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Astron. Astrophys. 343, 466-476 (1999)

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Modelling the spectral energy distribution and SED variability of the Carbon Mira R Fornacis *

A. Lobel 1, J.G. Doyle 1 and S. Bagnulo 2

1 Armagh Observatory, College Hill, Armagh BT61 9DG, Ireland
2 Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, A-1180 Wien, Austria

Received 2 July 1998 / Accepted 2 December 1998

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. 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 [FORMULA]=0.105 with [FORMULA]=3200[FORMULA]200 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 0.05[FORMULA]0.02 µm. The dust density distribution assumes a power law of [FORMULA] for a steady-state wind with a geometrical thickness ranging between [FORMULA] and [FORMULA] times the inner boundary shell radius and with a high gas mass-loss rate of 3-4 [FORMULA] [FORMULA] derived by radiation pressure onto the dust. We show that the optical and near-IR light curves are strongly affected by small changes of [FORMULA] 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

* Based on observations taken at UKIRT, CST, JCMT, SAAO, AAO and the IRAS Pointsource Catalogue

Send offprint requests to: A. Lobel (ajrl@star.arm.ac.uk)

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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