Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983ap%26ss..95..357f&link_type=abstract
Astrophysics and Space Science (ISSN 0004-640X), vol. 95, no. 2, Sept. 1983, p. 357-368.
Astronomy and Astrophysics
Astrophysics
11
Carbon Stars, R Coronae Borealis Stars, Stellar Atmospheres, Stellar Mass Ejection, Supergiant Stars, Variable Stars, Condensing, Graphite, Pressure Dependence, Stellar Temperature
Scientific paper
The existence of a critical mass loss rate is demonstrated. At this rate there is a sharp increase in the condensation degree of carbon through an increasing mass loss rate. When the mass loss rate exceeds the critical value, carbon transforms almost completely into solid state. The critical rate ranges from 10 to the -6th solar mass per yr for an effective temperature of 5300 K to 10 to the -5th solar mass per yr for an effective temperature of 8300 K. On the basis of the results, a model is proposed for R CrB stars. Semiregular pulsations of the R CrB stars are seen to be accompanied by running waes propagating through the stellar atmosphere. The mass flux caused by running waves is for a long time too small to bring on condensation of the bulk of the carbon. The oscillation amplitude sometimes increases so that the mass flux exceeds the critical mass loss rate; an optically thick dust shell then forms around the R CrB star. It is noted that this model explains the emission spectrum observed during the minimum of the visual brightness of R CrB stars as recombination radiation from the postshock zone.
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