Study of Circumstellar Envelopes Around Bright Carbon Stars. 2. MolecularAbundances

摘要

We have estimated and compared circumstellar and photospheric HCN, CN, and CSabundances for a sample of bright carbon stars. The circumstellar HCN and CS abundances roughly correlate with the photospheric ones, but the former appears to be systematically overestimated by a factor of five to ten. Although we cannot exclude the presence of a circumstellar chemistry that efficiently produces HCN and CS in the inner parts of these relatively hot circumstellar envelopes, we attribute this difference between photospheric and circumstellar abundances to errors in the circumstellar envelope model used. In particular, for the low mass loss rate objects a systematic underestimate of the mass loss rate may be suspected. In addition to this general trend stars that are in some sense peculiar also show circumstellar abundances that deviate significantly from the expected. There is evidence for the presence of maser features in the HCN(J=1-0) emission towards some of the stars. An attempt has been made to determine the chemistry in the, probably detached, envelope around R Scl. The data suggest a carbon-rich chemistry.