Critical Considerations on the Theories of Formation of Carbon Cosmic Grains.

Abstract

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Critical Considerations on the Theories of Formation of Carbon Cosmic Grains. Cernuschi, Felix ; Codina, Sayd Abstract Hoyle and Wickramasinghe (Monthly Notices Roy. Astron. Soc. 124, 417,1962) have proposed a cyclic mechanism for the formation of graphite grains, in the photosphere of class N stars. A more refined calculation on the conditions prevailing in the outer layers of carbon giant stars, by Tsuji (Ann. Tokyo Obs. 2nd Ser. 9, No. 1, 1964), gives a somewhat lower range of temperatures for graphite condensation than that estimated by Hoyle and Wickramasinghe. However, the theory of grain formation in the photospheric envelope of carbon stars in generally accepted as a very plausible one. We have analyzed critically the following aspects of Hoyle-Wickramasinghe theory: (1) The carbon partial pressure in the photo- sphere is overestimated, because their calculation is implicitly based [Hoyle and Wickramasinghe, Eq. (18)J on the assumption of total veiling of radiation by photospheric grains at the lowest temperature of the cycle, which lacks observational basis. The average variation of the apparent magnitude of class N stars catalogued by Kukarkin and Parenago (General Catalogue of Variable Stars, Academy of Sciences of the USSR, 1958) does not support in any way the above-mentioned implicit hypothesis. This variation is explained by changes of the surface temperature during the cycle assumed by Hoyle and Wickramasinghe. (2) A direct estimation of the evaporation rate of the grains, during the first stage of their outward journey, shows that the grains eventually formed in the photosphere are totally evaporated before they can reach an altitude of about 0.2 the star radius. (3) Hoyle and Wickramasinghe have calculated the total of carbon to the interstellar space supposing that 10~ class N stars in our Galaxy have been working cyclically, in the same way already mentioned, during 100 yr. In that way, they obtained a suitable fit to the average number of cosmic grains per cm3. But (even assuming that mixing processes over the entire volume of the star are extremely efficient, which is a very doubtful hypothesis) the admitted rate of carbon supply is so high that every star would have expelled all its carbon atoms in 106yr. The assumption that graphite is an important constituent of cosmic grains is very plausible, but a theory would be necessary which avoided the contradictions pointed out above. This might be provided by adapting for the formation of carbon grains the theory developed by Cernuschi et al. (Ann. Astrophys. 28, 860,1965). Publication: The Astronomical Journal Pub Date: September 1967 DOI: 10.1086/110569 Bibcode: 1967AJ.....72T.788C full text sources ADS | data products SIMBAD (3)