Annu. Rev. Astron. Astrophys. 1994. 32: 153-90 Copyright © 1994 by Annual Reviews. All rights reserved

5.2. p-Process Sites

The early papers on the p-process considered the proton capture mechanism (Burbidge et al 1957, Ito 1961, Macklin 1970, Truran & Cameron 1972, Audouze & Truran 1975). The site was imagined to be the hydrogen-rich envelope in massive stars undergoing a supernova explosion. The supernova shock passing through this region would heat up the material and proton capture reactions would produce the p-nuclei. However, the densities, temperatures, and timescales required are unrealistic for the hydrogen-rich envelope (see, for example, the discussion in Woosley & Howard 1978).

Arnould (1976) computed the p-process in the hydrostatic oxygen burning phase in stars. The timescales are longer in this site than in the supernova site and would allow for more proton capture. In this site, temperatures were high enough for disintegrations [especially (, n) reactions] to be important. A major challenge for this model is to eject the new p-nuclei without significantly modifying their abundances during the subsequent supernova explosion.

Woosley & Howard (1978) computed the p-process in the O/Ne shell in type II, that is, core-collapse, supernovae. The supernova shock heats up this shell and causes the partial melting of the nuclei. In this model, only disintegrations are important, hence the alternative name "gamma-process" (see Rayet et al 1990, Prantzos et al 1990, Rayet et al 1992 for extensions of this model). This model successfully reproduced most of the p-nuclei in their solar system proportions, although it seriously underproduced the light p-nuclei. As in the hydrostatic oxygen burning model, it was necessary to superimpose several abundance distributions to get a realistic distribution of p-nuclei. Type II supernovae should naturally give a distribution of conditions depending on the layers of the proto-SN considered that would naturally give a distribution of abundances. The inner regions of the O/Ne shell will achieve the highest temperatures and thus get closest to NSE. These regions make the lighter p-nuclei. Outer regions produce the heavier p-nuclei because the "melting" is less complete. Prantzos et al (1990) computed the p-process abundance distribution for a specific model of supernova 1987A and found that a distribution of conditions naturally arose and gave a solar system p-process abundance distribution, except for underproduction of light p-nuclei.

The problems with the underproduction of the light p-nuclei in the gamma-process led Howard et al (1991) to consider the production of p-nuclei in the outermost layers of a carbon-oxygen white dwarf star suffering a type Ia supernova explosion. In this model, s-processing prior to the explosion built up the abundances of A 90 nuclei. The high density during the explosion then allowed proton capture reactions to produce many of the light p-nuclei while the normal gamma-process made the heavier p-nuclei. Later calculations using the realistic type Ia models of Khoklov (1990) have not been as successful in producing the light p-nuclei (Howard & Meyer 1992). More studies of this promising site are required.

Some p-nuclei may also be produced in spallation reactions. Most notably, neutrinos may spall neutrons from heavy nuclei during type II supernovae to make p-nuclei. Such a process is only likely to produce significant amounts of the rarest p-nuclei such as ¹³⁸La and ¹⁸⁰Ta (Woosley et al 1990).