AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 13. Atmospheres, Winds, Envelopes, Disks and Planetaries
Display, Monday, May 31, 1999, 9:20am-6:30pm, Southeast Exhibit Hall

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[13.09] Ionization Structure and Mass-Loss for Rapidly-Rotating Near Main-Sequence B Stars

J.E. Bjorkman, B.P. Abbott (U Toledo)

Ultraviolet resonance line profiles of rapidly-rotating near main-sequence B stars indicate that the two-dimensional ionization structure of the circumstellar envelope can be crucial to our understanding of the mass-loss rate. We investigate these two-dimensional effects through the use of a radiation transfer model to construct piece-wise spherical ionization fractions for the wind-compressed disk model of a rotating stellar wind. Using these ionization fractions, we generate theoretical line profiles using a two-dimensional Monte Carlo simulation of the radiation transport for the UV resonance lines of Si~{\textsc{iii}}, Si~{\textsc{iv}}, C~{\textsc{iii}}, C~{\textsc{iv}}, and N~{\textsc{v}}.

For the B2.5~IV star chosen for this study, we find the mass-loss rate to be on the order of a few times 10-9 \, M\odot\textrm{\,yr}-1 and the X-ray emission measure \log EMX ~52.5 cm-3. This result is consistent with observed X-ray luminosities as well as UV resonance line profiles. Our mass-loss rate is higher by a factor of 5--10 over those predicted theoretically (via spherically symmetric radiation-driven wind theory) and observationally (due to uncertainties in the ionization fractions).

In addition, we also examine the effects of a latitudinal density gradient on the line profiles. We find that the line profiles are sensitive to two-dimensional effects, showing disproportionate amounts of emission versus absorption as the viewing location changes from pole to equator. Specifically, N~{\textsc{v}}, which is abundant in the polar regions and depleted in the equator, shows enhanced emission for an observer looking edge-on to the equatorial wind-compressed disk. We show that spherically symmetric models cannot account for the anomalously strong emission or absorption resulting from a latitudinal ionization gradient in the wind.

This work has been supported under NASA grants NAG5-3447 (BPA) and NAG5-3248 (JEB) to the University of Toledo.

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