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We studied the spectral features of Si II $\lambda\lambda$4130, 5972, 6355and S II W-trough for a large sample of Type Ia supernovae (SNe Ia). We findthat in NV (Normal-Velocity) subclass of SNe Ia, these features tend to reach amaximum line strength near maximum light, except for Si II $\lambda$5972.Spectral features with higher excitation energy, such as S II W-trough, arerelatively weak and have relatively low velocity. SNe Ia with larger$\Delta$m$_{15}$($B$) tend to have lower velocities especially at phases aftermaximum light. NV SNe show a trend of increasing line strength with increasing$\Delta$m$_{15}$($B$), while 91T/99aa-like SNe show an opposite trend. Nearmaximum light, the absorption depth of Si II $\lambda$5972 shows the strongestcorrelation with $\Delta$m$_{15}$($B$), while at early times the sum of thedepths of Si II $\lambda\lambda$4130 and 5972 shows the strongest correlationwith $\Delta$m$_{15}$($B$). The overall correlation between velocity and linestrength is positive, but within NV SNe the correlation is negative orunrelated. In normal SNe Ia, the velocity-difference and depth-ratio of alonger-wavelength feature to a shorter-wavelength feature tend to increase withincreasing $\Delta$m$_{15}$($B$). These results are mostly well explained withatomic physics, but some puzzles remain, possibly related to the effects of thesaturation, line competition or other factors.

A study of Si II and S II features in spectra of Type Ia supernova