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W.J. Fischer, N.D. Morrison (U. Toledo)
The star 6 Cas (A3 Ia+) is typical of the optically brightest stars in external galaxies, which have been proposed as extragalactic distance indicators through the wind momentum-luminosity relation (e.g., McCarthy et al. 1997, ApJ, 482, 757). In this method, usually a single high-resolution, long-exposure spectrum is modeled to derive the stellar parameters, including the wind momentum. Therefore, variability in the profile is a potentially significant source of error. The aims of the present study are to estimate the magnitude of this error by quantifying the H\alpha profile variability in this nearby example of the class and to gain better insight into the nature and cause of the wind variability. Our observational material consists of 28 high-resolution (R ~q 26,000) spectra of 6 Cas obtained between 1993 and 1998 and including H\alpha and several photospheric lines. Our preliminary results include the finding that the photospheric radial velocities and the positions and strengths of the absorption and the emission components H\alpha are variable, but no single, dominant periodicity appears in a PDM analysis. The absorption radial velocity is correlated with the emission equivalent width, more negative velocities being associated with stronger emission. The absorption component shows variable structure, which may be caused by blended, moving subcomponents; they sometimes emerge to appear as discrete absorption components (DACs). The temporal variance spectrum of H\alpha is similar in amplitude and shape to that of HD 92207 (Kaufer et al. 1996, A&A, 305, 887). If this similarity denotes a typical variability pattern in extremely luminous A-type supergiants, then it may provide a constraint on the variability-induced error in the distance estimates to extragalactic supergiants.
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