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Doublets and Double Peaks: Late-Time [O I] 6300, 6364 Line Profiles of Stripped-Envelope, Core-Collapse Supernovae

We present optical spectra of SN 2007gr, SN 2007rz, SN 2007uy, SN 2008ax, and SN 2008bo obtained in the nebular phase when line profiles can lead to information about the velocity distribution of the exploded cores. We compare these to 13 other published spectra of stripped-envelope core-collapse supernovae (Type IIb, Ib, and Ic) to investigate properties of their double-peaked [O I] 6300, 6364 emission. These 18 supernovae are divided into two empirical line profile types: (1) profiles showing two conspicuous emission peaks nearly symmetrically centered on either side of 6300 Angstroms and spaced approximately 64 Angstroms apart, close to the wavelength separation between the [O I] 6300, 6364 doublet lines, and (2) profiles showing asymmetric [O I] line profiles consisting of a pronounced emission peak near 6300 Angstroms plus one or more blueshifted emission peaks. Examination of these emission profiles, as well as comparison with profiles in the lines of [O I] 5577, O I 7774, and Mg I] 4571, leads us to conclude that neither type of [O I] double-peaked profile is necessarily the signature of emission from front and rear faces of ejecta arranged in a toroidal disk or elongated shell geometry as previously suggested. We propose possible alternative interpretations of double-peaked emission for each profile type, test their feasibility with simple line-fitting models, and discuss their strengths and weaknesses. The underlying cause of the observed predominance of blueshifted emission peaks is unclear, but may be due to internal scattering or dust obscuration of emission from far side ejecta.