The i-processes nucleosynthesis during the formation of He-rich hot-subdwarf stars

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Strasse 1, D-85748 Garching, Germany
² Instituto de Astrofísica de La Plata, Consejo Nacional de Investigaciones Científicas y Técnicas Avenida Centenario (Paseo del Bosque) S/N, B1900FWA La Plata, Argentina
³ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata Avenida Centenario (Paseo del Bosque) S/N, B1900FWA La Plata, Argentina
⁴ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany
⁵ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, E-08193 Cerdanyola del Valles, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Carrer Esteve Terradas, 1, Edifici RDIT, Campus PMT-UPC, E-08860 Castelldefels, Spain

^⋆ Corresponding author: tiara@mpa-garching.mpg.de

Received: 20 December 2024
Accepted: 28 March 2025

Abstract

Context. Intermediate neutron-capture processes are thought to occur in stellar environments where protons are ingested into a hot helium-burning convective region. It has been shown that proton ingestion episodes can happen in the formation of hot-subdwarf stars, and that neutron-capture processes are possible in those cases. Moreover, some helium-rich hot subdwarfs display extraordinarily high abundances of heavy elements such as Zr, Yr, and Pb on their surfaces. These elements can be produced by both slow and intermediate neutron-capture processes.

Aims. We explore under which conditions neutron-capture processes can occur in late helium core flashes, that is, those occurring in the cores of stripped red-giant stars. We explore the dependence of such processes on the metallicity of the star, and how much the star is stripped before the He flash takes place.

Methods. We computed evolutionary models through the helium core flash and the subsequent hydrogen ingestion episode in stripped red-giant stars. Stellar structure models were then used in post-processing to compute the detailed evolution of neutron-capture elements.

Results. We find that for metallicities of 10⁻³ and below, neutron densities can be as high as 10¹⁵ cm⁻³ and intermediate neutron-capture processes occur in some of our models. The results depend very strongly on the H-envelope mass that survives after the stripping, which alters the nucleosynthesis during the flashes, and therefore the behavior of the convective motions. Interestingly, we find that computed abundances in some of our models closely match the element abundances up to tin observed for EC 22536-5304, the only well-studied star for which the hot-flasher scenario assumed in our models is the most likely evolutionary path.

Conclusions. Intermediate neutron-capture processes can occur in the He-core flash experienced by the cores of some stripped red giants, and this might be connected to the abundances of heavy elements observed in some helium-rich hot-subdwarf stars. The agreement between the observed abundances in EC 22536-5304 and those of our models offers support to our nucleosynthesis calculations. Moreover, if confirmed, the idea that heavy element abundances retain signatures of the different evolutionary channels opens the possibility that heavy element abundances in iHe-sdOB stars can be used to infer their evolutionary origin.