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During the pulsational cycle of Miras, the total IR luminosity and the effective temperature of the star vary as shock waves pass out through the atmosphere and into the circumstellar shell. Since dust appears to form and evaporate near the inner edge of a circumstellar shell during the light cycle of Miras, we expect that the observed low resolution spectra (LRS) obtained by IRAS of the dust grain signatures should vary as a function of phase. Analysis of the individual spectral scans of M star Miras shows that the 12 $\mu$m PSC fluxes and the contrast of the 10 $\mu$m silicate feature vary in phase with the optical light curve. The contrast is defined as the ratio of observed flux at 10 $\mu$m to the black body flux at that wavelength.
We are currently modeling the dust emission using the Leung computer code (CSDUST3) for modeling radiative transfer in dust shells. The effects of varying model parameters (such as dust optical depth, stellar temperature and the inner radius of the dust shell) on the contrast of the observed silicate feature are being investigated and show, for example, that the contrast of the emission feature decreases as the temperature of the central star decreases. As T varies from 3500 K to 2000 K the contrast decreases from 2.6 to 1.4 (for $\tau$(10 $\mu$m ) = 0.1). A contrast of unity indicates no emission feature. The predictions of the models will be compared to the observed variations in PSC flux and contrast in order to model changes in the dust shell with phase.
This work is in part supported by NASA grant NAG5$-$1667 and Wellesley College.
