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J.J. Bochanski, E.M. Sion (Villanova University)
The true nature of Mira's companion star "Mira B" has intrigued and troubled investigators for the past 30 years. In the FUV spectral region, which is free of contamination from Mira A, the nature of the line spectrum and continuum energy distribution provide important clues. We have analyzed numerous IUE archival spectra of Mira B, in the wavelength range shorter than 2600 Å, with high gravity photosphere and accretion disk models with vertical structure. An optically thick accretion disk model at high inclination with an accretion rate of 10-10.5 Mo yr-1 reveals large disagreement with the observed continuum. Higher accretion rates, as proposed by Reimers and Cassatella (1985), are even worse. We find that the Lyman \alpha region and FUV continuum out to 2600 Åis best fit by a white dwarf with a surface temperature of 9000 K. Using the new Hipparcos distance to Mira AB (128 pc), our estimate of the observed FUV luminosity agrees with the bolometric luminosity of a 9000 K white dwarf. If this temperature is maintained by accretion, then the required accretion rate is only 5.45 x 10-12 Mo yr-1. If accretion is not a factor in the white dwarf temperature, then the cooling age of the white dwarf is 854 Myr, with a mass of 0.6 Mo. The details of our model fitting are presented together with a discussion of the implications for wind accretion efficiency and disk formation at large binary separations.
This research was supported by NSF grant AST 99-01955.