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.