Smithsonian/NASA ADS Astronomy Abstract Service


Title:
An Extremely Large Excess of 18O in the Hydrogen-deficient Carbon Star HD 137613
Authors:
Clayton, Geoffrey C.; Herwig, Falk; Geballe, T. R.; Asplund, Martin; Tenenbaum, Emily D.; Engelbracht, C. W.; Gordon, Karl D.
Affiliation:
AA(Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803; .; Maria Mitchell Observatory, 3 Vestal Street, Nantucket, MA 02554.), AB(Los Alamos National Laboratory, Los Alamos, NM 87544; .), AC(Gemini Observatory, 670 North A'ohoku Place, Hilo, HI 96720; .), AD(Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia; .), AE(Maria Mitchell Observatory, 3 Vestal Street, Nantucket, MA 02554.; Department of Chemistry, Pomona College, 645 North College Avenue, Seaver North, Claremont, CA 91711-6338 .), AF(Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065 , .), AG(Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065 , .)
Journal:
The Astrophysical Journal, Volume 623, Issue 2, pp. L141-L144. (ApJ Homepage)
Publication Date:
04/2005
Origin:
UCP
ApJ Keywords:
Stars: AGB and Post-AGB, Stars: Carbon, Stars: Evolution, Stars: Individual: Henry Draper Number: HD 137613
Abstract Copyright:
(c) 2005: The American Astronomical Society
DOI:
10.1086/430110
Bibliographic Code:
2005ApJ...623L.141C

Abstract

We report the discovery of a uniquely large excess of 18O in the hydrogen-deficient carbon (HdC) star HD 137613 based on a spectrum of the first-overtone bands of CO at 2.3-2.4 mum in which three strong absorption bands of 12C18O are clearly present. Bands of 12C16O also are present, but no bands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O <~ 1. The solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He shell of early-asymptotic giant branch (AGB) stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He shell could be lost, but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed show significant 18O.


Title:
Evolution and Yields of Extremely Metal-poor Intermediate-Mass Stars
Authors:
Herwig, Falk
Affiliation:
AA(Los Alamos National Laboratory, Los Alamos, NM 87544; and Department of Physics and Astronomy, University of Victoria, Canada; )
Journal:
The Astrophysical Journal Supplement Series, Volume 155, Issue 2, pp. 651-666. (ApJS Homepage)
Publication Date:
12/2004
Origin:
UCP
ApJ Keywords:
Nuclear Reactions, Nucleosynthesis, Abundances, Stars: AGB and Post-AGB, Stars: Evolution, Stars: Interiors
Abstract Copyright:
(c) 2004: The American Astronomical Society
DOI:
10.1086/425419
Bibliographic Code:
2004ApJS..155..651H

Abstract

Intermediate-mass stellar evolution tracks from the main sequence to the tip of the AGB for five initial masses (2-6 Msolar) and metallicity Z=0.0001 have been computed. The detailed one-dimensional structure and evolution models include exponential overshooting, mass loss, and a detailed nucleosynthesis network with updated nuclear reaction rates. The network includes a two-particle heavy neutron sink for approximating neutron density in the He-shell flash. It is shown how the neutron-capture nucleosynthesis is important in models of very low metallicity for the formation of light neutron-heavy species, like sodium or the heavy neon and magnesium isotopes. The models have high resolution, as required for modeling the third dredge-up. All sequences have been followed from the pre-main sequence to the end of the AGB when all envelope mass is lost. Detailed structural and chemical model properties as well as yields are presented. This set of stellar models is based on standard assumptions and updated input physics. It can be confronted with observations of extremely metal-poor stars and may be used to assess the role of AGB stars in the origin of abundance anomalies of some globular cluster members of correspondingly low metallicity.


Title:
Enhanced Extra Mixing in Low-Mass Red Giants: Lithium Production and Thermal Stability
Authors:
Denissenkov, Pavel A.; Herwig, Falk
Affiliation:
AA(Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada; .; On leave from Sobolev Astronomical Institute, St. Petersburg University, Universitetsky Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia.), AB(Los Alamos National Laboratory, T-6 Theoretical Astrophysics Group, MS B227, Los Alamos, NM 87545; .)
Journal:
The Astrophysical Journal, Volume 612, Issue 2, pp. 1081-1091. (ApJ Homepage)
Publication Date:
09/2004
Origin:
UCP
ApJ Keywords:
Stars: Chemically Peculiar, Stars: Evolution, Stars: Interiors, Stars: Late-Type, Stars: Rotation
Abstract Copyright:
(c) 2004: The American Astronomical Society
DOI:
10.1086/422575
Bibliographic Code:
2004ApJ...612.1081D

Abstract

We show that canonical extra mixing with a diffusion coefficient Dmix~109 cm2 s-1, which is thought to start working in the majority of low-mass stars when they reach the bump luminosities on the red giant branch (RGB), cannot lead to an Li flash or a thermal instability, as has been proposed. The abundance levels of 7Li measured in the most extreme Li-rich giants can be reproduced with models including enhanced extra mixing with a diffusion coefficient Dmix~1011 cm2 s-1. We propose that if extra mixing in RGB stars is driven by rotation, then enhanced extra mixing and Li enrichment in some of these stars can be triggered by their spinning up by an external source of angular momentum. As plausible mechanisms of the spinning up, we consider tidal synchronization of a red giant's spin and orbital rotation in a close binary system and engulfment of a massive planet. The most convincing theoretical argument in favor of our hypothesis is a finding that a 10-fold increase of the spin angular velocity of a solar metallicity upper RGB star results in appropriate changes of both extra-mixing depth and rate, exactly as required for efficient Li production. We regard the existence of binary and single RGB stars with rotational velocities approaching ~10% of their equatorial Keplerian velocities, as well as the much larger proportion of Li-rich giants (~50%) among rapidly rotating objects, as the observational support for our hypothesis.


Title:
The Abundance Evolution of Oxygen, Sodium, and Magnesium in Extremely Metal Poor Intermediate-Mass Stars: Implications for the Self-Pollution Scenario in Globular Clusters
Authors:
Denissenkov, Pavel A.; Herwig, Falk
Affiliation:
AA(Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada; , ), AB(Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada; , )
Journal:
The Astrophysical Journal, Volume 590, Issue 2, pp. L99-L102. (ApJ Homepage)
Publication Date:
06/2003
Origin:
UCP
ApJ Keywords:
Galaxy: Globular Clusters: General, Stars: Abundances, Stars: AGB and Post-AGB
Abstract Copyright:
(c) 2003: The American Astronomical Society
DOI:
10.1086/376748
Bibliographic Code:
2003ApJ...590L..99D

Abstract

We present full stellar evolution and parametric models of the surface abundance evolution of 16O, 22Ne, 23Na, and the magnesium isotopes in an extremely metal poor intermediate-mass star (MZAMS=5 Msolar, where ZAMS stands for the zero-age main sequence, and Z=0.0001). 16O and 22Ne are injected into the envelope by the third dredge-up following thermal pulses on the asymptotic giant branch. These species and the initially present 24Mg are depleted by hot bottom burning (HBB) during the interpulse phase. As a result, 23Na, 25Mg, and 26Mg are enhanced. If the HBB temperatures are sufficiently high for this process to deplete oxygen efficiently, 23Na is first produced and then depleted during the interpulse phase. Although the simultaneous depletion of 16O and enhancement of 23Na is possible, the required fine-tuning of the dredge-up and HBB casts some doubt on the robustness of this process as the origin of the O-Na anticorrelation observed in globular cluster stars. However, a very robust prediction of our models are low 24Mg/25Mg and 24Mg/26Mg ratios whenever significant 16O depletion can be achieved. This seems to be in stark contrast to recent observations of the magnesium isotopic ratios in the globular cluster NGC 6752.


Title:
Convective proton and 3He ingestion into helium burning: Nucleosynthesis during a post-AGB thermal pulse
Authors:
Herwig, F.; Langer, N.
Journal:
Nuclear Physics A, Volume 688, Issue 1-2, p. 221-224.
Publication Date:
05/2001
Origin:
ELSEVIER
Abstract Copyright:
(c) 2001 Elsevier Science B.V.
Bibliographic Code:
2001NuPhA.688..221H

Abstract

Electronic Article Available from Elsevier Science.


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