⁴¹Ca in presolar graphite of supernova origin

Five low-density graphite grains from the Murchison meteorite have large ⁴¹K excesses that can be attributed to the decay of ⁴¹Ca. The inferred ⁴¹Ca/⁴⁰Ca ratios, which range up to 1.7 × 10^-2, and other isotopic signatures, ¹⁵N, ¹⁸O, and ²³Si excesses and high ²⁶Al/²⁷Al ratios, indicate a Type II supernova origin for these grains and are evidence for heterogeneous mixing of supernova ejecta. ⁴¹Ca seems to mainly originate from neutron capture in the C-rich and O-rich supernova zones, with only minor contributions from the O-burning zone. Admixture of material from the O-rich zone to outer C-rich layers is required to explain the abundance patterns of the stable Ca isotopes. In contrast, the Ti isotopic compositions reflect that of the C-rich zone and do not require substantial amounts of material from the O-rich zone.