Mixing and Nucleosynthesis in Low- and Intermediate-Mass AGB Stars
Hollowell, David Earl
This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.
Permalink
https://hdl.handle.net/2142/70669
Description
Title
Mixing and Nucleosynthesis in Low- and Intermediate-Mass AGB Stars
Author(s)
Hollowell, David Earl
Issue Date
1988
Department of Study
Astronomy
Discipline
Astronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(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.
Use this login method if you
don't
have an
@illinois.edu
email address.
(Oops, I do have one)
IDEALS migrated to a new platform on June 23, 2022. If you created
your account prior to this date, you will have to reset your password
using the forgot-password link below.