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

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 yr in a convecting, carbon-burning region. I estimate the excitation of g-modes by convection during this phase and explore their possible effect 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_⊙. This raises the surface temperature by ≈4 × 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 ^(40)Ca, but some amount of ^(28)Si, ^(32)S, and ^(44)Ti may also be present. These ashes are expanded out with the subsequent explosion up to velocities of ~20, 000 km s^(–1), 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 ^(44)Ti production.