A strong neutron burst in jet-like supernovae of spinstars

¹ Department of Physics, Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto, Kobe, Hyogo 658-8501, Japan
e-mail: arthur.choplin@konan-u.ac.jp
² Geneva Observatory, University of Geneva, Maillettes 51, 1290 Sauverny, Switzerland
³ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
⁴ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA

Received: 16 March 2020
Accepted: 28 May 2020

Abstract

Context. Some metal-poor stars have abundance patterns, which are midway between the slow (s) and rapid (r) neutron capture processes.

Aims. We show that the helium shell of a fast rotating massive star experiencing a jet-like explosion undergoes two efficient neutron capture processes: one during stellar evolution and one during the explosion. It eventually provides a material whose chemical composition is midway between the s- and r-process.

Methods. A low metallicity 40 M_⊙ model with an initial rotational velocity of ∼700 km s⁻¹ was computed from birth to pre-supernova with an extended nuclear network following the slow neutron capture process. A two-dimensional hydrodynamic relativistic code was used to model a E = 10⁵² erg relativistic jet-like explosion hitting the stellar mantle. The jet-induced nucleosynthesis was calculated in post-processing with an optimised network of 1812 nuclei.

Results. During the star’s life, heavy elements from 30 ≲ Z ≲ 82 are produced thanks to an efficient s-process, which is boosted by rotation. At the end of evolution, the helium shell is largely enriched in trans-iron elements and in (unburnt) ²²Ne, whose abundance is ∼20 times higher than in a non-rotating model. During the explosion, the jet heats the helium shell up to ∼1.5 GK. It efficiently activates (α, n) reactions, such as ²²Ne(α, n), and leads to a strong n-process with neutron densities of ∼10¹⁹ − 10²⁰ cm⁻³ during 0.1 s. This has the effect of shifting the s-process pattern, which was built during stellar evolution, towards heavier elements (e.g. Eu). The resulting chemical pattern is consistent with the abundances of the carbon-enhanced metal-poor r/s star CS29528-028, provided the ejecta of the jet model is not homogeneously mixed.

Conclusions. The helium burning zones of rotating massive stars experience an efficient s-process during the evolution followed by an efficient n-process during a jet-like explosion. This is a new astrophysical site which can explain at least some of the metal-poor stars showing abundance patterns midway between the s- and r-process.