Circumstellar gas, dust emission, and mass loss from evolved carbon stars

A model to calculate the circumstellar dust emission of an evolved carbon s tar is developed, and the relations between the infrared flux densities at 2.2, 12, 25, and 60 mu m and the dust properties are described. The model i s combined with a preexisting model for CO emission, keeping physical quant ities consistent between the two models. The results are used to analyze th e infrared and CO data of a sample of 17 evolved carbon stars. The dust-she ll opacity determined from the 60 mu m flux density correlates well with th at determined from the [25]-[2.2], [60]-[12], and [25]-[12] colors over a r ange of almost 4 orders of magnitude within the sample. The scaled mass-los s rate hi is revealed through the CO line profile observational parameter z (dagger) = (M/1.5 x 10(-6) M. yr(-1)){([CO]/[H-2])/6.4 x 10(-4)}[(L-*/L.)/1 0(4)](-0.5), which ranges from 0.2 to 30 and averages 4.3. The envelope dus t-to-gas mass ratio, f, is found to be less than or similar to 10(-3) on th e assumption of usual values of [CO]/[H-2] and dust absorption efficiency. Typical grain radii a are found to be very small, with a geometric mean of less than or similar to 2 nm. It does not appear that the dust properties, such as f and a, have a direct, strong influence on the mass-loss rate, but the selected sample is, admittedly, small.