Volume 524, December 2010
|Number of page(s)||10|
|Section||Interstellar and circumstellar matter|
|Published online||23 November 2010|
The mass-loss return from evolved stars to the Large Magellanic Cloud
III. Dust properties for carbon-rich asymptotic giant branch stars
Institut d’Astrophysique de Paris,
98bis boulevard Arago,
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3 UCL-Institute of Origins, Astrophysics Group, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
4 UCL-Institute of Origins, Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
5 Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
6 NASA Ames Research Center, SOFIA Office, MS 211-3, Moffet Field, CA 94035, USA
7 Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
8 Department of Physics and Astronomy, Iowa State University, Ames, IA 50010, USA
9 Center for Radiophysics and Space Research, Cornell University, 222 Space Sciences Building, Ithaca, NY 14853, USA
Accepted: 12 September 2010
We present a radiative transfer model for the circumstellar dust shell around a Large Magellanic Cloud (LMC) long-period variable (LPV) previously studied as part of the Optical Gravitational Lensing Experiment (OGLE) survey of the LMC. OGLE LMC LPV 28579 (SAGE J051306.40–690946.3) is a carbon-rich asymptotic giant branch (AGB) star for which we have Spitzer broadband photometry and spectra from the SAGE and SAGE-Spec programs along with broadband UBVIJHKs photometry. By modeling this source, we obtain a baseline set of dust properties to be used in the construction of a grid of models for carbon stars. We reproduce the spectral energy distribution of the source using a mixture of amorphous carbon and silicon carbide with 15% SiC by mass. The grain sizes are distributed according to the KMH model, with γ = 3.5, amin = 0.01 μm and a0 = 1.0 μm. The best-fit model produces an optical depth of 0.28 for the dust shell at the peak of the SiC feature (11.3 μm), with an inner radius of about 1430 R⊙ or 4.4 times the stellar radius. The temperature at this inner radius is 1310 K. Assuming an expansion velocity of 10 km s-1, we obtain a dust mass-loss rate of 2.5 × 10-9 M⊙ yr-1. We calculate a 15% variation in this mass-loss rate by testing the sensitivity of the fit to variation in the input parameters. We also present a simple model for the molecular gas in the extended atmosphere that could give rise to the 13.7 μm feature seen in the spectrum. We find that a combination of CO and C2H2 gas at an excitation temperature of about 1000 K and column densities of 3 × 1021 cm-2 and 1019 cm-2 respectively are able to reproduce the observations. Given that the excitation temperature is close to the temperature of the dust at the inner radius, most of the molecular contribution probably arises from this region. The luminosity corresponding to the first epoch of SAGE observations is 6580 L⊙. For an effective temperature of about 3000 K, this implies a stellar mass of 1.5−2 M⊙ and an age of 1−2.5 Gyr for OGLE LMC LPV 28579. We calculate a gas mass-loss rate of 5.0 × 10-7 M⊙ yr-1 assuming a gas:dust ratio of 200. This number is comparable to the gas mass-loss rates estimated from the period, color and 8 μm flux of the source.
Key words: stars: AGB and post-AGB / stars: carbon / circumstellar matter / stars: mass-loss / Magellanic Clouds / stars: individual: OGLE LMC LPV 28579
© ESO, 2010
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