Neutronization During Type Ia Supernova Simmering

TL;DR

This paper investigates how neutronization during the simmering phase of a white dwarf prior to a Type Ia supernova affects the neutron excess, highlighting a baseline level influenced by carbon burning that can overshadow metallicity effects.

Contribution

It introduces the concept that neutronization during simmering sets a neutron excess floor, impacting supernova properties beyond metallicity dependence.

Findings

Neutronization during simmering adds to metallicity effects in neutron excess.

A neutronization floor exists, especially at low metallicities.

Neutronization effects can mask metallicity correlations in supernova observations.

Abstract

Prior to the incineration of a white dwarf (WD) that makes a Type Ia supernova (SN Ia), the star "simmers" for ~1000 years in a convecting, carbon burning region. We have found that weak interactions during this time increase the neutron excess by an amount that depends on the total quantity of carbon burned prior to the explosion. This contribution is in addition to the metallicity (Z) dependent neutronization through the 22Ne abundance (as studied by Timmes, Brown, & Truran). The main consequence is that we expect a floor to the level of neutronization that dominates over the metallicity contribution when Z/Z_\odot<2/3, and it can be important for even larger metallicities if substantial energy is lost to neutrinos via the convective Urca process. This would mask any correlations between SN Ia properties and galactic environments at low metallicities. In addition, we show that recent observations of the dependences of SNe Ia on galactic environments make it clear that metallicity alone cannot provide for the full observed diversity of events.