Millimetre observations and chemical processes in circumstellar envelopes and planetary nebulae

Abstract

After a brief review of the state of circumstellar chemistry, the results of some recent observational programs, performed mainly with the IRAM 30 m telescope are discussed, with respect to information they give on the molecular shells of late type stars and planetary nebulae and on the chemical processes which determine the observed molecular abundances. Several programs are in progress on: (1) Systematic multiline studies in IRC + 10216 (with detailed mapping) and CRL 2688; (2) Photodissociation of C₂H₂, C₂H, HCN and CN in various classes of C-rich stars.

Systematic surveys of lines SO, SO₂ and H₂S have been carried out recently on various classes of O-rich stars. The The results confirm the formation of SO and SO₂ by external photochemistry in most stars. However, one has still to explain the very large abundance and extension of SO in stars with relatively low mass-loss rates. As foresen, H₂S comes from the interior of the envelope. Its presence proves the low abundance of atomic hydrogen. Its apparent small extension is still difficult to explain.

The detection of strong millimetre lines of CN, HCO⁺, HCN and HNC has been recently reported in two evolved planetary nebulae with strong H₂ and CO, NGC 6072 and IC 4406. Abundance estimates show that, in comparison with red giant envelopes, CN is enhanced relative to HCN by about an order of magnitude. The HCO⁺ and HNC abundances, relative to HCN, are also much larger. The presence of these species in the envelopes of evolved planetary nebulae points to an ongoing peculiar chemistry which is probably dominated by photodissociation, shocks and ion–molecule reactions.