Hydrogen and helium burning in zero-metal asymptotic giant branch stars and the existence of thresholds (in core mass and CNO abundance) for the occurrence of helium shell flashes

Journal Article · Sat Dec 15 00:00:00 EST 1984 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/162733· OSTI ID:5887483

Fujimoto, M Y; Jr, Iben I; Chieffi, A; Tornambe, A

Using a semianalytical method, the properties of hydrogen and helium burning in shells above the electron-degenerate cores of asymptotic giant branch stars are studied as a function of core mass M/sub 1/ and of the abundance Z/sub e/ of CNO elements preexisting in the envelope. In zero-metal model stars (defined as stars in which Z/sub e/ is zero), the temperature T/sub 1/ in the hydrogen-burning shell is large enough that 3..cap alpha.. reactions produce locally sufficient /sup 12/C for hydrogen burning by the CN cycle to dominate in energy production, provided M/sub 1/> or approx. =0.5 M/sub sun/. Because of the increase in T/sub 1/ with increasing core mass, the stability of helium shell burning changes its nature at a core mass M/sub 1/(roughly-equal0.73 M/sub sun/. For M/sub 1/M/sub 1/(, helium burning is stable and settles into a steady state. For M/sub 1/>M/sub 1/(, there exists a critical envelope CNO abundance Z/sub e/( such that, if Z/sub e/>Z/sub e/(, helium shell flashes will be triggered. For M/sub 1/ in the range (0.8-1.4)M/sub sun/, Z/sub e/( increases exponentially with M/sub 1/ from 10/sup -7/ to 10/sup -4/. The evolution of zero-metal stars and their contribution to Galactic nucleosynthesis are discussed. For stars with a small total mass (< or approx. =4 M/sub sun/), the initial core mass at the onset of double shell burning is smaller than M/sub 1/(, and helium shell flashes occur.

• Research Organization: Univ. of Illinois at Urbana-Champaign, IL (United States)
• OSTI ID: 5887483
• Journal Information: Astrophys. J.; (United States), Vol. 287:2
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
GIANT STARS
HELIUM BURNING
HYDROGEN BURNING
ABUNDANCE
CARBON
GALACTIC EVOLUTION
MASS
MILKY WAY
NITROGEN
NUCLEOSYNTHESIS
OXYGEN
STABILITY
STAR EVOLUTION
THERMODYNAMIC PROPERTIES
ELEMENTS
GALAXIES
NONMETALS
PHYSICAL PROPERTIES
STAR BURNING
STARS
SYNTHESIS
640102* - Astrophysics &amp; Cosmology- Stars &amp; Quasi-Stellar
Radio &amp; X-Ray Sources