scholarly journals Heavy-Element Nucleosynthesis in AGB Stars

2000 ◽  
Vol 177 ◽  
pp. 443-448
Author(s):  
Verne V. Smith
Keyword(s):  
Neutron Source ◽  
Heavy Element ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Process Elements

The production of certain neutron-rich elements heavier than iron occurs during He shell-burning on the asymptotic giant branch (AGB). These neutron captures occur at rather low neutron densities and, thus, the resulting heavy-element nucleosynthesis is characterisitic of the so-called s-process. Abundance analyses of the s-process elements in AGB stars can reveal details of the neutron densities and the stage of AGB evolution at which s-processing occurs, as well as the nature of the neutron source. These details derived from observations can constrain models of stars evolving along the AGB. Recent results concerning the nature of the s-process as a function of metallicity and the nature of the neutron source are reviewed.

Rubidium in Barium Stars

2020 ◽  
Author(s):  
M P Roriz ◽  
M Lugaro ◽  
C B Pereira ◽  
N A Drake ◽  
S Junqueira ◽  
...  
Keyword(s):  
Neutron Source ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Neutron Density ◽  
Data Sets ◽  
Agb Stars ◽  
Branching Points ◽  
Theoretical Predictions ◽  
Low Mass ◽  
Chemically Peculiar Stars

Abstract Barium (Ba) stars are chemically peculiar stars that display in their atmospheres the signature of the slow neutron-capture (the s-process) mechanism that occurs in asymptotic giant branch (AGB) stars, a main contributor to the cosmic abundances. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories providing strong constraints for the nucleosynthesis of the s-process in AGB stars. In particular, rubidium (Rb) is a key element for the s-process diagnostic because it is sensitive to the neutron density and therefore its abundance can reveal the main neutron source for the s-process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high resolution spectra (R = 48000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from AGB s-process nucleosynthesis models. The target Ba stars in this study display [Rb/Zr] <0, showing that Rb was not efficiently produced by the activation of branching points. Model predictions from the Monash and FRUITY data sets of low-mass (≲ 4 M⊙) AGB stars are able to cover the Rb abundances observed in the target Ba stars. These observations indicate that the 13C(α,n)16O reaction is the main neutron source of the s-process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companion for IR/OH massive AGB stars.

scholarly journals Asymptotic Giant Branch Stars: Thermal Pulses, Carbon Production, and Dredge Up; Neutron Sources and s-Process Nucleosynthesis

1991 ◽  
Vol 145 ◽  
pp. 257-274
Author(s):  
Icko Iben
Keyword(s):  
Neutron Source ◽  
Lower Boundary ◽  
Source Model ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Core Mass ◽  
Convective Envelope ◽  
Thermal Pulses ◽  
Giant Branch Stars

A brief review is given of the structure of asymptotic giant branch (AGB) stars and of the characteristics of the thermal pulses which these stars experience. Following a pulse, model AGB stars with a large core mass easily dredge up fresh carbon, which is the main product of incomplete helium burning, and s-process isotopes, which are made as a consequence of the activation of the 22Ne neutron source. Model AGB stars of small core mass activate the 13C neutron source and produce s-process isotopes in nearly the solar system distribution. They also dredge up fresh carbon and s-process isotopes, but only if overshoot or some other form of “extra” mixing beyond the lower boundary of the convective envelope is invoked.

scholarly journals Discovery of technetium- and niobium-rich S stars: The case for bitrinsic stars

2020 ◽  
Vol 635 ◽  
pp. L6 ◽  
Author(s):  
S. Shetye ◽  
S. Van Eck ◽  
S. Goriely ◽  
L. Siess ◽  
A. Jorissen ◽  
...  
Keyword(s):  
Mass Transfer ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Chemical Abundances ◽  
Trace Evidence ◽  
Distinctive Features ◽  
The Third ◽  
Process Pattern ◽  
Process Elements ◽  
Hr Diagram

Context. S stars are late-type giants with overabundances of s-process elements. They come in two flavors depending on the presence or lack of presence of technetium (Tc), an element without stable isotopes. Intrinsic S stars are Tc-rich and genuine asymptotic giant branch (AGB) stars, while extrinsic S stars owe their s-process over abundances to the pollution from a former AGB companion, which is now a white dwarf (WD). In addition to Tc, another distinctive feature between intrinsic and extrinsic S stars is the overabundance of niobium (Nb) in the latter class. Indeed, since the mass transfer occurred long ago, 93Zr had time to decay into the only stable isotope of Nb, 93Nb, causing its overabundance. Aims. We discuss the case of the S stars BD+79°156 and o1 Ori, whose specificity lies in sharing the distinctive features of both intrinsic and extrinsic S stars, namely the presence of Tc along with a Nb overabundance. Methods. We used high-resolution HERMES optical spectra, MARCS model atmospheres of S stars, Gaia DR2 parallaxes, and STAREVOL evolutionary tracks to determine the stellar parameters and chemical abundances of the two S stars, and to locate them in the Hertzsprung-Russell (HR) diagram. Results. BD+79°156 is the first clear case of a bitrinsic star, that is, a doubly s-process-enriched object, first through mass transfer in a binary system and then through internal nucleosynthesis that is responsible for the Tc-enrichment in BD+79°156, which must, therefore, have reached the AGB phase of its evolution. This hybrid nature of the s-process pattern in BD+79°156 is supported by its binary nature and its location in the HR diagram that is just beyond the onset of the third dredge-up on the AGB. The Tc-rich, binary S-star o1 Ori with a WD companion was another long-standing candidate for a similar hybrid s-process enrichment. However, the marginal overabundance of Nb derived in o1 Ori does not allow one to trace evidence of large amounts of pollution coming from the AGB progenitor of its current WD companion unambiguously. As a side product, the current study offers a new way of detecting binary AGB stars with WD companions by identifying their Tc-rich nature along with a Nb overabundance.

scholarly journals Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models

2009 ◽  
Vol 26 (3) ◽  
pp. 314-321 ◽  
Author(s):  
Sara Bisterzo ◽  
Roberto Gallino ◽  
Oscar Straniero ◽  
Wako Aoki
Keyword(s):  
Mass Transfer ◽  
Binary System ◽  
Neutron Capture ◽  
Molecular Cloud ◽  
Main Sequence ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Spectroscopic Observations ◽  
Theoretical Predictions ◽  
Process Elements

AbstractAsymptotic Giant Branch (AGB) stars play a fundamental role in s-process nucleosynthesis during their thermal pulsing phase. The theoretical predictions obtained by AGB models at different masses, s-process efficiencies, dilution factors and initial r-enrichment, are compared with spectroscopic observations of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s), collected from the literature. We discuss here five stars as example, CS 22880-074, CS 22942-019, CS 29526-110, HE 0202-2204 and LP 625-44. All these objects lie on the main sequence or on the giant phase, clearly before the thermally pulsing AGB stage. The hypothesis of mass transfer from an AGB companion, would explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are CEMP(s + r) objects, and are interpreted assuming that the molecular cloud, from which the binary system formed, was already enriched in r-process elements by SNII pollution. In several cases, the observed s-process distribution may be accounted for by AGB models of different initial masses with proper 13C-pocket efficiencies and dilution factors. Na (and Mg), produced via the neutron capture chain starting from 22Ne, may provide an indicator of the initial AGB mass.

scholarly journals Comparing the Nucleosynthesis Parameters of s+r Stars and Ba Stars

2008 ◽  
Vol 4 (S252) ◽  
pp. 113-114
Author(s):  
Wen-Yuan Cui ◽  
Dong-Nuan Cui ◽  
Bo Zhang
Keyword(s):  
Neutron Source ◽  
Strong Correlation ◽  
Binary Systems ◽  
Parametric Model ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
R Process ◽  
Process Component

AbstractIn this paper, we use a parametric model of the asymptotic giant branch (AGB) stars, in which the 13C neutron source is activated in radiative conditions during the interpulse periods, to calculate the nucleosynthesis in 29 very metal-poor double-enhanced stars (i.e. s+r stars) and 26 barium stars (i.e. Ba stars), respectively. Through a statistical analyzing on the corresponding parameters obtained for the above stars, we get the possible conditions which the s+r stars formed in. We find that the value of neutron exposures of most s+r stars is greater than that of Ba stars. In the very metal-poor stars, the Ba stars stars should belong to the binary systems with large initial orbital separation, by comparing the s-process-component coefficient (Cs) values with those of s+r stars. For s+r stars, there is strong correlation between their Cs and Cr (r-process-component coefficient) but no correlation for Ba stars. This strongly confirms the possibility that the s+r stars should form through the accretion-induced collapse (AIC) or type 1.5 supernova mechanism.

scholarly journals Monash Chemical Yields Project (Monχey) Element production in low- and intermediate-mass stars

2015 ◽  
Vol 11 (A29B) ◽  
pp. 164-165
Author(s):  
Carolyn Doherty ◽  
John Lattanzio ◽  
George Angelou ◽  
Simon W. Campbell ◽  
Ross Church ◽  
...  
Keyword(s):  
Chemical Evolution ◽  
Heavy Element ◽  
Internal Surface ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Galactic Chemical Evolution ◽  
Intermediate Mass ◽  
Mixing Processes ◽  
Intermediate Mass Stars ◽  
Solar Metallicity

AbstractThe Monχey project will provide a large and homogeneous set of stellar yields for the low- and intermediate- mass stars and has applications particularly to galactic chemical evolution modelling. We describe our detailed grid of stellar evolutionary models and corresponding nucleosynthetic yields for stars of initial mass 0.8 M⊙ up to the limit for core collapse supernova (CC-SN) ≈ 10 M⊙. Our study covers a broad range of metallicities, ranging from the first, primordial stars (Z = 0) to those of super-solar metallicity (Z = 0.04). The models are evolved from the zero-age main-sequence until the end of the asymptotic giant branch (AGB) and the nucleosynthesis calculations include all elements from H to Bi. A major innovation of our work is the first complete grid of heavy element nucleosynthetic predictions for primordial AGB stars as well as the inclusion of extra-mixing processes (in this case thermohaline) during the red giant branch. We provide a broad overview of our results with implications for galactic chemical evolution as well as highlight interesting results such as heavy element production in dredge-out events of super-AGB stars. We briefly introduce our forthcoming web-based database which provides the evolutionary tracks, structural properties, internal/surface nucleosynthetic compositions and stellar yields. Our web interface includes user- driven plotting capabilities with output available in a range of formats. Our nucleosynthetic results will be available for further use in post processing calculations for dust production yields.

scholarly journals Understanding AGB Carbon Star Nucleosynthesis from Observations

2003 ◽  
Vol 20 (4) ◽  
pp. 314-323 ◽  
Author(s):  
C. Abia ◽  
I. Domínguez ◽  
R. Gallino ◽  
M. Busso ◽  
O. Straniero ◽  
...  
Keyword(s):  
Carbon Star ◽  
Solar Neighborhood ◽  
Asymptotic Giant Branch ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Carbon Stars ◽  
The Core ◽  
Standard Scenario ◽  
Low Mass ◽  
Process Elements

AbstractRecent advances in the knowledge of the evolutionary status of asymptotic giant branch (AGB) stars and of the nucleosynthesis processes occurring in them are discussed, and used to interpret abundance determinations for s-process elements, lithium and CNO isotopes in several types of AGB stars. We focus our attention mainly on carbon-rich AGB stars. By combining these different constraints we conclude that most carbon stars in the solar neighborhood are of low mass (M≤3 M⊙), their abundances being a consequence of the operation of thermal pulses and the third dredge-up. However, the observed abundances in carbon stars of the R and J types cannot be explained by this standard scenario. These stars may not be on the AGB, but possibly in the core-He burning phases; their envelopes may have been polluted with nuclear ashes of the core-He flash, followed by CNO re-processing enhancing 13C. Observational evidence suggesting the operation of non-standard mixing mechanisms during the AGB phase is also discussed.

scholarly journals The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars

Universe ◽  
2021 ◽  
Vol 7 (11) ◽  
pp. 446
Author(s):  
Devika Kamath ◽  
Hans Van Winckel
Keyword(s):  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Chemical Abundance ◽  
Major Development ◽  
Upper Limits ◽  
The Galaxy ◽  
Process Elements ◽  
Detailed Chemical ◽  
Theoretical Developments

Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. For the subset of post-AGB stars with low metallicities the determined upper limits based on detailed chemical abundance studies are much lower than what is predicted. Recent theoretical studies have pointed to the occurrence of the i-process to explain the observed chemical patterns, especially of Pb. A major development, in the observational context, is the release of the GAIA EDR3 parallaxes of the post-AGBs in the Galaxy, which has opened the gateway to systematically studying the sample of stars as a function of current luminosities (which can be linked to their initial masses). In this paper, we succinctly review the Pb discrepancy in post-AGB stars and present the latest observational and theoretical developments in this research landscape.

scholarly journals The Chemical Composition of Asymptotic Giant Branch Stars-the s-process

1991 ◽  
Vol 145 ◽  
pp. 299-316
Author(s):  
David L. Lambert
Keyword(s):  
Chemical Composition ◽  
Solar System ◽  
Neutron Source ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Intermediate Mass ◽  
Low Mass ◽  
Giant Branch Stars

This review discusses the chemical composition of AGB stars in the light of predictions for intermediate-mass (3-8 M⊙, 22Ne(α,n) = the neutron source) and low-mass (< 3 M⊙, 13C(α,n) = the neutron source) stars. LM-AGB models can be constructed with envelopes having a composition quite similar to that of solar system material, the SiC grains recently discovered in meteorites, and real AGB stars in the sequence of spectral types M → S → C. Stellar counterparts of the IM-AGB models have yet to be discovered.

scholarly journals When binaries keep track of recent nucleosynthesis

2018 ◽  
Vol 14 (S343) ◽  
pp. 438-440
Author(s):  
D. Karinkuzhi ◽  
S. Van Eck ◽  
A. Jorissen ◽  
S. Goriely ◽  
L. Siess ◽  
...  
Keyword(s):  
Neutron Source ◽  
Stellar Evolution ◽  
Asymptotic Giant Branch ◽  
Heavy Elements ◽  
Agb Stars ◽  
Present Sample ◽  
Elemental Abundances ◽  
Operation Temperature ◽  
Process Operation

AbstractWe determine Zr and Nb elemental abundances in barium stars to probe the operation temperature of the s-process that occurred in the companion asymptotic giant branch (AGB) stars. Along with Zr and Nb, we derive the abundances of a large number of heavy elements. They provide constraints on the s-process operation temperature and therefore on the s-process neutron source. The results are then compared with stellar evolution and nucleosynthesis models. We compare the nucleosynthetic profile of the present sample stars with those of CEMP-s, CEMP-rs and CEMP-r stars. One barium star of our sample is potentially identified as the highest-metallicity CEMP-rs star yet discovered.

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