The quasi-static evolution of ONeMg cores - Explosive ignition densities and the collapse/explosion alternative

The quasi-static evolution of an electron-degenerate ONeMg core is followed up to the point of explosive Ne-O ignition triggered by electron capture. Recently calculated electron-capture rates on 24Mg, 24Na, 20Ne, and 20F are used, and the ignition density is found to be about 8.5 x 10 exp 9 g/cu cm (when the Ledoux criterion for convection is adopted), instead of about 10 exp 10 g/cu cm as in earlier calculations. Such a comparatively low ignition density should lead to complete explosive disruption of the core even if the thermonuclear burning propagated all the way as a conductive, laminar flame. The dependence of the ignition density on the treatment adopted for mixing in the semiconvective region and the astronomical implications of our results are briefly discussed.