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Title:Mixing and Nucleosynthesis in Low- and Intermediate-Mass AGB Stars
Author(s):Hollowell, David Earl
Department / Program:Astronomy
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Astronomy and Astrophysics
Abstract:The existence of carbon stars brighter than M$\sb {\rm bol}$ = $-4$ 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 = 0.001), low-envelope mass (0.1 M$\sb\odot$), low-core mass (0.6 M$\sb\odot$) AGB star can transform into a carbon star with a brightness $-5.0$ $<$ M$\sb{\rm bol}$ $<$ $-4.2$. In models that use a mixing length to pressure scale height ratio of 1.5 (with no overshoot), the transport of $\sp $C outward stops (a few) $\cdot$ 10$\sp{-5}$ M$\sb\odot$ 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 $-4.9$ $<$ M$\sb{\rm bol}$ $<$ $-4.1$.
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 $\sp1$H and $\sp $C in such a way that a layer of $\sp $C and $\sp $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 $\sp $C is rapidly converted into $\sp $O and neutrons. The rate of neutron production is mediated by the rate at which the $\sp $C layer is engulfed by the convective shell, and the $\sp $C neutron source typically provides a neutron density of (a few) $\cdot$ 10$\sp9$ n/cm$\sp3$ at the convective shell base. This neutron source also produces approximately 20-50 neutrons per iron-seed nucleus at the convective shell base, but only 10%-20% of these neutrons are captured by the iron-seed nuclei as over one half of these neutrons are absorbed by $\sp{22}$Ne.
The neutron exposure $\Delta\tau$ that the $\sp $C source provides each thermal pulse is 0.14 mb$\sp{-1}$ $<$ $\Delta\tau$ $<$ 0.20 mb$\sp{-1}$. Given the thermal pulse overlap, the average neutron exposure of this matter is 3.6 mb $<$ $\Lambda$ $<$ 4.5 mb. This is remarkably similar to the neutron exposure observed in some AGB stars and in solar-system material.
Issue Date:1988
Description:183 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
Other Identifier(s):(UMI)AAI8823147
Date Available in IDEALS:2014-12-16
Date Deposited:1988

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