Production of Silicon on Mass-increasing White Dwarfs: Possible Origin of High-velocity Features in Type Ia Supernovae

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 _⊙, 1.2 M _⊙, and 1.35 M _⊙ CO WDs accreting helium-rich matter with several mass-accretion rates, ranging from 1 ×10^-7 M _⊙ yr^-1 to 7.5 ×10^-7 M _⊙ yr^-1. For the 1.35 M _⊙ WD with the accretion rate of 1.6 ×10^-7 M _⊙ yr^-1, the surface layer developed as helium burning ash and consisted of 40% ²⁴Mg, 33% ¹²C, 23% ²⁸Si, and a few percent of ²⁰Ne by weight. For a higher mass-accretion rate of 7.5 ×10^-7 M _⊙ yr^-1, the surface layer consisted of 58% ¹²C, 31% ²⁴Mg, and 0.43% ²⁸Si. For the 1.2 M _⊙ WDs, silicon is produced only for lower mass-accretion rates (2% for 1.6 ×10^-7 M _⊙ yr^-1). No substantial silicon (<0.07%) is produced on the 1.0 M _⊙ 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 (1.38 M _⊙).