MASS-LOSS FROM OH/IR STARS - MODELS FOR THE INFRARED-EMISSION OF CIRCUMSTELLAR DUST SHELLS

We have modeled the infrared emission of a sample of 24 OH/IR stars deriving the properties of circumstellar dust and mass-loss rate of the central star. Our results show that for some sources the observations of the far-infrared emission is well fitted with a lambda--1 law, while some have a steeper index of 1.5. At 60-mu-m, this corresponds to kappa-60 of 240 and 160 cm2 g-1 for the former and latter, respectively. For a few sources, detailed studies of the observed 10-mu-m feature suggest the presence of circumstellar ice grains. Dust mass-loss rates have been determined from detailed studies for all the stars in this sample. They range from 6.0 x 10(-10) M. yr-1 for an optically visible Mira to 2.2 x 10(-6) M. yr-1 for a heavily obscured OH/IR star. These dust mass-loss rates are then compared to those calculated from IRAS photometry using 12, 25, and 60-mu-m fluxes. The dust mass-loss rates are also compared to gas mass-loss rates determined from OH and CO observations. For stars with tenuous shells, a dust-to-gas ratio of 0.001 is obtained. For heavily obscured stars, however, both OH and CO seem to underestimate the mass-loss rate considerably. This may be due to a recent (< 10(2) yr) enhancement in mass-loss rates.