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Astron. Astrophys. 345, 233-243 (1999)

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8. Concluding remarks

The major result of this work is that [FORMULA] ratios have quantitatively been determined for a large sample of J-type carbon stars. The distribution of the [FORMULA] ratios shows a rather broad peak from 1 to 6. A significant fraction of the program stars have [FORMULA] ratios smaller than the value expected at the equilibrium of the CN-cycle. Two stars, VX And and HD 52432, are found to have moderately high [FORMULA] ratios.

The cool bottom processing seems to be adequate for explaining the low [FORMULA] ratios derived here, but nitrogen abundances should be analyzed for more J-type carbon stars, in order to examine this scenario further. The analysis of the elemental abundances of carbon and oxygen as well as oxygen isotope ratios is also important in the understanding of the nuclear mechanism responsible for the formation of J-type carbon stars.

The five silicate carbon stars in our sample show no peculiar [FORMULA] ratios as compared with other J-type carbon stars. This result suggests that the mechanism which lowers [FORMULA] ratios in silicate carbon stars might not be different from that operating in other J-type carbon stars. Though the scenario in which the oxygen-rich material shed at the He-core flash is stored in the accretion disk around the companion has still some drawbacks, it might be possible that some of R-type carbon stars would evolve and be observed as silicate carbon stars, when the conditions for the survival of the accretion disk are met.

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999