Production of Silicon on Mass Increasing White Dwarfs -- Possible Origin of High-Velocity-Features in Type Ia Supernovae

TL;DR

This study suggests that silicon-rich layers formed on near-Chandrasekhar-mass white dwarfs through helium shell flashes could explain high-velocity features observed in early spectra of Type Ia supernovae, offering insights into their origin.

Contribution

The paper presents detailed nuclear yield calculations showing silicon production on accreting white dwarfs, linking surface silicon layers to observed supernova spectral features, which is a novel explanation for HVFs.

Findings

Silicon-rich layers form on white dwarfs prior to explosion.

Surface silicon layers are consistent with high-velocity features in spectra.

Silicon production depends on white dwarf mass and accretion rate.

Abstract

Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon-oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 $M_\odot$, 1.2 $M_\odot$, and 1.35 $M_\odot$ CO WDs accreting helium-rich matter with several mass-accretion rates ranging from $1 \times 10^{-7}~M_\odot$ yr$^{-1}$ to $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$. For the $1.35~M_\odot$ WD with the accretion rate of $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer developed as helium burning ash and consisted of 40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne by weight. For a higher mass accretion rate of $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer consisted of 58% $^{12}$C, 31% $^{24}$Mg, and 0.43% $^{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lower mass accretion rates (2% for $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$). No substantial silicon ($< 0.07%$) is produced on the $1.0~M_\odot$ WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si II HVFs, its characteristics are consistent with that of mass increasing WDs. We also discuss possible Ca production on very massive WDs ($ \gtrsim 1.38~M_\odot$).