The superwind mass-loss rate of the metal-poor carbon star LI-LMC 1813 in the LMC cluster KMHK 1603

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 UBVIJHK s 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, a min = 0.01 μm and a 0 = 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 C 2 H 2 gas at an excitation temperature of about 1000 K and column densities of 3 × 10 21 cm −2 and 10 19 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.

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Variability of halo carbon stars

Battinelli

Demers

2012

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Context. Carbon stars are among the brightest intermediate-age stars. Over one hundred have been identified in the Galactic halo. Since the halo consists essentially in an old stellar population, we believe that these C stars are trespassers and belong to streams left over by disrupted dwarf spheroidal galaxies. Aims. By performing photometric monitoring we intend to detect long-period variables among halo carbon stars. We should be in position to identify Mira, semi-regular, and irregular variables and determine their period and age group. Methods. We obtained, over several semesters, K, J, and I images centered on the C stars in order to determine their variation and periodicity. Results. We establish the period of 14 program stars and discover 13 Miras among them. Most of them belong to the 1-3 Gyr age group. Conclusions. The period distribution of the halo Miras closely matches that of the Miras of Fornax. The lack of old Miras suggests that the majority of the halo Miras likely do not originate from the Sagittarius dwarf spheroidal galaxy, which is believed to be older than 5 Gyr.

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The superwind mass-loss rate of the metal-poor carbon star LI-LMC 1813 in the LMC cluster KMHK 1603

Cited by 20 publications

References 46 publications

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Variability of halo carbon stars

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