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 (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 angstrom in type. I spectra are due to more than Si II lambda 6355. However, while an interpretation of conspicuous Si II lambda 6355 can approximate 6150 angstrom absorption features for all SNe Ia during the first month of free expansion, similar identifications applied to 6250 angstrom features of SNe Ib and Ic have not been as successful. When the corresponding synthetic spectra are compared with high-quality 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 angstrom; 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 II lambda 6355 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 angstrom features are possibly influenced by either trace amounts of hydrogen or blueshifted absorption and emission in H alpha, the latter being an effect which is frequently observed in the spectra of hydrogen-rich, SNe II.