Wind circulation in selected rotating magnetic early-B stars

Citation
Ma. Smith et D. Groote, Wind circulation in selected rotating magnetic early-B stars, ASTRON ASTR, 372(1), 2001, pp. 208-226
Citations number
65
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY & ASTROPHYSICS
ISSN journal
1432-0746 → ACNP
Volume
372
Issue
1
Year of publication
2001
Pages
208 - 226
Database
ISI
SICI code
1432-0746(200106)372:1<208:WCISRM>2.0.ZU;2-Q
Abstract
The rotating magnetic B stars are a class of variables consisting of He-str ong and some beta Cep stars which have oblique dipolar magnetic fields. Suc h stars develop co-rotating, torus-shaped clouds by channeling wind particl es from their magnetic poles to circumstellar regions centered around the p lane of their magnetic equators. The rotation of the cloud-star complex per mits the study of absorptions from the cloud as it occults the star. In thi s paper we describe a quantitative analysis of archival IUE data to map the properties of these clouds over four stars (HD184927, sigma Ori E, beta Ce p, and HR6684). By computing spectral synthesis models for these stars, we find that only beta Cep has a solar-like metallicity. Our analysis also sho ws that the metal composition across the surfaces of all these stars is at least approximately homogeneous. Using the Hubeny code CIRCUS, we demonstra te that the periodic variations of broad-band ultraviolet continuum fluxes can be explained fully by the absorptions of the co-rotating cloud. We show next that among selected lines, those arising from low-excitation states a re selectively affected by cloud absorption and turbulence. Our analysis al so quantifies the cloud temperatures and column densities required to match the absorptions of a number of weak to moderate strength resonance lines. These temperatures increase with the ionization potential of the parent ion s of these various lines, a result which is consistent with radiative equil ibrium models in which temperature increases with proximity to the star's s urface. Although these attributes appear stable from one epoch to another, dynamic processes are nonetheless at work. Both the strengths and widths of resonance lines at occultation phases indicate the presence of a turbulenc e in the cloud which increases inwards. The spectroscopic hallmark of this stellar class is the presence of strong CIV and NV resonance line absorptio ns at occultation phases and of redshifted emissions of these lines at magn etic pole-on phases. The emissions have characteristics which seem most com patible with their generation by high-energy shocks at the wind-cloud inter face, as predicted recently by Babel (1998).