Three-Dimensional Hydrodynamical Simulations of a Proton Ingestion Episode in a Low-Metallicity Asymptotic Giant Branch Star

Stancliffe, Richard J.; Dearborn, David S. P.; Lattanzio, John C.; Heap, Stuart A.; Campbell, Simon

doi:10.1088/0004-637x/742/2/121

Cited by 100 publications

(165 citation statements)

References 35 publications

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“…There are as yet no detailed models of this process for low-metallicity AGB stars, although PIE events have been identified as a a possible physical site (the i-process, Cowan & Rose 1977;Herwig et al 2011). Overshoot leading to PIEs is uncertain, but known to be favoured in AGB stars of mass ∼1 M and [Fe/H] < 2 (Fujimoto et al 1990;Campbell & Lattanzio 2008;Cristallo et al 2009;Lugaro et al 2012) with recent 3D models supporting these results (Stancliffe et al 2011;Herwig et al 2014;Woodward et al 2015). We stress that one-dimensional hydrostatic models of PIE events with artificial proton distributions like those presented here are not expected to describe the ingestion and the mixing correctly, and this may have a strong effect on the resulting abundance patterns.…”

Section: Discussionmentioning

confidence: 91%

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

Lugaro

Campbell

Winckel

et al. 2015

A&A

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Aims. We explore modifications to the current scenario for the slow neutron-capture process (the s-process) in asymptotic giant branch (AGB) stars to account for the Pb deficiency observed in post-AGB stars of low metallicity ([Fe/H] −1.2) and low initial mass ( 1−1.5 M ) in the Large and Small Magellanic Clouds. Methods. We calculated the stellar evolution and nucleosynthesis for a 1.3 M star with [Fe/H] = −1.3 and tested different amounts and distributions of protons leading to the production of the main neutron source within the 13 C-pocket and proton ingestion scenarios. Results. No s-process models can fully reproduce the abundance patterns observed in the post-AGB stars. When the Pb production is lowered, the abundances of the elements between Eu and Pb, such as Er, Yb, W, and Hf, are also lowered to below those observed. Conclusions. Neutron-capture processes with neutron densities intermediate between the s and the rapid neutron-capture processes may provide a solution to this problem and be a common occurrence in low-mass, low-metallicity AGB stars.

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Section: Discussionmentioning

confidence: 91%

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

Lugaro

Campbell

Winckel

et al. 2015

A&A

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“…This point is very relevant, as it has recently been suggested that α should vary during the evolution of the star - Meakin & Arnett (2007). Moreover, it is possible that mixing length theory cannot adequately describe the convection of the actual star - Stancliffe et al (2011). Alternatively, if we could find some observational constraints on these models, we might be able to use these to discriminate between different convection theories or different values of α for this phase of the evolution.…”

Section: Discussionmentioning

confidence: 98%

The end of super AGB and massive AGB stars

Lau

Gil‐Pons

Doherty

et al. 2012

A&A

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Context. The literature is rich in analysis and results related to thermally pulsing-asymptotic giant branch (TP-AGB) stars, but the problem of the instabilities that arise and cause the divergence of models during the late stages of their evolution is rarely addressed. Aims. We investigate the physical conditions, causes and consequences of the interruption in the calculations of massive AGB stars in the late thermally-pulsing AGB phase. Methods. We have thoroughly analysed the physical structure of a solar metallicity 8.5 M star and described the physical conditions at the base of the convective envelope (BCE) just prior to divergence. Results. We find that the local opacity maximum caused by M-shell electrons of Fe-group elements lead to the accumulation of an energy excess, to the departure of thermal equilibrium conditions at the base of the convective envelope and, eventually, to the divergence of the computed models. For the 8.5 M case we present in this work the divergence occurs when the envelope mass is about 2 M . The remaining envelope masses range between somewhat less than 1 and more than 2 M for stars with initial masses between 7 and 10 M and, therefore, our results are relevant for the evolution and yields of super-AGB stars. If the envelope is ejected as a consequence of the instability we are considering, the occurrence of electron-capture supernovae would be avoided at solar metallicity.

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“…Splitting of the HeCZ (shell) is also found in PIE resulting from very late thermal pulses in post-AGB stars (Herwig 2001;Althaus et al 2005) and in simulations of the early phases of the TP-AGB in very metal-poor stars (Fujimoto et al 2000;Serenelli 2006;Cristallo et al 2009). On the other hand, recent hydrodynamic 3-dimensional simulations of the TP-AGB phase for a low-mass star (Stancliffe et al 2011) have not shown splitting of the HeCZ, although the authors argued this might be an artifact of the low resolution used in the calculations.…”

Section: Figmentioning

confidence: 96%

S-process in extremely metal-poor, low-mass stars

Cruz¹,

Serenelli²,

Weiß³

2013

A&A

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Context. Extremely metal-poor (EMP), low-mass stars experience an ingestion of protons into the helium-rich layer during the core He-flash, resulting in the production of neutrons through the reactions 12 C(p, γ) 13 N(β) 13 C(α, n) 16 O. This is a potential site for the production of s-process elements in EMP stars, which does not occur in more metal-rich counterparts. The signatures of s-process elements in the two most iron deficient stars observed to date, HE1327-2326 & HE0107-5240, still await for an explanation. Aims. We investigate the possibility that low-mass EMP stars could be the source of s-process elements observed in extremely iron deficient stars, either as a result of self-enrichment or in a binary scenario as the consequence of a mass transfer episode. Methods. We present evolutionary and post-processing s-process calculations of a 1 M stellar model with metallicities of Z = 0, 10 −8 , and 10 −7 . We assess the sensitivity of nucleosynthesis results to uncertainties in the input physics of the stellar models with particular regard to the details of convective mixing during the core He-flash. Results. Our models provide the possibility of explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the result of mass-transfer from a low-mass EMP star. The drawback of our model is that nitrogen would be overproduced and the 12 C/ 13 C abundance ratio would be underproduced in comparison to the observed values if mass would be transferred before the primary star enters the asymptotic giant branch phase. Conclusions. Our results show that low-mass EMP stars cannot be ruled out as companion stars that might have polluted HE1327-2326 and HE0107-5240 and produced the observed s-process pattern. However, more detailed studies of the core He-flash and the proton ingestion episode are needed to determine the robustness of our predictions.

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Three-Dimensional Hydrodynamical Simulations of a Proton Ingestion Episode in a Low-Metallicity Asymptotic Giant Branch Star

Cited by 100 publications

References 35 publications

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

The end of super AGB and massive AGB stars

S-process in extremely metal-poor, low-mass stars

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