tokens: include in our list lines with intensity values that are above some fraction (typically 50%) of the maximum value. Of course, the choice of this threshold is arbitrary, but we find it works well for our needs, and can be easily modified as needed. In a companion paper (A. Gal-Yam, O. Yaron N. Knezevic 2019, in preparation), we describe in more detail our survey of other methods to compile line lists from the NIST databases and other tests and applications of this method. With the line lists in hand, we attempt to determine for each spectrum the expansion velocity by matching features to ions with numerous transitions (see. e.g., Figure 1 ). Once we have determined a convincing value for the expansion velocity, we inspect other ions using the fixed velocity (i.e., we assume explicitly that the photosphere is narrow). Figure 1. High S/N red spectra of the nearby SLSN-I PTF12dam (left; from Nicholl et al. 2013 ) and Gaia16apd (right; from Yan et al. 2017b ). Both spectra (black curves) have been binned and host narrow emission lines have been excised; the original spectra are shown in blue. Uncorrected Telluric features in the spectrum of PTF12dam are marked. The spectrum of PTF12dam shows pronounced lines of O i and C ii with distinct sharp absorption troughs, all at the same expansion velocity of V exp = 11,000 km s −1 ; O ii lines probably begin to shape the spectrum blueward of 5000 Å while C iii may also contribute. The same is true for Gaia16apd (right) though absorption lines are of lower contrast and have higher expansion velocity, V exp = 14,000 km s −1 . 2.1. Application to Peak Spectra of SLSNe-I We begin our examination of SLSN-I peak spectra by inspecting high signal-to-noise ratio (S/N) spectra of nearby events. In particular, we begin by focusing on the red side of the optical range ( λ > 5000 Å). This wavelength range has been relatively neglected in studies of SLSN-I spectra as it does not contain strong features, and is redshifted out of visible-light spectra of SLSNe at moderate or high redshifts, which are typical for SLSNe. Figure 1 (left) shows our analysis results for a high S/N spectrum of PTF12dam from Nicholl et al. ( 2013 ) obtained on 2012 May 24, 25 restframe days prior to peak according to the analysis of Vreeswijk et al. ( 2017 ). This and all other spectra discussed here are available from WISeREP (Yaron Gal-Yam 2012 ). 5 5 http://wiserep.weizmann.ac.il/ Multiple features consistent with O i and C ii lines all align using a single expansion velocity of V exp = 11,000 km s −1 ; these two ions explain most features in this wavelength range. C iii may explain two weak features, and may also contribute to the observed spectrum. O ii has a distinct line spectrum with a multitude of lines all with very similar intensities. While a few single lines seen around 6500 Å appear to be negligible compared to the stronger C ii lines, accumulations of multiple overlapping O ii lines begin to shape the spectrum bluewards of 5000
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