[Paper Review] The intermediate neutron capture process. I. Development of the i-process in low-metallicity low-mass AGB stars

A. Choplin, L. Siess+1 more|arXiv (Cornell University)|Feb 17, 2021

Stellar, planetary, and galactic studies59 references4 citations

TL;DR

This study demonstrates that the intermediate neutron capture process (i-process) can efficiently occur in low-metallicity, low-mass asymptotic giant branch (AGB) stars via proton ingestion during the third thermal pulse, producing neutron densities up to ~4.3×10¹⁴ cm⁻³ and reproducing the trans-iron element abundances observed in r/s-stars. The resulting nucleosynthesis, confirmed robust across numerical resolutions, is best traced by specific isotopic ratios of Ba, Nd, Sm, and Eu.

ABSTRACT

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. 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. 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 $\approx 4.3 imes 10^{14}$ 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. 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.

Motivation & Objective

• To investigate whether the i-process can develop during the early AGB phase of low-metallicity, low-mass stars.
• To determine if proton ingestion in a helium-burning convective zone can produce the observed intermediate neutron capture process abundances in r/s-stars.
• To assess the robustness of i-process nucleosynthesis against numerical resolution and transport processes.
• To identify unique isotopic signatures that distinguish i-process from s- and r-process nucleosynthesis.
• To compare model predictions with observed abundance patterns in 14 r/s-stars using a new selection method.

Proposed method

• Stellar evolution was modeled using the STAREVOL code for a 1 M☉ star at [Fe/H] = -2.5.
• A nuclear network of up to 1091 isotopes was coupled to hydrodynamics and transport processes, including diffusion and convection.
• Proton ingestion was simulated during the third thermal pulse, triggering a neutron burst in the helium-burning convective zone.
• Numerical convergence tests were performed by varying temporal and spatial resolution to assess stability of nucleosynthesis results.
• Isotopic ratios (e.g., ¹³⁷Ba, ¹⁴⁴Nd, ¹⁵⁴Sm, ¹⁵¹Eu, ¹⁵³Eu) were analyzed as potential i-process tracers.
• Model abundances were compared with observed r/s-stars using the selection method of Karinkuzhi et al. (2021).

Experimental results

Research questions

• RQ1 Can the i-process be initiated and sustained in low-metallicity, low-mass AGB stars via proton ingestion in a helium-burning convective zone?
• RQ2 How do temporal and spatial resolution affect the predicted i-process nucleosynthesis and abundance yields?
• RQ3 Which isotopic ratios uniquely identify i-process contributions in stellar abundances?
• RQ4 To what extent can the i-process in AGB stars reproduce the observed abundance patterns in r/s-stars?
• RQ5 How do uncertainties in mixing, mass loss, and nuclear physics affect i-process predictions?

Key findings

• The third thermal pulse triggers a major proton ingestion event, producing a neutron burst with peak density of ~4.3×10¹⁴ cm⁻³.
• The i-process significantly enhances trans-iron elements, with predicted abundances matching observed r/s-stars within ~±0.3 dex uncertainty.
• The surface carbon abundance increases by more than 3 dex, raising opacity and triggering strong mass loss that terminates further thermal pulses.
• Isotopic ratios f¹³⁷Ba, f¹⁴⁴Nd, f¹⁵⁴Sm, f¹⁵¹Eu, and f¹⁵³Eu are robust tracers of i-process nucleosynthesis, differing markedly from s- and r-process values.
• Numerical convergence tests confirm that the i-process is not a numerical artifact, though abundances show ±0.3 dex uncertainty due to resolution effects.
• The model successfully reproduces the abundance patterns of 14 r/s-stars, though minor discrepancies remain for Sr, Y, and Zr due to close-atomic-number variations.