Electron capture supernova (ECSN)

collapse or explode,

thats the question...

Stars with a mass from 8 - 10 solar mass develop a massive O-Ne-Mg degenerate core close to the Chandrasekhar mass (about ~1.37 solar mass). The central density can reach as high as 1e10 g/cc, where electron capture of isotopes including Ne-20 and Mg-24 takes place. The gamma ray released during electron capture can trigger the nuclear runaway (flame) of O-Ne rich matter.

Whether an ECSN collapses and forms a neutron star (e.g. Crab) or explodes like a Type Iax supernova, is unclear due to the competition of multi-dimensional physical processes (e.g., turbulent flame) in the star. Major competition happens between:

Energy input by nuclear reaction
Energy loss by electron capture and its neutrino loss

My research includes using the realistic stellar evolution models of O-Ne-Mg core and studying their final fate by multi-dimensional simulations. Based on the most updated physics and stellar evolution models, I show that the majority of ECSN models collapses and forms neutron stars. But the details are sensitive to the input physics.

Evolution of the central density of the O-Ne-Mg core after nuclear deflagration has started. The two models have very similar initial masses but their final fate bifurcates. The model with a slightly higher mass collapses while the lower mass one expands. It shows the sensitivity of the final fate on the input physics

Explodes like a Type-Iax supernova?

When energy production by the O+Ne deflagration dominates the process, the star enters direct expansion similar to a sub-luminous Type Ia(x) supernova. The lack of detonation transition makes the white dwarf partially disrupted. The synthesized elements may mix with the O+Ne rich matter and are ejected. A O+Si+Fe white dwarf leaves behinds as the final remnant.

Collapses into a neutron star?

When electron capture dominates the process, the core contract due to the drop of the degeneracy pressure. The flame (deflagration) cannot freely propagate. As a result, the flame is confined to the core until the collapse starts. Notice in the figure the hot zone (red -- burnt by deflagration) concentrated in the very inner part of the star.

Effects of O-Ne-Mg core mass

The exact mass of the O-Ne-Mg core and its corresponding central density, when the flame starts, is a matter of debate. It is subject to a number of uncertainties, namely from input physics.

Depending on their strength, they may change the initial runaway density from (in log10 scale) 9.95 to 10.2 (g/cc) (Corresponding to a mass of 1.373 - 1.384 solar mass O-Ne-Mg core). In this figure I show how the initial runaway density leads to completely different results. For slightly lower runaway density (log10 rho = 9.85), the central density goes down (star expands). A slightly higher runaway density leads to central density going up (star collapses).

Effects of initial flame size

With a more efficient convection, the temperature profile may be flattened and approaches adiabatic. The initial flame can be smeared instead of a local instantaneous runaway. In this figure I show the central density evolution of O-Ne-Mg cores at the same central density (i.e. mass) but different initial flame. We can see that the core with a larger initial flame expands, but those with a smaller flame collapses.

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