Neutrino Processes and Pair Formation in Massive Stars and Supernovae.

The present paper traces the physical properties of matter inside highly evolved stars, on the assumption that the whole material of the star is non-degenerate and that the star is in quasi-hydrostatic equilibrium. When these conditions are satisfied, the physical evolution of a particular element of material is insensitive to the stellar model but not to the total mass of the star. Our considerations refer explicitly to stars of mass greater than 10 M0 but less than 106 M0, at which point general relativistic considerations become paramount. In Parts I and II neutrino-loss processes and neutrino-loss rates are examined. We conclude that e + e+ H p + p is the most important neutrino process in massive stars. In Part III a method is developed for calculating the product j as a function of temperature when electron-positron pair formation is taken into account. In this product is the mean molecular weight and p the ratio of gas pressure to gas plus radiation pressure. The results are used to derive relations of the form p (Mo/M)112T3 for massive stars. In Part IV we consider the internal energy of matter, again as a function of temperature, and including the effects of pair formation. Parts V, VI, and VII are concerned with nuclear reactions, in particular with oxygen burning, the a-process, and the e-process. In the final Parts VIII and Ix we consider the onset of a supernova of Type II in which the central core im lodes while the mantle and envelope of the star explode. These considerations are tentative be- cause t e discussion now involves the structure of the whole star, and hence of the stellar model. It is emphasized that massive stars do not necessarily become Type II supernovae but can collapse to general relativistic singularities. In the case that some form of braking mechanism, such as rotation, internal turbulence, or an entrained magnetic field, leads to core implosion followed by mantle-envelope explosion, our two main conclusions are as follows: 1. Although neutrino losses greatly speed up evolution when the temperature exceeds 10 K, the loss rate is not sufficient to produce a free-fall implosion. Free fall must await the phase change of iron group nuclei first to helium and free neutrons and finally to free protons and neutrons. Up to that point nuclear reactions which transform hydrogen into the most stable nuclei near iron are exoergic and supply the energy lost through radiation and neutrino rocesses. 2. Burbidge, Burbidge, Fowler, andHoyle 1957) showed that the observed relative abundance of the iron group nuclei could be understood in terms of an equilibrium process, provided two parameters were appropriately chosen-the temperature and the ratio of the densities of free neutrons and protons. Other choices for these parameters did not lead to a satisfactory correspondence with the observed abundances. In this early work, no explanation could be given of why the two parameters should take the values necessary to explain the observed abundances. In Part VII we arrive at an e lanation in terms of the evolution time scale set by neutrino losses due to pair annihilation. We conclu e in part: The terrestrial iron-group isotopic ahundance ratios strongly indicate the operation in massive stars of an energy-loss mechanism having a loss rate of the same order of magnitude as that calculated for e+ + e H p + p on the basis of the universal Fermi interaction strength. Detailed theoretical derivations and numerical results have been relegated to three appendices. Appendix A treats beta-interaction rates under stellar conditions, Appendix B treats the effects of electron- positron pair formation on stellar structure and evolution, while Appendix C presents a summary of current estimates concerning nuclear-reaction rates.