The neutrino-induced neutron source in helium shell and r-process nucleosynthesis

TL;DR

This paper investigates neutrino-induced neutron production in helium shells during stellar collapse and its potential role in r-process nucleosynthesis, finding it insufficient to explain observed heavy element abundances without nonstandard stellar models.

Contribution

It provides a detailed kinetic analysis of neutrino-driven neutron flux and its impact on r-process nucleosynthesis in helium shells, highlighting limitations and conditions for effectiveness.

Findings

Neutrino-induced neutron flux can drive r-process in metal-poor stars.

Heavy element production is limited at radii greater than 10^9 cm.

Standard models cannot account for observed solar r-process abundances.

Abstract

The huge neutrino pulse that occurs during the collapse of a massive stellar core, is expected to contribute to the origination of a number of isotopes both of light chemical elements and heavy ones. It is shown that, in general, the heating of stellar matter due to the neutrino scattering off electrons and the heat released from the neutrino-helium breakup followed by the thermonuclear reactions should be taken into account. On the base of kinetic network, using all the important reactions up to Z=8, the main features and the time-dependent character of the neutrino- driven neutron flux are investigated. The time-dependent densities of free neutrons produced in helium breakup, Y_n(t), were used to calculate the r-process nucleosynthesis with another full kinetic network for 3200 nuclides. It was found that in the case of metal-deficient stars, Z < 0.01 Z(solar), the resulting density of free neutrons seems to be high enough to drive the r-process efficiently under favorable conditions. But it is impossible to obtain a sufficient amount of heavy nuclei in neutrino-induced r-process in a helium shell at radii R > R_cr \approx 10^9 cm. We speculate that to make the neutrino-induced r-process work efficiently in the shell, one has to invoke nonstandard presupernova models in which helium hopefully is closer to the collapsed core owing, for instance, to a large scale mixing or/and rotation and magnetic fields. Apart from this exotic possibility, the neutrino-induced nucleosynthesis in the helium shell is certainly not strong enough to explain the observed solar r-process abundances.