A STUDY OF CIRCUMSTELLAR ENVELOPES AROUND BRIGHT CARBON STARS .2. MOLECULAR ABUNDANCES

We have estimated and compared circumstellar and photospheric HCN, CN, and CS abundances for a sample of bright carbon stars. The circumstel lar HCN and CS abundances roughly correlate with the photospheric ones , but the former appear to be systematically overestimated by a factor of 5-10. Although we cannot exclude the presence of a circumstellar c hemistry that efficiently produces HCN and CS in the inner parts of th ese relatively hot circumstellar envelopes, we attribute this differen ce between photospheric and circumstellar abundances to errors in the circumstellar envelope model used. In particular, for the low mass-los s 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 signifi cantly from the expected. There is evidence for the presence of maser features in the HCN (J = 1-0) emission toward some of the stars. The e stimated circumstellar CN abundances suggest that this species is a ph otodissociation product of HCN. A search for circumstellar HNC, SiS, a nd HC3N shows that molecular emission from species other than CO, HCN, CN, and CS is very weak in these low mass-loss rate objects. An attem pt has been made to determine the chemistry in the (probably detached) envelope around R Scl. The data suggest a carbon-rich chemistry. Fina lly, it is shown that the circumstellar (CO/CO)-C-12-C-13 and (HCN/HCN )-C-12-C-13 intensity ratios roughly correlate with the photospheric C -12/C-13 isotope ratios for a small number of low mass-loss rate objec ts, although the former ratios are systematically lower than the latte r by a factor of about 2.