computer Science
2
Scientific paper
This dissertation examines the spectroscopic properties of Wolf-Rayet stars in the Galaxy and Magellanic Clouds over ultraviolet and near-infrared wavelengths, with emphasis on the stars of the nitrogen (WN) sequence. International Ultraviolet Explorer satellite and ground-based CCD spectrophotometry (~ 0.1-1.0 μm) are analyzed from both observational and basic theoretical viewpoints to (1) explore issues of spectrum formation under generalized conditions appropriate for observed, collective properties, and (2) determine extrinsic observables of stellar distances and interstellar reddening in order to quantify more accurately the stellar and wind parameters needed to evaluate local environmental conditions. First, we exploit HeII emission line flux ratios to derive the color excesses E_{B-V } of Galactic WN stars, relying on the LMC WN stars less-affected by reddening for calibration. The effects of anomalous line-of-sight reddening properties on the E_{B-V} values are diagnosed with the variable extinction law of Cardelli et al. (1989). Next, we recognize from observations and published non-LTE model atmospheres that the intrinsic, extended continuum energy distribution of Wolf-Rayet stars of both nitrogen and carbon (WC) sequences is well-approximated by a power law of the form F_λ~lambda^{-alpha}. By correcting the observed (reddened) distributions to fit this model (via chi^2 minimization), we obtain color excesses E_ {B-V} and spectral indices alpha for WR stars in the Galaxy and Magellanic Clouds. The value of alpha is interpreted to be a measure of radial extension of the continuum-forming layers, and a simple model helium atmosphere (most applicable to the WN stars) with a rho^2 -dependent continuum opacity is constructed to show how this depends on the radius and temperature of the star, the mass-loss rate, and the wind temperature and density profiles. A comparison between these predictions and the observed values of alpha shows that there is a gradual transition between strong-lined and weak-lined WN stars. We also find that the observed range of alpha is qualitatively consistent with the theoretical range expected from more detailed, non-LTE model atmospheres. But on closer examination, we see that these models predict a bimodal distribution of the values of alpha with smaller values of alpha~eq 2.7, i.e. corresponding to flatter energy distributions, for the strong-lined stars and larger values of alpha~eq 3.1, corresponding to steeper energy distributions, for the weak-lined stars. New insights on the empirical relationship between the emission lines and the continuum fluxes are found with the discovery of negative correlations between the Line equivalent widths and the continuum flux at line center among WN stars of the LMC. This effect had previously been observed only in the spectra of quasars and some Seyfert I galaxies (the "Baldwin Effect"). Its occurrence in Wolf -Rayet stars suggests some similarities in spectrum formation mechanisms at work in these different environments. We again resort to our simple model atmosphere for an empirical understanding of the effect in Wolf-Rayet stars, and exploit the HeII equivalent width-continuum correlations obtained from LMC observations to determine distances and absolute magnitudes of Galactic and SMC WN stars. (Abstract shortened by UMI.).
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