The final fate of stars that ignite neon and oxygen off-center: electron capture or iron core-collapse supernova?

TL;DR

This paper investigates the off-center neon and oxygen burning in stars of 8.8-9.5 solar masses, exploring how flame propagation influences whether the star undergoes electron capture supernova or iron core-collapse.

Contribution

It provides detailed analysis of off-center neon-oxygen flame behavior and emphasizes the role of mixing and flame propagation in stellar core evolution.

Findings

Neon flame forms similarly to super-AGB carbon flames but with narrower width.

Convective mixing disrupts flame propagation, leading to shell flashes instead of core ignition.

Core contraction and electron fraction reduction influence the star's final collapse pathway.

Abstract

In the ONeMg cores of $8.8-9.5~{\rm M}_\odot$ stars, neon and oxygen burning is ignited off-center. Whether the neon-oxygen flame propagates to the center is critical to determine whether these stars undergo Fe core collapse or electron capture induced ONeMg core collapse. We present more details of stars that ignite neon and oxygen burning off-center. The neon flame is established in a similar manner to the carbon flame of super-AGB stars, albeit with a narrower flame width. The criteria for establishing a flame are able to be met if the strict Schwarzschild criterion for convective instability is adopted. Mixing across the interface of the convective shell disrupts the conditions for the propagation of the burning front and instead the shell burns as a series of inward-moving flashes. While this may not directly affect whether the burning will reach the center (as in super-AGB stars), the core is allowed to contract between each shell flash. Reduction of the electron fraction in the shell reduces the Chandrasekhar mass and the center reaches the threshold density for the URCA process to activate and steer the remaining evolution of the core. This highlights the importance of a more accurate treatment of mixing in the stellar interior for yet another important question in stellar astrophysics - determining the properties of stellar evolution and supernova progenitors at the boundary between electron capture supernova and iron core-collapse supernova.