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The r-Process in Collapsing O/Ne/Mg Cores

Several circumstantial arguments point to the formation of the third r-process peak at A~190, near platinum, in stars of mass of ~8-10 M☉: (1) the delayed production of europium with respect to iron imposes a timescale that restricts the progenitor stars to M≲10 M; (2) the r-process demands a dominant robust mechanism at least for Z≥56, barium, and above, since the relative abundance pattern of those r-process elements in the low-metallicity stars, [Fe/H] < -2, is consistent with the solar pattern; (3) stars of ~8-10 M☉ produce nearly identical degenerate O/Ne/Mg cores that collapse because of electron capture; and (4) the resulting low-mass cores may produce both an r-process in a prompt explosion and a subsequent r-process in a neutrino-driven wind. A special case of the r-process singles out low entropies for initial Ye ~ Z̄/A, where Z̄ is the mean atomic number of the seed nuclei and A is the atomic weight of the target. For Z̄ ≃ 35 and Ye≃0.18, the A≃190 peak results in a natural way. The prompt explosion of an O/Ne/Mg core yields a low entropy, S~15, and a low electron fraction, Ye≃0.2, and hence may produce a reasonable r-process peak at A≃190, as well as all of the r-process elements with Z≥56. The possible differences in the ν-driven wind and associated r-process due to the low-mass neutron stars expected in this mass range are also discussed.