Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989a%26a...209..135w&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 209, no. 1-2, Jan. 1989, p. 135-140.
Astronomy and Astrophysics
Astrophysics
11
Red Giant Stars, Stellar Evolution, Stellar Models, Nuclear Fusion, Stellar Luminosity, Stellar Mass, Thermodynamic Equilibrium
Scientific paper
A criterion for the onset of the evolution of stars to red giants, which has been introduced by Renzini (1984), is investigated. This criterion states that a thermal runaway occurs in the envelope of stars above about 1.5 solar masses when a quantity, W, that links changes in the radius of a mass shell with the change of the luminosity transmitted by this shell, is negative. This instability causes a star to expand until convection sets in. W can be calculated from the radiative transport equation. The investigation of standard stellar evolution models for M = 1, 5, 10 and 15 solar masses shows that in most cases W is not linked to expansion or contraction in the way described above. By performing a numerical experiment - evolving a star with constant opacity - it is also shown that the opacity is not, as emphasized by Renzini (1984), the main cause for W to become negative. The question of a possibly restricted validity of the criterion is discussed and a more general interpretation of W is presented.
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