Impact Of Forbidden Electron Capture In Intermediate-Mass Stars

Intermediate-mass stars (7-11 M-circle dot) 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 Ne-20(e(-), nu(e))F-20(e(-), nu(e))O-20 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 Ne-20 and F-20 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 Na-24 and Ne-24 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.