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

Abstract

We studied the spectral features of Si II $\lambda \lambda$4130, 5972, 6355 and S II W-trough for a large sample of Type Ia supernovae (SNe Ia). We find that in NV (Normal-Velocity) subclass of SNe Ia, these features tend to reach a maximum line strength near maximum light, except for Si II $\lambda$5972. Spectral features with higher excitation energy, such as S II W-trough, are relatively weak and have relatively low velocity. SNe Ia with larger $\Delta$m${}_{15}$($B$) tend to have lower velocities especially at phases after maximum 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. Near maximum light, the absorption depth of Si II $\lambda$5972 shows the strongest correlation with $\Delta$m${}_{15}$($B$), while at early times the sum of the depths of Si II $\lambda \lambda$4130 and 5972 shows the strongest correlation with $\Delta$m${}_{15}$($B$). The overall correlation between velocity and line strength is positive, but within NV SNe the correlation is negative or unrelated. In normal SNe Ia, the velocity-difference and depth-ratio of a longer-wavelength feature to a shorter-wavelength feature tend to increase with increasing $\Delta$m${}_{15}$($B$). These results are mostly well explained with atomic physics, but some puzzles remain, possibly related to the effects of the saturation, line competition or other factors.

14 pages, 13 figures. Accepted for publication in MNRAS

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