Some effects of Urca reactions upon degenerate carbon burning

The ignition of 12C in degenerate cores is studied using a realistic reaction network. Convection is taken into account, and Urca neutrino losses are calculated explicitly. Urca losses from inhibit 12C detonation during a temporary hydrostatic stage. Collapse is initiated by the increase of e caused by 12C burning with subsequent electron captures. A significant fraction of the initial 12C remains at the time the core evolution becomes rapid. These results are insensitive to the exact rate of the 12C + 12C reaction; they apply to both Population I and Population II objects. Sitbject headings: nuclear reactions - stellar evolution - supernovae