ELECTRON - CAPTURE SUPERNOVAE AS ORIGIN OF 48 Ca

We report that electron-capture supernovae (ECSNe), arising from collapsing oxygen-neonmagnesium cores, are a possible source of Ca, whose origin has remained a long-standing puzzle. Our two-dimensional, self-consistent explosion model of an ECSN predicts ejection of neutron-rich matter with electron fractions Ye ≈ 0.40–0.42 and relatively low entropies, s ≈ 13–15 kB per nucleon (kB is the Boltzmann constant). Post-processing nucleosynthesis calculations result in appreciable production of Ca in such neutron-rich and low-entropy matter during the quasi-nuclear equilibrium and subsequent freezeout phases. The amount of ejected Ca can account for that in the solar inventory when we consider possible uncertainties in the entropies. ECSNe could thus be a site of Ca production in addition to a hypothetical, rare class of high-density Type Ia supernovae. Subject headings: nuclear reactions, nucleosynthesis, abundances — stars: abundances — supernovae: general