Episodic Dust Emission from Alpha Orionis
Session 22 -- Late-Type and Variable Stars
Oral presentation, Monday, 2:30-4:00, Dwinelle 145 Room

[22.04] Episodic Dust Emission from Alpha Orionis

W.C. Danchi (UCB), L.J. Greenhill (SAO), M. Bester (UCB), C. Degiacomi (Spectrospin), C.H. Townes (UCB)

The spatial distribution of dust surrounding $\alpha$ Orionis has been observed with the Infrared Spatial Interferometer (ISI) operating at a wavelength of 11.15 $\mu$m. Radiative transfer modeling of the visibility curves obtained by the ISI has yielded estimates of the physical parameters of the dust surrounding the star and new details of the dust distribution. The visibility curves taken in 1992 can be fitted best by a model with two dust shells. One shell has an inner radius of 1.0$\pm$ 0.1${ }^{\prime\prime}$, a thickness between 50-200 milliarcsec, and a temperature of about 380 K. The second shell has an inner radius of 2.0$\pm$0.1${ }^{\prime\prime}$, a thickness less than about 200 milliarcsec, and a temperature of 265 K. These results are consistent with the recent spatially resolved spectroscopy of $\alpha$ Orionis reported by Sloan et al. (1993, Ap.J., 404, 303).

The dust was modelled with the MRN size distribution with radius varying from 0.005--0.25 $\mu$m. The star was assumed to be a blackbody with a temperature of 3500 K and angular radius of 21.8 milliarcsec, consistent with recent interferometric determinations of its diameter (cf. Dyck et al., 1992, A.J., 104, 1992).

For an adopted distance of 150 pc, the model for the 1992 data was evolved backward in time for a comparison with previous visibility data of Sutton (1979, Ph.D. Thesis, U.C. Berkeley) and Howell et al. (1981, Ap.J., 251, L21). The velocities, 11 ${\rm km \ s^{-1}}$ and 18 ${\rm km \ s^{-1}}$, were used for the first and second shells respectively, which are the CO velocities measured by Bernat et al. (1979, Ap.J.,233, L135). We find excellent agreement if the dust shells were at approximately 0.80${ }^{\prime\prime}$ and 1.67${ }^{\prime\prime}$ at the epoch of the previous measurements. The data are consistent with the hypothesis that inner dust shell was emitted during the unusual variations in radial velocity and visual magnitude in the early 1940's, described by Goldberg (1984, PASP, 96, 366).