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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-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$).
High-velocity features (HVFs) are spectral features in Type Ia supernovae (SNe Ia) that have minima indicating significantly higher (by greater than about 6000 km/s) velocities than typical photospheric-velocity features (PVFs). The PVFs are absorpti
The carbon-oxygen white dwarf (CO WD) + He star channel has been thought to be one of the promising scnarios to produce young type Ia supernovae (SNe Ia). Previous studies found that if the mass-accretion rate is greater than a critical value, the He
The high-velocity features (HVFs) in optical spectra of type Ia supernovae (SNe Ia) are examined with a large sample including very early-time spectra (e.g., t < -7 days). Multiple Gaussian fits are applied to examine the HVFs and their evolutions, u
Due to the increasing number of observations Type Ia supernovae are nowadays regarded as a heterogeneous class of objects consisting of several subclasses. One of the largest of these is the class of Type Iax supernovae (SNe Iax) which have been sugg
High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here we show that the hot, convec