Neutronization During Carbon Simmering In Type Ia Supernova Progenitors

TL;DR

This study uses detailed stellar modeling to quantify neutronization during the carbon simmering phase of Type Ia supernova progenitors, revealing that progenitor metallicity primarily determines neutron excess in the supernova fuel.

Contribution

It provides the first comprehensive calculation of neutron excess during simmering, accounting for full density and temperature profiles, refining previous estimates.

Findings

Neutron excess from simmering is about 3 x 10^{-4}.

Progenitor metallicity dominates neutron excess for Z > 1/3 Z_sun.

Neutron excess sets a metallicity-dependent floor at lower metallicities.

Abstract

When a Type Ia supernova (SN Ia) progenitor first ignites carbon in its core, it undergoes ${\sim} \,10^{3}-10^{4} \,$yr of convective burning prior to the onset of thermonuclear runaway. This carbon simmering phase is important for setting the thermal profile and composition of the white dwarf. Using the \texttt{MESA} stellar evolution code, we follow this convective burning and examine the production of neutron-rich isotopes. The neutron content of the SN fuel has important consequences for the ensuing nucleosynthesis, and, in particular, for the production of secondary Fe-peak nuclei like Mn and stable Ni. These elements have been observed in the X-ray spectra of SN remnants like Tycho, Kepler, and 3C 397, and their yields can provide valuable insights into the physics of SNe Ia and the properties of their progenitors. We find that weak reactions during simmering can at most generate a neutron excess of ${\approx} \, 3 \times 10^{-4}$. This is ${\approx} \, 8 \times 10^{-4}$ lower than that found in previous studies that do not take the full density and temperature profile of the simmering region into account. Our results imply that the progenitor metallicity is the main contributor to the neutron excess in SN Ia fuel for $Z \gtrsim 1/3 \, Z_{\odot}$. Alternatively, at lower metallicities, this neutron excess provides a floor that should be present in any centrally-ignited SN~Ia scenario.