The Mass-Loss Return from Evolved Stars to the Large Magellanic Cloud II: Dust Properties for Oxygen-Rich Asymptotic Giant Branch Stars

摘要

We model multi-wavelength broadband UBVIJHKs and Spitzer IRAC and MIPSphotometry and IRS spectra from the SAGE and SAGE-Spec observing programs oftwo oxygen-rich asymptotic giant branch (O-rich AGB) stars in the LargeMagellanic Cloud (LMC) using radiative transfer models of dust shells aroundstars. We chose a star from each of the bright and faint O-rich AGB populationsfound by earlier studies of the SAGE sample in order to derive a baseline setof dust properties to be used in the construction of an extensive grid ofradiative transfer models of the O-rich AGB stars found in the SAGE surveys.From the bright O-rich AGB population we chose HV 5715, and from the faintO-rich AGB population we chose SSTISAGE1C J052206.92-715017.6 (SSTSAGE052206).We found the complex indices of refraction of oxygen-deficient silicates fromOssenkopf et al (1992) and a "KMH"-like grain size distribution with gamma of-3.5, a_min of 0.01 microns, and a_0 of 0.1 microns to be reasonable dustproperties for these models. There is a slight indication that the dust aroundthe faint O-rich AGB may be more silica-rich than that around the bright O-richAGB. Simple models of gas emission suggest a relatively extended gas envelopefor the faint O-rich AGB star modeled, consistent with the relatively largedust shell inner radius for the same model. The models compute the dustmass-loss rates for the two stars to be 2.0x10^-9 M_Sun/yr and 2.3x10^-9M_Sun/yr, respectively. When a dust-to-gas mass ratio of 0.002 is assumed forSSTSAGE052206 and HV 5715, the dust mass-loss rates imply total mass-loss ratesof 1.0x10^-6 M_Sun/yr and 1.2x10^-6 M_Sun/yr, respectively. These properties ofthe dust shells and stars, as inferred from our models of the two stars, arefound to be consistent with properties observed or assumed by detailed studiesof other O-rich AGB stars in the LMC and elsewhere.