Modelling the Spectral Energy Distribution of Carbon and Oxygen-rich stars from stellar model spectra
Abstract
We have developed a new method to determine the physical properties and the local circumstances of dust shells surrounding Carbon- and Oxygen-rich stars for a given pulsation phase. The observed mid-IR dust emission feature(s), in conjunction with IRAS BB photometry and coeval optical and near-IR BB photometry, are modelled from radiative transport calculations through the dust shell using a grid of detailed synthetic model input spectra for M-S-C giants to the DUSTY code. From its application to the optical Carbon Mira R For we find that the temperature of the inner shell boundary exceeds 1000 K, ranging between 1200 K and 1400 K. The optical depth of the shell at 11.3 microns is determined at tau11 = 0.105 with Teff = 3200 +- 200 K for the central star in the considered phase of variability (Jul.-Aug. '95). By-products of the analysis are the shell composition of 90% amorphous carbon and only 10% SiC grains with rather small average radii of 0.05 +- 0.02 microns. The dust density distribution follows a power law of r^{-2} for a steady-state wind with a geometrical thickness ranging between 10^4 and 5 10^4 times the inner boundary shell radius. The density gradient of r^{-2} results from the full dynamic radiative transport calculations which include the momentum transfer from the radiation field. This elaborate method enables the modelling of the variable shape of the silicate feature of o Cet and other oxygen-rich stars in conjunction with the SED observed at various pulsation phases. We discuss the changes in the CSE and the related ranges of the mass-loss rate and terminal outflow velocity. A brief comparison of high-resolution optical spectra of R For, o Cet and spectra of other carbon- and oxygen-rich stars, with the selected model input spectra is also provided.
- Publication:
-
IAU Symposium
- Pub Date:
- 1998
- Bibcode:
- 1998IAUS..191P.307L