Mixing and nucleosynthesis in low- and intermediate-mass AGB stars
The existence of carbon stars brighter than M{sub bol} = {minus}4 can be understood in terms of dredge up in thermally pulsing asymptotic giant branch (AGB) stars. Models in this study include the effect that carbon recombination has upon opacity, and it is shown that a low-metallicity (Z = 0.002), low-envelope mass (0.1 M{sub {circle dot}}), low-core mass (0.6 M{sub {circle dot}}) AGB star can transform into a carbon star with a brightness {minus}5.0 < M{sub bol} < {minus}4.2. In models that use a mixing length to pressure scale height ratio of 1.5 (with no overshoot), the transport of {sup 12}C outward stops (a few) {center dot} 10{sup {minus}5} M{sub {circle dot}} below the convective envelope. If a limited amount of convective overshoot is included in these models, dredge up of interior matter will occur when the post-pulse luminosity is {minus}4.9 < M{sub bol} < {minus}4.1. Neutron capture occurs on iron-seed nuclei during a shell flash, and the products of this nucleosynthesis may be carried to the stellar surface during the dredge-up phase. The carbon recombination that induces dredge up in these models also induces mixing of {sup 1}H and {sup 12}C in such a way that a layer of {sup 13}C and {sup 14}N is formed in the region between the hydrogen-burning and helium-burning shells. During a subsequent thermal pulse this matter is engulfed into a hot, convective shell and the {sup 13}C is rapidly converted into {sup 16}O and neutrons. The rate of neutron production is mediated by the rate at which the {sup 13}C layer is engulfed by the convective shell, and the {sup 13}C neutron source typically provides a neutron density of (a few) {center dot} 10{sup 9} n/cm{sup 3} at the convective shell base.
- Research Organization:
- Illinois Univ., Urbana, IL (USA)
- OSTI ID:
- 5097078
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GENERAL PHYSICS
CARBON STARS
NUCLEOSYNTHESIS
CALCULATION METHODS
CARBON
HYDROGEN
MATHEMATICAL MODELS
MIXING
OXYGEN
ELEMENTS
MAIN SEQUENCE STARS
NONMETALS
STARS
SYNTHESIS
640100* - Astrophysics & Cosmology
640102 - Astrophysics & Cosmology- Stars & Quasi-Stellar
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