The intermediate neutron capture process

Abstract

Context. Results from observations report a growing number of metal-poor stars showing an abundance pattern midway between the s- and r-processes. These so-called r/s-stars raise the need for an intermediate neutron capture process (i-process), which is thought to result from the ingestion of protons in a convective helium-burning region, but whose astrophysical site is still largely debated. Aims. We investigate whether an i-process during the asymptotic giant branch (AGB) phase of low-metallicity low-mass stars can develop and whether it can explain the abundances of observed r/s-stars. Methods. We computed a 1 M⊙ model at [Fe/H] = −2.5 with the stellar evolution code STAREVOL, using a nuclear network of 1091 species (at maximum) coupled to the transport processes. The impact of the temporal and spatial resolutions on the resulting abundances was assessed. We also identified key elements and isotopic ratios that are specific to i-process nucleosynthesis and carried out a detailed comparison between our model and a sample of r/s-stars. Results. At the beginning of the AGB phase, during the third thermal pulse, the helium driven convection zone is able to penetrate the hydrogen-rich layers. The subsequent proton ingestion leads to a strong neutron burst with neutron densities of ∼4.3 × 1014 cm−3 at the origin of the synthesis of i-process elements. The nuclear energy released by proton burning in the helium-burning convective shell strongly affects the internal structure: the thermal pulse splits and after approximately ten years the upper part of the convection zone merges with the convective envelope. The surface carbon abundance is enhanced by more than 3 dex. This leads to an increase in the opacity, which triggers a strong mass loss and prevents any further thermal pulse. Our numerical tests indicate that the i-process elemental distribution is not strongly affected by the temporal and spatial resolution used to compute the stellar models, but typical uncertainties of ±0.3 dex on individual abundances are found. We show that specific isotopic ratios of Ba, Nd, Sm, and Eu can represent good tracers of i-process nucleosynthesis. Finally, an extended comparison with 14 selected r/s-stars show that the observed composition patterns can be well reproduced by our i-process AGB model. Conclusions. A rich i-process nucleosynthesis can take place during the early AGB phase of low-metallicity low-mass stars and explain the elemental distribution of most of the r/s-stars, but cannot account for the high level of enrichment of the giant stars in a scenario involving pollution by a former AGB companion.

Magnetic-buoyancy-induced mixing in AGB stars: Fluorine nucleosynthesis at different metallicities

“…[ls/Fe], and [hs/Fe] indexes, which is much lower than the typical uncertainties affecting spectroscopic observations of s-processrich stars. Stellar models of lower metallicity are expected to produce even minor variations (Cristallo et al 2009a;Karakas 2010) unless their mass and metallicity are so low (M 1.3 M and [Fe/H] −2.5) that they experience a proton ingestion episode at the first fully developed TP (see Cristallo et al 2009b;Campbell et al 2010;Choplin et al 2021). In Figs.…”

The Gaia-ESO Survey: a new approach to chemically characterising young open clusters

“…Different types of stars have been proposed as possible stellar hosts of the i-process: post-AGB stars (Herwig et al 2011) and low-mass AGB stars (e.g. Lugaro et al 2015;Cristallo et al 2016;Choplin et al 2021), super-AGB stars (Jones et al 2016), rapidly-accreting white dwarfs (e.g. Denissenkov et al 2017Denissenkov et al , 2019Côté et al 2018) and massive stars (Roederer et al 2016;Clarkson et al 2018;Banerjee et al 2018).…”

The intermediate neutron capture process II.Nuclear uncertainties

“…The timesteps as well as the size of the grid mesh needed to ensure that the integration variables remain correctly discretised must be accurately determined. As discussed in Choplin et al (2021), the temporal and spatial resolutions can be tuned by two parameters in the STAREVOL code. For the present study, these are set to α = 0.01 and max = 0.04, respectively.…”

“…Various astrophysical sites have been suggested to be responsible for the i-process nucleosynthesis. These include: the early asymptotic giant branch (AGB) phase of metal-poor low-mass stars, in which the entropy barrier between the hydrogen-and helium-rich zones can be surmounted by the energy released by the thermal pulse (Fujimoto et al 2000;Chieffi et al 2001;Siess et al 2002;Iwamoto et al 2004;Cristallo et al 2009;Suda & Fujimoto 2010;Cristallo et al 2016;Choplin et al 2021); the core helium flash of very low-metallicity low-mass stars (Fujimoto et al 2000;Schlattl et al 2001;Suda & Fujimoto 2010;Campbell et al 2010;Cruz et al 2013); the very late thermal pulses of post-AGB stars (Herwig et al 2011); rapidly accret-ing carbon-oxygen (C-O) or oxygen-neon (O-Ne) white dwarfs (RAWDs) in close binary systems (Denissenkov et al 2017(Denissenkov et al , 2019; and super-AGB stars (7 M M ini 10 M ) at low metallicity (Z < ∼ 10 −3 ) (Jones et al 2016). ; the so-called dredgeout in super-AGB stars, when, at the end of carbon-burning, a helium-driven convective shell merges with the descending convective envelope (Siess 2007); or the helium shell of very lowor zero-metallicity massive stars (M ini > 10 M ) during central carbon-burning (or later stages) (Banerjee et al 2018;Clarkson et al 2018;Clarkson & Herwig 2020).…”