On the evolution of an intermediate-mass zero-metal star which does not experience thermal instabilities during the double shell burning phase

Details On the evolution of an intermediate-mass zero-metal star which does not experience thermal instabilities during the double shell burning phase On the evolution of an intermediate-mass zero-metal star which does not experience thermal instabilities during the double shell burning phase

Dec 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984apj...287..745c&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 287, Dec. 15, 1984, p. 745-748.

Astronomy and Astrophysics

Astrophysics

26

Stellar Cores, Stellar Envelopes, Stellar Evolution, Stellar Mass, Thermal Stability, Abundance, Giant Stars, Hertzsprung-Russell Diagram, Pre-Main Sequence Stars, Stellar Luminosity, Stellar Models

Scientific paper

The evolution of a 5 Msun zero-metal star has been followed from the pre-main sequence to the asymptotic giant branch (AGB) phase, special attention being paid to the behavior of the nuclear burning regions during the AGB phase. We find that (a) both hydrogen and helium central burning phases occur at the blue side of the H-R diagram; (b) during the AGB phase, double shell burning proceeds at a steady state without being interrupted by thermal instabilities of the kind found in Population I and II stellar models. As a consequence of point (a), the "first" dredge-up phenomenon does not occur, whereas, following helium exhaustion at the center, the dredge-up phenomenon usually referred to as the "second" dredge-up does occur and brings to the surface an unusually large amount of helium. The third dredge-up phase does not occur, due to the failure of thermal pulses to take place. We suggest that the physical reason for the nonoccurrence of thermal pulses is that, due to the lack of CNO nuclei, the 3α reactions must be active, even if partially, in the hydrogen-burning shell in order to produce just the slight number of carbon nuclei necessary to sustain hydrogen burning via the CNO cycle. At the same time, if the 3α reactions are active in the hydrogen-burning shell, a fortiori they will be active in the inner (and hotter) helium shell, and the centers of the two shells must move outward in mass at the same rate. The mass of the helium-burning region does not increase with time, a factor which is essential for the onset of a thermal instability; such an instability can therefore not occur. We find that our models spontaneously follow Iben's 1977 prescriptions for steady-state burning, the ratio between the helium and the hydrogen luminosities being of the order of 0.20.

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