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David McDavid (Limber Obs.), Karen Bjorkman (Ritter Obs.), Dietrich Baade (ESO), Geraldine Peters (USC)
In mid-1997 the Ritter Observatory H-alpha line profile of o And was dominated by a deep shell line, with weak emission partially filling in the photospheric absorption profile. The intrinsic polarization was barely measurable, with a peak value of about 0.23% in the B bandpass recorded at Limber Observatory. By mid-1998 the H-alpha shell line was slightly stronger, but the line profile was dominated by broad double-peaked emission. The difference spectrum shows that apart from the increase in peak emission, a general filling in of the photospheric profile also occurred, extending to +/-500 km/s from the H-alpha core. This velocity is almost twice vsini of the star (260 km/s) and is comparable to the Keplerian orbital velocity at the stellar surface. The intrinsic polarization had increased substantially, to a peak value of about 0.53% in B. Remarkably, the UBVRI wavelength dependence of the polarization remained almost the same.
We can see from these observations that even strong shell absorption is not necessarily associated with polarization. The increased polarization appears instead to correlate with enhancement of the emission line wings, which is usually attributed to electron scattering but may also involve a component of Keplerian rotation of the circumstellar disk. This supports the hypothesis that the polarization is mostly produced by electron scattering in the inner region of the disk, where the stellar flux, the electron density, and the orbital velocity are all expected to be the greatest. If the same electron scattering responsible for the polarization is also responsible for the emission wings, which is not necessarily true in general, it would make this star an ideal target for high resolution spectropolarimetry of the H-alpha profile. The wavelength dependence of the polarization, due to neutral hydrogen absorption, may well have remained the same in the presence of a significant overall polarization increase simply because of the general weakness of both emission and polarization in o And compared to other Be-shell stars. However, this makes it possible to observe correlations which are often obscured in the more spectacular Be stars because of the optical thickness of the circumstellar material.
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The author(s) of this abstract have provided an email address for comments about the abstract: mcdavid@limber.org