Abstracts Category : Other

Evolution of Degenerate Oxygen-Neon Cores

by Heiko Mller

Mass-accreting electron-degenerate stellar cores that are composed primarily of the carbon-burning ashes 16-O and 20-Ne (ONe cores) appear in several astrophysical scenarios. On the one hand, they can be formed during the late evolution of intermediate-mass stars with 8 to 10 solar masses. On the other hand, they can occur in the context of the accretion-induced collapse (AIC) of ONe white dwarfs, where the collapse is induced by mass transfer from a companion star in a binary system or due to cooling of the outer layers of a white dwarf-white dwarf merger remnant. Their evolution is critically depending on electron capture (EC) reactions on nuclei with mass number A ~ 20 that become relevant above a density of 109 g/cm3. Besides removing electrons (the main pressure support) from the core, EC are also responsible for releasing or absorbing heat. In the canonical picture, the accreting ONe core will undergo compression until EC on the abundant 20-Ne are activated. As a consequence, the core becomes gravitationally unstable. Simultaneously, ECs release sufficient heat to ignite oxygen in a thermonuclear runaway, launching an outwards traveling deflagration wave. Nevertheless, the energy release from oxygen fusion is insufficient to halt gravitational collapse. Otherwise, the star would be destroyed by a thermonuclear explosion. Subsequent to collapse, a neutron star is formed and the stellar envelope explodes in an electron-capture supernova (ECSN).Recent 3D hydrodynamic simulations of the oxygen deflagration in ONe cores have suggested that the outcome of such events (either ECSNe or thermonuclear explosions) depends critically on the ignition density of oxygen. Depending on the treatment of convection, the ignition density is estimated to be ~ 2x1010 g/cm3 in case core convection sets in prior to ignition and ~ 1010 g/cm3, if not. It has been suggested that models corresponding to the first case lead to a collapse, while models corresponding to the second case result in a thermonuclear explosion of the star.We study accreting ONe cores in the AIC scenario, focusing on open questions regarding the evolution of the core prior to ignition. By including the secondary carbon-burning products 23-Na and 25-Mg in the initial models, new insights can be gained concerning Urca cooling. While it seems well established that EC processes do not trigger convection in the ONe core, the poorly understood phenomenon of overstable convection could alter this picture and will be assessed by us, in detail. Furthermore, modifications at high densities to the standard set of nuclear reactions, responsible for neon and oxygen burning, are investigated. Previously, reaction channels that become possible due to the presence of 20-O, formed by double-EC on 20-Ne, have not been considered. Neon burning is modified by the reaction 20-O(a,g)24-Ne and oxygen burning can additionally proceed by the fusion involving neutron-rich oxygen isotopes: 20-0 + 16/20-O -> 36/40-S*. In neither case experimental data is available. FurtherAdvisors/Committee Members: Martnez-Pinedo, Gabriel (advisor), Langanke, Karlheinz (advisor).