(Pre)-white dwarf stars as measuring tools for yields of AGB nucleosynthesis

2019 ◽  
Vol 15 (S357) ◽  
pp. 158-161
Author(s):  
Lisa Löbling
Keyword(s):  
White Dwarf ◽  
Neutron Capture ◽  
Convection Zone ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Heavy Elements ◽  
Agb Stars ◽  
Subsequent Evolution ◽  
Dwarf Stars ◽  
White Dwarf Stars

AbstractIn the helium-rich intershell region of asymptotic giant branch (AGB) stars, slow neutron-capture nucleosynthesis produces heavy elements beyond iron. If the stars experience a final-flash of the He-burning shell, a pulse-driven convection zone establishes, the stars become hydrogen-deficient and exhibit former intershell material at their surfaces. In their subsequent evolution towards the white-dwarf cooling sequence, but still at constant luminosity, a strong stellar wind prevents diffusion to wipe out the information about AGB yields. We present and interpret the analysis results of hydrogen-rich and -deficient post-AGB stars, discuss difficulties in their analysis and review the implications on the understanding of post-AGB evolution.

The role of AGB stars in Galactic and cosmic chemical enrichment

2018 ◽  
Vol 14 (S343) ◽  
pp. 247-257
Author(s):  
Chiaki Kobayashi ◽  
Christopher J. Haynes ◽  
Fiorenzo Vincenzo
Keyword(s):  
Electron Capture ◽  
Galaxy Formation ◽  
Neutron Capture ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Capture Process ◽  
Chemical Enrichment ◽  
Cno Abundances

AbstractThe role of asymptotic giant branch (AGB) stars in chemical enrichment is significant for producing 12,13C, 14N, F, 25,26Mg, 17O and slow neutron-capture process (s-process) elements. The contribution from super-AGB stars is negligible in classical, one-zone chemical evolution models, but the mass ranges can be constrained through the contribution from electron-capture supernovae and possibly hybrid C+O+Ne white dwarfs, if they explode as Type Iax supernovae. In addition to the recent s-process yields of AGB stars, we include various sites for rapid neutron-capture processes (r-processes) in our chemodynamical simulations of a Milky Way type galaxy. We find that neither electron-capture supernovae or neutrino-driven winds are able to adequately produce heavy neutron-capture elements such as Eu in quantities to match observations. Both neutron-star mergers (NSMs) and magneto-rotational supernovae (MRSNe) are able to produce these elements in sufficient quantities. Using the distribution in [Eu/(Fe, α)] – [Fe/H], we predict that NSMs alone are unable to explain the observed Eu abundances, but may be able to together with MRSNe. In order to discuss the role of long-lifetime sources such as NSMs and AGB stars at the early stages of galaxy formation, it is necessary to use a model that can treat inhomogeneous chemical enrichment, such as in our chemodynamical simulations. In our cosmological, chemodynamical simulations, we succeed in reproducing the observed N/O-O/H relations both for global properties of galaxies and for local inter-stellar medium within galaxies, without rotation of stars. We also predict the evolution of CNO abundances of disk galaxies, from which it will be possible to constrain the star formation histories.

scholarly journals Heavy elements in planetary nebulae: A theorist's gold mine

2011 ◽  
Vol 7 (S283) ◽  
pp. 127-130
Author(s):  
Amanda I. Karakas ◽  
Maria Lugaro
Keyword(s):  
Neutron Capture ◽  
Planetary Nebulae ◽  
Asymptotic Giant Branch ◽  
Heavy Elements ◽  
Gold Mine ◽  
Agb Stars ◽  
Capture Process ◽  
Wide Range ◽  
Model Predictions

AbstractObservations of planetary nebulae have revealed a wealth of information about the composition of heavy elements synthesized by the slow neutron capture process (the s process). In some of these nebulae the abundances of neutron-capture elements are enriched by factors of 10 to 30 times the solar value, indicating that these elements were produced in the progenitor star while it was on the asymptotic giant branch (AGB). In this proceedings we summarize results of our recent full s-process network predictions covering a wide range of progenitor masses and metallicities. We compare our model predictions to observations and show how this can provide important insights into nucleosynthesis processes occurring deep within AGB stars.

scholarly journals Pulsations of White Dwarf Stars with Thick Hydrogen or Helium Surface Layers

1988 ◽  
Vol 123 ◽  
pp. 333-337
Author(s):  
Arthur N. Cox ◽  
Sumner G. Starrfield ◽  
Russell B. Kidman ◽  
W. Dean Pesnell
Keyword(s):  
Stellar Evolution ◽  
White Dwarf ◽  
Convection Zone ◽  
Helium Surface ◽  
Surface Layers ◽  
Dwarf Stars ◽  
Surface Mass ◽  
White Dwarf Stars ◽  
Surface Convection ◽  
Nonradial Pulsation

In order to see if there could be agreement between results of stellar evolution theory and those of nonradial pulsation theory, calculations of white dwarf models have been made for hydrogen surface masses of 10−4M⊙. Earlier results by Winget et al. (1982) indicated that surface masses greater than 10−8M⊙ would not allow nonradial pulsations, even though all the driving and damping is in surface layers only 10−12 of the mass thick. We show that the surface mass of hydrogen in the pulsating white dwarfs (ZZ Ceti variables) can be any value as long as it is thick enough to contain the surface convection zone.

scholarly journals Characterizing the companion AGBs using surface chemical composition of barium stars

2019 ◽  
Vol 492 (3) ◽  
pp. 3708-3727 ◽  
Author(s):  
J Shejeelammal ◽  
Aruna Goswami ◽  
Partha Pratim Goswami ◽  
Rajeev Singh Rathour ◽  
Thomas Masseron
Keyword(s):  
Neutron Capture ◽  
Detailed Comparison ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Capture Process ◽  
Origin And Evolution ◽  
Element Abundances ◽  
The Galaxy ◽  
Low Mass

ABSTRACT Barium stars are one of the important probes to understand the origin and evolution of slow neutron-capture process elements in the Galaxy. These are extrinsic stars, where the observed s-process element abundances are believed to have an origin in the now invisible companions that produced these elements at their asymptotic giant branch (AGB) phase of evolution. We have attempted to understand the s-process nucleosynthesis, as well as the physical properties of the companion stars through a detailed comparison of observed elemental abundances of 10 barium stars with the predictions from AGB nucleosynthesis models, FRUITY. For these stars, we have presented estimates of abundances of several elements, C, N, O, Na, Al, α-elements, Fe-peak elements, and neutron-capture elements Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu. The abundance estimates are based on high resolution spectral analysis. Observations of Rb in four of these stars have allowed us to put a limit to the mass of the companion AGB stars. Our analysis clearly shows that the former companions responsible for the surface abundance peculiarities of these stars are low-mass AGB stars. Kinematic analysis has shown the stars to be members of Galactic disc population.

scholarly journals White Dwarf Stars: Evolution of the Envelope Composition

1992 ◽  
Vol 9 ◽  
pp. 639-641
Author(s):  
James Liebert
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Standard Theory ◽  
Agb Stars ◽  
Dwarf Stars ◽  
Convective Mixing ◽  
White Dwarf Stars ◽  
Stars Evolution

AbstractOver the last several years, evidence has been unfolding that the surface abundances of white dwarfs may evolve as the stars cool. It is possible that some post-AGB stars enter the white dwarf sequence with far less hydrogen and helium than predicted by standard theory of the quenching of shell sources, and with atmospheres dominated by C and O. The interplay of diffusion and episodes of convective mixing may lead first to He-rich atmospheres, then H-rich, and finally He-rich again.

Hubble Space Telescope observations of cool white dwarf stars: Detection of new species of heavy elements

1995 ◽  
Vol 109 ◽  
pp. 1231 ◽  
Author(s):  
H. L. Shipman ◽  
M. Barnhill ◽  
J. Provencal ◽  
S. Roby ◽  
I. Bues ◽  
...  
Keyword(s):  
New Species ◽  
White Dwarf ◽  
Hubble Space Telescope ◽  
Heavy Elements ◽  
Dwarf Stars ◽  
Space Telescope ◽  
White Dwarf Stars

Convection zone of white-dwarf stars

1992 ◽  
Vol 187 (1) ◽  
pp. 27-36
Author(s):  
Z. M. Atweh ◽  
D. Eryurt-Ezer
Keyword(s):  
White Dwarf ◽  
Convection Zone ◽  
Dwarf Stars ◽  
White Dwarf Stars

scholarly journals White Dwarf Rotation: Observations and Theory

2004 ◽  
Vol 215 ◽  
pp. 561-570 ◽  
Author(s):  
Steven D. Kawaler
Keyword(s):  
Angular Momentum ◽  
White Dwarf ◽  
White Dwarfs ◽  
Main Sequence ◽  
Observational Constraints ◽  
Agb Stars ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
The Many ◽  
Limiting Case

White dwarfs rotate. The angular momentum in single white dwarfs must originate early in the life of the star, but also must be modified (and perhaps severely modified) during the many stages of evolution between birth as a main–sequence star and final appearance as a white dwarf. Observational constraints on the rotation of single white dwarf stars come from traditional spectroscopy and from asteroseismology, with the latter providing hints of angular velocity with depth. Results of these observational determinations, that white dwarfs rotate with periods ranging from hours to days (or longer), tells us that the processes by which angular momentum is deposited and/or drained from the cores of AGB stars are complex. Still, one can place strong limits on these processes by considering relatively simple limiting cases for angular momentum evolution in prior stages, and on subsequent angular momentum evolution in the white dwarfs. These limiting-case constraints will be reviewed in the context of the available observations.

scholarly journals The slow-neutron capture process in low-metallicity asymptotic giant branch stars

2009 ◽  
Vol 5 (S265) ◽  
pp. 57-60
Author(s):  
Amanda I. Karakas ◽  
Maria Lugaro ◽  
Simon W. Campbell
Keyword(s):  
Neutron Capture ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Observational Constraints ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Capture Process ◽  
Low Metallicity ◽  
Giant Branch Stars ◽  
Full Network

AbstractElements heavier than iron are produced in asymptotic giant branch (AGB) stars via the slow neutron capture process (s process). Recent observations of s-process-enriched Carbon Enhanced Metal-Poor (CEMP) stars have provided an unprecedented wealth of observational constraints on the operation of the s-process in low-metallicity AGB stars. We present new preliminary full network calculations of low-metallicity AGB stars, including a comparison to the composition of a few s-process rich CEMP stars. We also discuss the possibility of using halo planetary nebulae as further probes of low-metallicity AGB nucleosynthesis.

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