A study of Si ii and S ii features in spectra of Type Ia supernovae

Abstract

ABSTRACT We studied the spectral features of Si ii λλ4130, 5972, 6355 and the S ii W-trough for a large sample of Type Ia supernovae (SNe Ia). We found that in the NV (normal-velocity) subclass of SNe Ia, these features tend to reach a maximum line strength near maximum light, except for Si ii λ5972. Spectral features with higher excitation energy, such as the S ii W-trough, are relatively weak and have a relatively low velocity. SNe Ia with larger Δm15(B) tend to have lower velocities, especially at phases after maximum light. NV SNe show a trend of increasing line strength with increasing Δm15(B), while 91T/99aa-like SNe show an opposite trend. Near maximum light, the absorption depth of Si ii λ5972 shows the strongest correlation with Δm15(B), while at early times the sum of the depths of Si ii λλ4130 and 5972 shows the strongest correlation with Δm15(B). The overall correlation between velocity and line strength is positive, but within NV SNe the correlation is negative or unrelated, depending on the phase. In normal SNe Ia (including the high-velocity SNe), the velocity difference and depth ratio of a longer-wavelength feature to a shorter-wavelength feature tend to increase with increasing Δm15(B). These results are mostly well explained with atomic physics, but some puzzles remain, possibly related to the effects of saturation, line competition or other factors.