Impact of forbidden electron capture in intermediate-mass stars

Intermediate-mass stars (7-11 M⊙) form degenerate ONe cores following carbon burning. In some cases the cores grow dense enough to trigger electron capture on various nuclei. The double electron capture 20Ne(e-, v e)20F(e-, νe )20O releases enough heat to trigger runaway oxygen burning. Depending on the conditions at ignition the outcome is either a collapse to a neutron star or a thermonuclear explosion with an ONeFe remnant. We have investigated the impact of two forbidden transitions on the ignition conditions using the MESA stellar evolution code. The 0+ → 2+ transition between the ground states of 20Ne and 20F has been measured experimentally and is found to decrease the ignition density while pushing the point of ignition off-centre. We have also studied the 2+ → 4+ transition between 24Na and 24Ne through a shell model calculation. Its impact is found to be marginal in our simulations, but it might play a role in the development of convectional instabilities.