Abstract:
Here we revisit line identifications of type I supernovae (SNe I) and highlight trace amounts of unburned hydrogen
as an important free parameter for the composition of the progenitor. Most one-dimensional stripped-envelope
models of supernovae indicate that observed features near 6000–6400 Å in typeI spectra are due to more than
Si IIλ6355. However, while an interpretation of conspicuous Si IIλ6355 can approximate 6150 Å absorption
features for all SNe Ia during the first month of free expansion, similar identifications applied to 6250 Å features of
SNe Ib and Ic have not been as successful. When the corresponding synthetic spectra are compared with highquality timeseries observations, the computed spectra are frequently too blue in wavelength. Some improvement
can be achieved with Fe II lines that contribute redward of 6150 Å; however, the computed spectra either remain
too blue or the spectrum only reaches a fair agreement when the rise-time to peak brightness of the model conflicts
with observations by a factor of two. This degree of disagreement brings into question the proposed explosion
scenario. Similarly, a detection of strong Si IIλ6355 in the spectra of broadlined Ic and super-luminous events of
type I/R is less convincing despite numerous model spectra used to show otherwise. Alternatively, we suggest
6000–6400 Å features are possibly influenced by either trace amounts of hydrogen or blueshifted absorption and
emission in Hα, the latter being an effect which is frequently observed in the spectra of hydrogen-rich, SNe II.
