G-Mode Excitation During the Pre-explosive Simmering of Type Ia Supernovae

TL;DR

This paper investigates how g-mode oscillations excited by convection in a white dwarf prior to a Type Ia supernova can lead to surface helium ignition, influencing observable features and progenitor scenarios.

Contribution

It estimates g-mode excitation and breaking in white dwarfs, linking surface helium ignition to supernova features and progenitor differences.

Findings

G-modes grow and break near the surface, depositing energy.

Helium ignition occurs, producing specific isotopes like 28Si and 44Ti.

High velocity features in supernovae may originate from this process.

Abstract

Prior to the explosive burning of a white dwarf (WD) that makes a Type Ia supernova (SN Ia), the star "simmers" for ~10^3 yrs in a convecting, carbon burning region. I estimate the excitation of g-modes by convection during this phase and explore their possible affect on the WD. As these modes propagate from the core of the WD toward its surface, their amplitudes grow with decreasing density. Once the modes reach nonlinear amplitudes, they break and deposit their energy into a shell of mass ~10^{-4}M_\odot. This raises the surface temperature by 6*10^8 K, which is sufficient to ignite a layer of helium, as is expected to exist for some SN Ia scenarios. This predominantly synthesizes 28Si, 32S, 40Ca, and some 44Ti. These ashes are expanded out with the subsequent explosion up to velocities of ~20,000 km/s, which may explain the high velocity features (HVFs) seen in many SNe Ia. The appearance of HVFs would therefore be a useful discriminant for determining between progenitors, since a flammable helium-rich layer will not be present for accretion from a C/O WD as in a merger scenario. I also discuss the implications of 44Ti production.