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 top of 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 _ { \sun } , 1.2 M _ { \sun } , and 1.35 M _ { \sun } CO WDs accreting helium-rich matter with several mass-accretion rates ranging from 1 \times 10 ^ { -7 } ~ { } M _ { \sun } yr ^ { -1 } to 7.5 \times 10 ^ { -7 } ~ { } M _ { \sun } yr ^ { -1 } .
For the 1.35 ~ { } M _ { \sun } WD with the accretion rate of 1.6 \times 10 ^ { -7 } ~ { } M _ { \sun } 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 _ { \sun } yr ^ { -1 } , the surface layer consisted of 58 % ^ { 12 } C , 31 % ^ { 24 } Mg , and 0.43 % ^ { 28 } Si .
For the 1.2 ~ { } M _ { \sun } WDs , silicon is produced only for lower mass accretion rates ( 2 % for 1.6 \times 10 ^ { -7 } ~ { } M _ { \sun } yr ^ { -1 } ) .
No substantial silicon ( < 0.07 \% ) is produced on the 1.0 ~ { } M _ { \sun } WD independently of the mass-accretion rate .
If the silicon-rich surface layer is the origin of \ion Si2 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 _ { \sun } ) .