Weak Interactions in Degenerate Oxygen-Neon Cores

Dag Isak August Fahlin Strömberg
Stars with an initial mass of roughly seven to eleven times the mass of our sun are known as intermediate-mass stars. They go through central hydrogen, helium and carbon burning before entering their Super-AGB phase. At this point they have a degenerate oxygen-neon core in their centre consisting mostly out of 16O and 20Ne, with smaller amount of other nuclei such as 23Na, 24Mg, 25Mg and 27Al. Mass is added to the core through thermal pulses, causing it to contract. In some cases the density
more » ... ws to a point where electron capture processes are triggered. Alternatively, an oxygen-neon white dwarf is formed that can reach similar high densities by accreting mass from a binary companion. Eventually heating from the double electron capture 20Ne(e^− , ν_e )20F(e^− ,ν_e)20O triggers a runaway oxygen burning. Known as an electron-capture supernova, such an event results in either a collapse to a neutron star or a thermonuclear explosion with an oxygen-neon-iron white dwarf remnant. The outcome depends, among other things, on the conditions in the core when ignition occurs. In particular, if the central density is larger than a certain critical value the core is believed to collapse. In this work we focus on weak interaction rates in the pre-ignition phase. Due to the relatively low temperatures ( T < 1 GK ) at this stage only low-lying states ( E < 100 keV ) are thermally populated. The rates are thus fully determined by the small set of transitions involving these states. Typically only allowed transitions are considered, but it has been shown that the second-forbidden non-unique transition between the ground states of 20Ne and 20F might have a significant impact on the rate. We seek to constrain the rate of any relevant forbidden transitions and evaluate their effect on the evolution of the core. We use shell model calculations to determine the relevant nuclear matrix elements. Due to cancellations this approach results in one of the matrix elements being identically zero. We get a more realistic value by relat [...]
doi:10.25534/tuprints-00013302 fatcat:57af5zhqwfbk5kkw7tlyg3qyaq