The γ-process nucleosynthesis in core-collapse supernovae

I. A novel analysis of γ-process yields in massive stars

¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
² CSFK, MTA Centre of Excellence, Budapest, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: lorenzo.roberti@csfk.org
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ E. A. Milne Centre for Astrophysics, University of Hull, Hull HU6 7RX, UK
⁵ Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
⁶ Triangle Universities Nuclear Laboratory, Duke University, Durham, NC 27710, USA
⁷ Max-Planck Institute for Astrophysics, Postfach 1317, 85741 Garching, Germany
⁸ Institute for Nuclear Research (ATOMKI), 4001 Debrecen, Hungary
⁹ Eötvös Loránd University, Institute of Physics, Budapest 1117, Pázmány Péter sérány 1/A, Hungary
¹⁰ School of Physics and Astronomy, Monash University, VIC 3800, Australia

Received: 31 March 2023
Accepted: 15 June 2023

Abstract

Context. The γ-process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average yields of γ-process nucleosynthesis from massive stars are still insufficient to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are a factor of ten lower than the yields of the other γ-process nuclei.

Aims. We investigate the γ-process in five sets of core-collapse supernova models published in the literature with initial masses of 15, 20, and 25 M_⊙ at solar metallicity.

Methods. We compared the γ-process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass relative to their solar values. Further, we investigated the contribution of C–O shell mergers in the supernova progenitors as an additional site of the γ-process.

Results. Our analysis shows that a large scatter among the different models exists for both the γ-process integrated yields and the isotopic ratios. We find only ten ratios that agree with their solar values, all the others differ by at least a factor of three from the solar values in all the considered sets of models. The γ-process within C–O shell mergers mostly influences the isotopic ratios that involve intermediate and heavy proton-rich isotopes with A > 100.

Conclusions. We conclude that there are large discrepancies both among the different data sets and between the model predictions and the solar abundance distribution. More calculations are needed; particularly updating the nuclear network, because the majority of the models considered in this work do not use the latest reaction rates for the γ-process nucleosynthesis. Moreover, the role of C–O shell mergers requires further investigation.