Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aj....118..539k&link_type=abstract
The Astronomical Journal, Volume 118, Issue 1, pp. 539-548.
Astronomy and Astrophysics
Astronomy
4
Polarization, Stars: Individual (Hd 165763), Stars: Mass Loss, Stars: Wolf-Rayet
Scientific paper
We have obtained spectropolarimetric data of HD 165763 (WR 111, WC 5) with a spectral resolution of 1.24 Å, covering the wavelength range from 4950 to 6200 Å. The continuum is polarized at a level of 0.39% at 5805 Å, but there is no polarization variation across the emission lines. The latter indicates that most of the polarization arises from the interstellar medium. It further suggests that any global deviation of the atmosphere from spherical symmetry, if it exists, is small. Radiative transfer calculations of axisymmetric stellar wind models are used to predict polarization changes across the very strong C IV (lambda5805) emission line. We fitted the observational data with the models by using the continuum polarization as a constraint and by treating the interstellar polarization as a free parameter instead of using unreliable values of interstellar polarization estimated from analysis of field stars. The results from the chi^2 testing of the model suggest that the global deviation from spherical symmetry of this object is no larger than 20%, and it is probably less than 10%. In our formulation, the ratio of the equatorial density and the polar density (rho_eq/rho_pole) corresponding to the 20% upper limit is about 1.25. A similar conclusion is obtained from comparison of ``continuum-minus-line'' polarization of the observations with that of our models. None of the single WC stars (except for WR 103) with spectropolarimetric data show a variation in polarization across emission lines. Therefore, global deviations from spherical symmetry of WC stars are expected to be small in general. The relatively low value of the upper limit for WR 111 indicates that mass-loss enhancement due to rotation is unlikely to explain the difference between the observed and the predicted WC mass-loss rates. It also suggests that a significant amount of angular momentum is removed by mass loss during the pre-WC star stage of stellar evolution. A low value for the upper limit of deviation from spherical geometry is important, since it validates the assumption of spherical symmetry used in many current evolutionary and atmospheric models of single WC stars.
Hillier Desmond John
Kurosawa Ryuichi
Schulte-Ladbeck Regina E.
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