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

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 $1 \times 10<sup>{-7}~M_\odot$ yr$<sup>{-1}$ to $7.5 \times 10<sup>{-7}~M_\odot$ yr$<sup>{-1}$. For the $1.35~M_\odot$ WD with the accretion rate of $1.6 \times 10<sup>{-7}~M_\odot$ yr$<sup>{-1}$, the surface layer developed as helium burning ash and consisted of 40% $<sup>{24}$Mg, 33% $<sup>{12}$C, 23% $<sup>{28}$Si, and a few percent of $<sup>{20}$Ne by weight. For a higher mass accretion rate of $7.5 \times 10<sup>{-7}~M_\odot$ yr$<sup>{-1}$, the surface layer consisted of 58% $<sup>{12}$C, 31% $<sup>{24}$Mg, and 0.43% $<sup>{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lower mass accretion rates (2% for $1.6 \times 10<sup>{-7}~M_\odot$ yr$<sup>{-1}$). No substantial silicon ($&lt; 0.07%$) 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 ($\gtrsim 1.38~M_\odot$).