Astronomy and Astrophysics – Astronomy
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........6s&link_type=abstract
PhD Thesis, Stockholm Observatory, ISBN 91-7265-111-3
Astronomy and Astrophysics
Astronomy
14
Stars: Abundances, Stars: Agb And Post-Agb, Stars: Carbon, Stars: Circumstellar Matter, Stars: Late-Type
Scientific paper
Red giant stars are key objects in many astronomical fields of research. However, they are complicated objects and many physical and chemical processes combine to determine their properties. This makes it difficult to interpret the observations made towards these important stars, and to use them in a quantitative way when probing stellar evolution or galactic structure. A key process for their final evolution is mass loss from the surface, and this also has implications for the nucleosynthesis and the chemical evolution of galaxies. The mass loss leads to the formation of extended circumstellar envelopes of gas and dust particles, and their emission may be used to determine the mass loss characteristics. We have performed extensive modelling of molecular line emission from the circumstellar envelopes using a detailed radiative transfer analysis. In particular, we present results on carbon monoxide radio line emission from a large sample of optically bright carbon stars. Assuming spherically symmetric envelopes created by a continuous, smooth expanding, mass loss, we are able to reproduce the observational results for about 90% of the sample stars. The derived mass loss rates span almost four orders of magnitude from roughly 5×10-9 up to 2×10-5 solar masses per year. The physical conditions prevailing in the circumstellar envelopes vary considerably over such a large range of mass loss rates, posing a challenge to the model. A comparison between the stellar and circumstellar properties is also provided. The majority of the remaining stars have known detached circumstellar envelopes, suggesting episodic mass loss. Carbon stars are probably important in returning processed gas to the interstellar medium. We estimate that carbon stars of the type considered here annually return on the order of 0.05 solar masses of gas to the Galaxy, but more extreme carbon stars may contribute an order of magnitude more. However, as for the total carbon budget of the Galaxy, carbon stars appear to be of only minor importance. We present estimates of the 12C/13C-ratio towards a number of carbon stars. This carbon isotope ratio is an important probe of stellar evolution and, if known accurately, should increase our understanding of the processes that lead to the formation of carbon stars. With the Infrared Satellite Observatory a new window for observing late-type stars was opened up, and this allowed studies of gas much closer to the star than was previously possible using, mainly, ground-based radio telescopes. By combining infrared and radio observations of carbon monoxide rotational lines we have been able to probe the circumstellar envelope characteristics over a large radial range. This provides a form of archaeology, giving a time resolved mass loss history of the object under study. In a similar way we have analysed stellar light scattered in circumstellar rotational-vibrational lines of carbon monoxide. Finally, we have also performed interferometric observations of the circumstellar line emission from hydrogen cyanide and cyanide, molecules with more complex spectra. Using a detailed treatment of the more complicated radiative transfer in these cases we are able to strengthen the observational constraints put on models describing the circumstellar chemistry.
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