The mass-loss return from evolved stars to the Large Magellanic Cloud

Abstract

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.

Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

“…For the O-rich stars HV 5715 and sagemcj052206, Sargent et al (2010) fitted the SED and IRS spectra, and their results agree well with the GRAMS-based results of Sargent et al (2011) and Riebel et al (2012) (see Table 3). The fitting method of Jones et al (2012), however, led to a much higher estimate of dust MLRs.…”

“…They also accounted for inclination angle of the LMC disk which leads to some differences in luminosity and MLR (Jones et al, 2012, private communication Srinivasan et al (2010) 4810, 6580 2.5 (2.4-2.9) Srinivasan et al (2011) 6170 2. 4 Riebel et al (2012) 7080 ± 700 2.12 ± 0.42 present paper 4740 3.2 HV 5715 Sargent et al (2010) 36 000 ± 4000 2.3 (1.1-4.1) Sargent et al (2011) 33 000 1.5 Riebel et al (2012) 33 700 ± 5960 1.56 ± 0.43 Jones et al (2012) 28 800 19.6 Jones et al (2014) 19 230 ± 4300 0.63 ± 0.14 present work 28 200 0.25 sagemcj052206 Sargent et al (2010) 5100 ± 500 2.0 (1.1-3.1) Sargent et al (2011) Ossenkopf et al (1992) for single-sized grains of 0.15 µm and compared those to the grains that best fit HV 5715 and sagemcj052206 in the present work. The grains in the present work are larger, and the ratio of opacities at 1 and 2 µm are 1.5-2.6 and 2-6, respectively, consistent with the differences in MLRs between the present work and most of the works based on the M-star GRAMS grid.…”

Surprising detection of an equatorial dust lane on the AGB star IRC+10216

“…These values are typical values for a carbonaceous dusty outflow (for e.g. Srinivasan et al 2010;Hony et al 2002). By fitting the SED and comparing the result with the scattered light images, we can constrain the albedo and density structure of the grains.…”

The mass-loss return from evolved stars to the Large Magellanic Cloud

“…A more precise estimate for the injection rate requires detailed radiative transfer (RT) modeling of the spectral energy distribution (SED) of each star in the candidate list. Many authors have computed such detailed models for LMC carbon stars (see, e.g., van Loon et al 1999;Groenewegen et al 2009;Srinivasan et al 2010). The computation of individual models becomes time-consuming for large samples such as the SAGE dataset.…”

“…We circumvent this assumption by adopting a more general treatment -we specify the inner radius as input to the modeling and this automatically determines the temperature of the dust in the shell when the model is computed. The output SEDs are very sensitive to the inner radius R in (see, e.g., Srinivasan et al 2010) and we incorporate this dependence in our grid by computing models for different R in values. We thus provide a large grid of models that is complementary to the currently available grids constructed by other authors.…”