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Astron. Astrophys. 335, 207-217 (1998)


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

D.K. Nadyozhin 1, I.V. Panov 1, 2 and S.I. Blinnikov 1

1 Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya St. 25, 117259 Moscow, Russia
2 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, Postfach 1523, D-85740 Garching, Germany

Received 5 March 1997 / Accepted 10 February 1998

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. In particular, evaporation of neutrons from helium nuclei excited by neutrino-nuclear inelastic collisions, may result in the r-process as it was first discussed by Epstein et al. (1988). Here we consider mainly the possibility to obtain the considerable amount of neutrons owing to the neutrino breakup of helium nuclei. 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 [FORMULA], 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, [FORMULA], were used to calculate the r-process nucleosynthesis with another full kinetic network for [FORMULA] nuclides. It was found that in the case of metal-deficient stars, [FORMULA], 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 [FORMULA] 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.

Key words: nuclear reactions – nucleosynthesis – abundances – supernovae: general

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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