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Molecular Abundances in the Envelopes Around Evolved Stars

Published online by Cambridge University Press:  12 April 2016

Hans Olofsson
Hans Olofsson*
Affiliation:
Onsala Space Observatory, S-43992 Onsala, Sweden

Extract

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Red giant stars on the asymptotic giant branch (AGB), AGB-stars, lose copious amounts of matter in a slow stellar wind (Olofsson 1993). Mass loss rates in excess of 10-4 M⊙ yr-1 have been measured. The primary observational consequence of this mass loss is the formation of an expanding envelope of gas and dust, a circumstellar envelope (CSE), that surrounds the star. This is a truly extended atmosphere that continues thousands of stellar radii away from the star. At the highest mass loss rates (which probably occur at the end of the AGB evolution) the CSE becomes so opaque that the photosphere is hidden and essentially all information about the object stems from the circumstellar emission. At some point on the AGB a star may change from being O-rich (i.e., the abundance of O is higher than that of C) to becoming C-rich (i.e., a carbon star where the abundance of C is higher than that of O) as a result of nuclear-processed material being dredged up to the surface. The chemical composition of the CSE will follow that of the central star, although with some time delay so that there may be some rare cases of O-rich CSEs around carbon stars. The mass loss decreases and changes its nature as the star leaves the AGB and starts its post-AGB evolution. Eventually the star becomes hot enough to ionize the inner part of the AGB-CSE and a planetary nebula (PN) is formed. The ultimate fate of the star is a long life as a slowly cooling white dwarf. The CSE will gradually disperse and its metal-enriched matter will mix with the interstellar medium, and thereby it contributes to the chemical evolution of a galaxy. The intense mass loss makes it possible for stars as massive as 8 M⊙, i.e., the bulk of all stars in a galaxy, to follow this evolutionary sequence. Similar CSEs are also found around supergiants.

Type
Research Article
Information
International Astronomical Union Colloquium , Volume 146: Molecules in the Stellar Environment , 1994 , pp. 113 - 133
Copyright
Copyright © Springer-Verlag 1994

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