THE s-PROCESS IN ASYMPTOTIC GIANT BRANCH STARS OF LOW METALLICITY AND THE COMPOSITION OF CARBON-ENHANCED METAL-POOR STARS

By M. Lugaro, A. Karakas, R. Stancliffe, Carlos Rijs. 2011

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We present models for the slow neutron-capture process (s-process) in asymptotic giant branch stars of metallicity [Fe/H] = −2.3 and masses 0.9–6 M☉. We encountered different regimes of neutron-capture nucleosynthesis listed here increasing in importance as the stellar mass decreases: the 22Ne(α, n)25Mg reaction activated during the thermal pulses (TPs), the 13C(α, n)16O reaction activated in radiative conditions during the interpulse periods, and the 13C(α, n)16O reaction activated during the TPs, also as a result of mild proton-ingestion episodes. The models where the 13C burns radiatively (masses ≃2 M☉) produce an overall good match to carbon-enhanced metal-poor (CEMP) stars showing s-process enhancements (CEMP-s), except they produce too much Na and F. On the other hand, none of our models can provide a match to the composition of CEMP stars also showing rapid-process enhancements (CEMP-s/r). The models fail to reproduce the observed Eu abundances, and they also fail to reproduce the correlation between the Eu and Ba abundances. They also cannot match the ratio of heavy-to-light s-process elements observed in many CEMP-s/r stars, which can be more than 10 times higher than in the solar system. To explain the composition of CEMP-s/r stars we need to invoke the existence of a different “s/r” neutron-capture process either with features in between the s- and the r-processes, or generated by superpositions of different neutron-capture processes in the same astrophysical site or in sites linked to each other—for example, in multiple stellar systems.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/58e94293aeed3a1d1e152e224fa0b9797d8337e4

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Citations in the corpus

THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] ⩽ −3.5,, (2012, background): “…elements from AGB stars are strongly dependent on mass and metallicity, such that they may be good candidates for explaining part, or perhaps most, of the large range of C ands-process element abundances (e.g., Karakas & Lattanzio 2007; Karakas 2010; Cristallo et al. 2011; Lugaro et al. 2012).”
STELLAR YIELDS FROM METAL-RICH ASYMPTOTIC GIANT BRANCH MODELS (2016, methodology): “The numerical method used for the post-processing numerical calculations is the same as described in previous studies including Lugaro et al. (2012), Fishlock et al. (2014) and Shingles et al. (2015).”
NUGRID STELLAR DATA SET. I. STELLAR YIELDS FROM H TO BI FOR STARS WITH METALLICITIES Z = 0.02 and Z = 0.01 (2013, background): “…the spectroscopic observations of stars at different metallicities (Lambert et al. 1995; Busso et al. 2001; Abia et al. 2002; Masseron et al. 2010; Bisterzo et al. 2011; Lugaro et al. 2012; Maiorca et al. 2012) and planetary nebulae (Karakas et al. 2007; Pignatari et al. 2008; Karakas et al. 2009).”
The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, methodology): “2M⊙, regardless of Z (see also Cristallo et al. 2009; Lugaro et al. 2012).”
GALACTIC CHEMICAL EVOLUTION AND SOLAR s-PROCESS ABUNDANCES: DEPENDENCE ON THE 13C-POCKET STRUCTURE (2014, background): “A range of 13C-pocket strengths is needed in order to explain this spread (see, e.g., Busso et al. 2001; Sneden et al. 2008; Käppeler et al. 2011; Bisterzo et al. 2011; Lugaro et al. 2012).”
s-process production in rotating massive stars at solar and low metallicities (2015, background): “Recent catalogues may be found by Masseron et al. (2010), Allen et al. (2012), Bisterzo et al. (2012), Lugaro et al. (2012), Norris et al. (2013), Bonifacio et al. (2015) and Hansen et al. (2015).”
Binarity in carbon-enhanced metal-poor stars (2014, background): “…stars (e.g., Masseron et al. 2010; Allen et al. 2012), although this might mean that a different neutron-capture process with features in between the r- and s-process will need to be invoked (Lugaro et al. 2012, Herwig et al., in preparation) to provide for its particular chemical signatures.”