On the formation and evolution of super-asymptotic giant branch stars with cores processed by carbon burning. 1: SPICA to Antares

Details On the formation and evolution of super-asymptotic giant branch stars with cores processed by carbon burning. 1: SPICA to Antares On the formation and evolution of super-asymptotic giant branch stars with cores processed by carbon burning. 1: SPICA to Antares

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...434..306g&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 434, no. 1, p. 306-318

Other

55

Asymptotic Giant Branch Stars, Carbon, Helium, Nuclear Fusion, Star Formation, Stellar Composition, Stellar Cores, Stellar Evolution, Stellar Models, Abundance, Main Sequence Stars, Red Giant Stars, Stellar Luminosity

Scientific paper

A 10 solar mass model of Population I composition is evolved from the hydrogen-burning main sequence onto the early 'super'-asymptotic giant branch (ESAGB), where hydrogen does not burn, the dominant source of surface luminosity is shell helium burning, and carbon burning in the helium-exhausted core converts all of the initial C-12 and Ne-22 and some of the O-16 in this core into isotopes of oxygen, neon, sodium, magnesium, and other heavy elements. The entire ESAGB phase lasts approximately 4.6 x 104 yr, and helium burning is the primary source of surface luminosity for all but approximately 3000 yr. The carbon-burning phase lasts approximately 2 x 104 yr. In comparison, the main-sequence phase lasts approximately 2 x 107 yr, and the core helium-burning phase lasts approximately 4 x 106 yr. At the end of the core carbon-burning phase, dredge-up decreases the mass of the hydrogen-exhausted core from approximately 2.46 solar mass to approximately 1.315 solar mass. During the dredge-up episode, in addition to products of complete hydrogen burning, products of incomplete helium burning are mixed into the hydrogen-rich convective enevelope. At the end of the second dredge-up phase, the model has an electron-degenerate carbon-exhausted core of mass approximately 1.285 solar mass. Between the carbon-exhausted core and the hydrogen-rich convective envelope are three layers which are each of mass approximately 0.01 solar mass and which contain (1) C-12 and O-16 at comparable abundances by mass; (2) C-12 and He-4 at comparable abundances by mass; and (3) mostly He-4 with some C-12. The abundances by mass of the main constituents of the carbon-exhausted core vary signifcantly through the core. At the center, abundances of mass are X(O-16) approximately 0.80, X(Ne-20) approximately 0.11, X(Na-22) approximately 0.03, X(Mg-24) approximatelly 0.0075, and X(Mg-26) approximately 0.01, whereas, near the edge of the core, X(O-16) approximately 0.19, X(Ne-20) approximately 0.50, X(Na-23) approximately 0.04, and X(Mg-24) approximately 0.27. The abundance of Na-23 is large enough that, if the mass of the SAGB core can grow to approximately 1.37 solar mass, core collapse into a neutron star will be initiated by electron capture on Na-23 rather than on Mg-24.

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