Circumstellar shells resolved in iras survey data. II. Analysis

The IRAS survey data for 512 red giant stars and young planetary nebulae, most of which have been detected in rotational transitions of CO, were examined. The data for those objects which were not located in highly confused regions of the sky were processed using a computer program which fitted the data to an idealized model of a circumstellar shell. Of these stars 76 were found to have a circumstellar shell (CSS) that is resolved in the 60 μm survey data. Some of these stars are also resolved in the 100 μm survey data. Of the 76 stars, 40% are carbon stars. Three of the carbon stars, W Pic, RY Dra, and R CrB, have shells with radii of 3 pc or more. Only two of the resolved objects detectable in CO, NGC 2346 and IRC + 10216, are surrounded by shells with masses ≳1 M⊙. Most of the nearby stars which are resolved are semiregular variables, while most of the nearby unresolved stars are Miras. No evidence was found that the shapes of the shells were distorted by interaction with the ISM. The average dust temperature in the outer region of the shells is ∼35 K. Using this temperature and the mass-loss rates derived from observations of the inner shell, it is shown that the expansion velocity of the dust cannot greatly exceed that of the gaseous material. Some of the shells appear to be detached from the central star. The distribution of the inner radii of the detached shells indicates that CO is dissociated at a distance of ∼0.3 pc from the star. Analysis of the colors of the shells resolved in both the 60 and 100 μm bands shows that the central star is the dominant dust heating source at radii less than ∼0.5 pc, whereas an external source dominates beyond ∼1 pc. Thirteen of the stars found to be extended are Mira variables. A simple model of the evolution of these shells, involving the interaction of the expelled material with the ISM, suggests that the period during which Mira variables lose mass lasts for ∼10⁵ yr. The same model indicates that carbon stars shed mass for ∼2 × 10⁵ yr. For the largest shells the expansion velocity of the outer shell will typically be lower than the expansion velocity obtained from CO observations, by a factor of 3-5.

Duke Scholars

Author Ken H Young Pathology