Journal Article

Dust shells around carbon Mira variables

M. A. T. Groenewegen, P. A. Whitelock, C. H. Smith and F. Kerschbaum

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 293, issue 1, pages 18-42
Published in print January 1998 | ISSN: 0035-8711
Published online January 1998 | e-ISSN: 1365-2966 | DOI:
Dust shells around carbon Mira variables

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The spectral energy distributions and mid-infrared spectra of 44 carbon Mira variables are fitted using a dust radiative transfer model. The pulsation periods of these stars cover the entire range observed for carbon Miras. The luminosities are derived from a period—luminosity relation. Parameters derived are the distance, the temperature of the dust at the inner radius, the dust mass-loss rate and the ratio of silicon carbide to amorphous carbon dust. The total mass-loss rate is derived from a modified relation between the photon momentum transfer rate (L/c) and the momentum transfer rate of the wind (̇Mv). Mass-loss rates between 1 × 10−8 and 4 × 10−5 M yr−1 are found. We find good correlations between mass-loss rate and pulsation period (log ̇M=4.08 log P−16.54), and between mass-loss rate and luminosity (log ̇M=3.94 log L−20.79). These relations are not independent, as we assumed a PL relation. If we had assumed a constant luminosity for all stars, there still would be a significant relation between ̇M and P. The dust-to-gas ratio appears to be almost constant up to periods of about 500 d, corresponding to about 7900 L, and then to increase by a factor of 5 towards longer periods and higher luminosities. A comparison is made with radiation-hydrodynamical calculations including dust formation. The mass-loss rates predicted by these models are consistent with those derived in this paper. The main discrepancy is in the predicted expansion velocities for models with luminosities below ∽5000 L. The radiation-hydrodynamical calculations predict expansion velocities which are significantly too large. This is related to the fact that these models need to be calculated with a large C/O ratio to get an outflow in the first place. Such a large C/O ratio is contrary to observational evidence. It indicates that a principal physical ingredient in these radiation-hydrodynamical calculations is still missing. Possibly the winds are ‘clumpy’, which may lead to dust formation on a local scale, or there is an additional outwards directed force, possibly radiation pressure on molecules.

Keywords: stars: AGB and post-AGB; stars: carbon; circumstellar matter; stars: mass-loss; stars: variables: other; infrared: stars

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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