A strong neutron burst in jet-like supernovae of spinstars

Abstract

Some metal-poor stars have abundance patterns which are midway between the slow (s) and rapid (r) neutron capture processes. 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. A low metallicity 40~$M_{\odot }$ model with an initial rotational velocity of $\sim 700$~km~s${}^{-1}$ was computed from birth to pre-supernova with a nuclear network following the slow neutron capture process. A 2D hydrodynamic relativistic code was used to model a $E=10^{52}$~erg relativistic jet-like explosion hitting the stellar mantle. The jet-induced nucleosynthesis was calculated in post-processing with a network of 1812 nuclei. During the star's life, heavy elements from $30\lesssim Z\lesssim 82$ 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) ${}^{22}$Ne, whose abundance is $\sim 20$ times higher than in a non-rotating model. During the explosion, the jet heats the helium shell up to $\sim 1.5$ GK. It efficiently activates ($\alpha ,n$) reactions, such as ${}^{22}$Ne($\alpha ,n$), and leads to a strong n-process with neutron densities of $\sim 10^{19}-10^{20}$~cm${}^{-3}$ during $0.1$~second. This has the effect of shifting the s-process pattern 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. 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.

9 pages, 12 figures, accepted in A&A

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