title: Weak Interactions in Degenerate Oxygen-Neon Cores
Creators: name: Fahlin Strömberg, Dag Isak August
Publisher: name: UNSPECIFIED
Year: 2020

description: 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 grows 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 &lt; 1 GK ) at this stage only low-lying states ( E &lt; 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 relating it to one of the non-zero matrix elements via the conserved vector current (CVC) theory. We benchmark our approach against the second-forbidden non-unique beta decay of 36Cl and 24Na. For 36Cl our predicted rate is more than a factor five too large, but our theoretical spectrum agrees well with experiment if the CVC relation is used. There is no experimental spectrum available for 24Na but our calculated rate is within 50% of the measured value. To constrain the forbidden transition between 20 Ne and 20 F we collaborated with experimentalists who measured the high-energy tail of the 20 F decay spectrum. In this particular decay the important nuclear matrix elements are essentially constrained by the CVC relation and the analogue gamma decay in 20Ne. The resulting spectrum agrees with the 20F measurement within the experimental uncertainties. We find that the forbidden transition is quite close to its previously known upper limit, increasing the capture rate on 20Ne by several orders of magnitude in a critical density range. To evaluate the impact of the new rate we use the stellar evolution code MESA. The forbidden transition tends to reduce the ignition density and push the ignition away from the centre. We demonstrate that the off-centre ignition is due to the forbidden transition slowly depleting 20Ne in the centre of the core, ultimately leading to outer regions with more 20Ne left heating at a higher rate. We also calculate the strength of two additional forbidden transitions: between 24Na and 24Ne and between 27Al and 27Mg. We find that the 24Na/24Ne transition only has a minor impact on the ignition conditions. However, it may have an impact on convective stability and could be important for cores with substantial residual carbon. The transition between 27Al and 27Mg does not seem to have any significant effects.