Modelling the spectral energy distribution and SED variability of the Carbon Mira R Fornacis *
Received 2 July 1998 / Accepted 2 December 1998
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. 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 µm is determined at =0.105 with =3200200 K for the central star in the considered phase of variability. By-products of the analysis are the shell composition of 90% amorphous carbon and only 10% SiC grains with rather small average radii of 0.050.02 µm. The dust density distribution assumes a power law of for a steady-state wind with a geometrical thickness ranging between and times the inner boundary shell radius and with a high gas mass-loss rate of 3-4 derived by radiation pressure onto the dust. We show that the optical and near-IR light curves are strongly affected by small changes of and of the shell optical depth with pulsation. A comparison of high resolution optical spectra of R For and medium/low resolution spectra of other carbon stars with the selected model input spectrum is also provided.
Key words: stars: AGB and post-AGB stars: mass-loss stars: individual: R For stars: carbon
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© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999