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We present a simple and well defined prescription to compare absorption linesin supernova (SN) spectra with lists of transitions drawn from the NationalInstitute of Standards and Technology (NIST) database. The method is designedto be applicable to simple spectra where the photosphere can be mostlydescribed by absorptions from single transitions with a single photosphericvelocity. These conditions are plausible for SN spectra obtained shortly afterexplosion. Here we show that the method also works well for spectra ofhydrogen-poor (Type I) superluminous supernovae (SLSNe-I) around peak. Analysisof high signal to noise spectra leads to clear identification of numerousspectroscopic features arising from ions of carbon and oxygen, that account forthe majority of absorption features detected in the optical range, suggestingthe outer envelope of SLSN-I progenitors is dominated by these elements. Wefind that the prominent absorption features seen in the blue are dominated bynumerous lines of OII, as previously suggested, and that the apparentabsorption feature widths are dominated by line density and not by dopplerbroadening. In fact, we find that while the expansion velocities of SLSNe-Iaround peak are similar to those of normal SNe, the apparent velocitydistribution (manifest as the width of single transition features) is muchlower (~1500 km/s) indicating emission from a very narrow photosphere invelocity space that is nevertheless expanding rapidly. We inspect thecontroversial case of ASASSN-15lh, and find that the early spectrum of thisobject is not consistent with those of SLSNe-I. We also show that SLSNe thatinitially lack hydrogen features but develop these at late phases, such asiPTF15esb and iPTF16bad, also differ in their early spectra from standardSLSNe-I.

A simple analysis of Type I superluminous supernova peak spectra: composition, expansion velocities, and dynamics