Line Identifications of Type I Supernovae: On the Detection of Si II for these Hydrogen-poor Events

Here we revisit line identifications of type I supernovae and highlight trace amounts of unburned hydrogen as an important free parameter for the composition of the progenitor. Most 1-dimensional stripped-envelope models of supernovae indicate that observed features near 6000-6400 Ang in type I spectra are due to more than Si II 6355. However, while an interpretation of conspicuous Si II 6355 can approximate 6150 Ang absorption features for all type Ia supernovae during the first month of free expansion, similar identifications applied to 6250 Ang features of type Ib and Ic supernovae have not been as successful. When the corresponding synthetic spectra are compared to high quality time-series observations, the computed spectra are frequently too blue in wavelength. Some improvement can be achieved with Fe II lines that contribute red-ward of 6150 Ang, however the computed spectra either remain too blue, or the spectrum only reaches 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 II 6355 in the spectra of broad-lined 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 Ang features are possibly influenced by either trace amounts of hydrogen, or blue-shifted absorption and emission in Halpha, the latter being an effect which is frequently observed in the spectra of hydrogen-rich, type II supernovae.