  Several circumstantial arguments point to the formation of the third
r-process peak at A about 190, near platinum, in stars of mass of about 8-10
solar masses: 1) The delayed production of europium with respect to iron
imposes a time scale that restricts the progenitor stars to less than about 10
solar masses; 2) the r-process demands a dominant robust mechanism at least for
barium and above, since the relative abundance pattern of those r-process
elements in low-metallicity stars is consistent with the solar pattern; 3)
stars of about 8-10 solar masses produce nearly identical degenerate O/Ne/Mg
cores that collapse due to 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. The prompt explosion of an O/Ne/Mg core
yields low entropy and low electron fraction, and hence may produce a
reasonable r-process peak at A about 190 as well as all of the r-process
elements with Z greater than 56. The possible differences in the
neutrino-driven wind and associated r-process due to the low-mass neutron stars
expected in this mass range are also discussed.
