Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apjs...87..305o&link_type=abstract
Astrophysical Journal Supplement v.87, p.305
Astronomy and Astrophysics
Astronomy
70
Circumstellar Matter, Molecular Processes, Stars: Abundances, Stars: Carbon
Scientific paper
We have estimated and compared circumstellar and photospheric HCN, CN, and CS abundances for a sample of bright carbon stars. The circumstellar 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 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 toward some of the stars. The estimated circumstellar CN abundances suggest that this species is a photodissociation product of HCN. A search for circumstellar HNC, SiS, and 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 attempt has been made to determine the chemistry in the (probably detached) envelope around R Scl. The data suggest a carbon-rich chemistry. Finally, it is shown that the circumstellar 12CO/13CO and H12CN/H13CN intensity ratios roughly correlate with the photospheric 12C/13C isotope ratios for a small number of low mass-loss rate objects, although the former ratios are systematically lower than the latter by a factor of about 2.
Carlstroem U.
Eriksson Kimmo
Gustafsson Bengt
Olofsson Hans
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