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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-richlayer on a carbon-oxygen (CO) core before it explodes as an SN Ia. Wecalculated 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-richmatter with several mass-accretion rates ranging from $1 \times10^{-7}~M_\odot$ 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 of40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne byweight. 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, and0.43% $^{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lowermass accretion rates (2% for $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$). Nosubstantial silicon ($< 0.07%$) is produced on the $1.0~M_\odot$ WDindependently of the mass-accretion rate. If the silicon-rich surface layer isthe origin of Si II HVFs, its characteristics are consistent with that of massincreasing WDs. We also discuss possible Ca production on very massive WDs ($\gtrsim 1.38~M_\odot$).

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