New insights on Ba overabundance in open clusters.★ Evidence for the intermediate neutron-capture process at play?

Mishenina, T. V.; Pignatari, M.; Carraro, G.; Kovtyukh, V. V.; Monaco, L.; Коротин, С. А.; Shereta, E. P.; Yegorova, I.; Herwig, Falk

doi:10.1093/mnras/stu2337

Cited by 87 publications

(132 citation statements)

References 79 publications

(179 reference statements)

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“…On the other hand, in the case of the i process, there is an increase of around a factor of 3 between the production of Sr and that of Zr; and a factor of 10 decrease between Ba and La and/or Ce. In fact, the overproduction of Ba with respect to La resulting from the i process has been pointed out as the possible source of the Ba excess observed in open clusters (Mishenina et al 2015).…”

Section: Resultsmentioning

confidence: 99%

The Intermediate Neutron-Capture Process and Carbon-Enhanced Metal-Poor Stars

Hampel

Stancliffe

Lugaro

et al. 2016

ApJ

176

245

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Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display enrichments in heavy elements associated with either the s (slow) or the r (rapid) neutron-capture process (e.g., barium and europium respectively), and in some cases they display evidence of both. The abundance patterns of these CEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling since the s and the r processes require neutron densities that are more than ten orders of magnitude apart, and hence are thought to occur in very different stellar sites with very different physical conditions. We investigate whether the abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of the intermediate neutron-capture process (the i process), which is characterised by neutron densities between those of the s and the r processes. Using nuclear network calculations, we study neutron capture nucleosynthesis at different constant neutron densities n ranging from 10 7 to 10 15 cm −3 . With respect to the classical s process resulting from neutron densities on the lowest side of this range, neutron densities on the highest side result in abundance patterns that show an increased production of heavy s-process and r-process elements but similar abundances of the light s-process elements.Such high values of n may occur in the thermal pulses of asymptotic giant branch (AGB) stars due to proton ingestion episodes. Comparison to the surface abundances of 20 CEMP-s/r stars show that our modelled i-process abundances successfully reproduce observed abundance patterns that could not be previously explained by s-process nucleosynthesis. Because the i-process models fit the abundances of CEMP-s/r stars so well, we propose that this class should be renamed as CEMP-i.

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Section: Resultsmentioning

confidence: 99%

The Intermediate Neutron-Capture Process and Carbon-Enhanced Metal-Poor Stars

Hampel

Stancliffe

Lugaro

et al. 2016

ApJ

176

245

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“…Evidence for i-process nucleosynthesis has also been reported in the anomalous Ba enrichment in some open clusters (Mishenina et al 2015), while Dardelet et al (2014) showed that nucleosynthesis in i-process conditions provided a remarkable match to the abundances in several CEMP-r/s (carbon enhanced metal poor with simultaneous presence of s elements and Eu) stars. Possible sites for the i process include the He-core flash (e.g.…”

Section: Introductionmentioning

confidence: 81%

i-process Nucleosynthesis and Mass Retention Efficiency in He-shell Flash Evolution of Rapidly Accreting White Dwarfs

Denissenkov

Herwig

Battino

et al. 2017

ApJL

102

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Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs in close binary systems are an astrophysical site for the intermediate neutron-capture process. During recurrent and very strong He-shell flashes in the stable H-burning accretion regime H-rich material enters the He-shell flash convection zone.12 C(p, γ) 13 N reactions release enough energy to potentially impact convection, and i process is activated through the 13 C(α, n) 16 O reaction. The H-ingestion flash may not cause a split of the convection zone as it was seen in simulations of He-shell flashes in post-AGB and low-Z AGB stars. We estimate that for the production of first-peak heavy elements this site can be of similar importance for galactic chemical evolution as the s-process production by low-mass AGB stars. The He-shell flashes result in the expansion and, ultimately, ejection of the accreted and then i-process enriched material, via super-Eddington luminosity winds or Roche-lobe overflow. The white dwarf models do not retain any significant amount of the accreted mass, with a He retention efficiency of 10% depending on mass and convective boundary mixing assumptions. This makes the evolutionary path of such systems to supernova Ia explosion highly unlikely.

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“…Since stars in this mass range are common, this would have important implications for the stellar yields that drive the chemical evolution of stellar clusters and galaxies. For example, these long-lived lowmass stars may be the i-process source required to match observations of [Ba/Fe] and [Ba/La] overabundances in open clusters (Mishenina et al 2015). If the i-process is confirmed to be responsible for the abundances observed in CEMP-s/r and post-AGB stars, similarities and differences in the neutron-capture pattern of the two groups, which sample different metallicity ranges, will provide fundamental constraints to determine its metallicity dependence and its effect on the chemical evolution of stellar systems.…”

Section: Discussionmentioning

confidence: 99%

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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New insights on Ba overabundance in open clusters.★ Evidence for the intermediate neutron-capture process at play?

Cited by 87 publications

References 79 publications

The Intermediate Neutron-Capture Process and Carbon-Enhanced Metal-Poor Stars

The Intermediate Neutron-Capture Process and Carbon-Enhanced Metal-Poor Stars

i-process Nucleosynthesis and Mass Retention Efficiency in He-shell Flash Evolution of Rapidly Accreting White Dwarfs

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

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