Long-duration Superluminous Supernovae at Late Times

We present nebular-phase observations and spectral models of Type Ic superluminous supernovae. LSQ14an and SN 2015bn both display late-time spectra similar to SN 2007bi, and the class shows strong similarity with broad-lined Type Ic SNe such as SN 1998bw. Near-infrared observations of SN 2015bn at +315d show a strong Ca II triplet, O I 9263, O I 1.13 μm and Mg I 1.50 μm, but no strong He, Si or S lines. Spectral models of oxygen-zone emission are investigated to put constraints on the emitting region. Models require M(O) 10 M to produce enough [O I] 6300, 6364 luminosity to match observed levels, irrespective of the powering situation and the density. This is an argument against shell collisions from pair-instability pulsations for explaining the powering, as these shells are limited to a few solar masses. The high oxygen-zone mass, supported by estimated magnesium masses, points to explosions of massive CO cores, requiring MZAMS 40 M . [O III] lines emerge naturally in many O-zone models, which strengthens the identification of broad [O III] 4363 and [O III] 4959, 5007 in some spectra. A small filling factor f ≤ 0.01 for the O/Mg zone is needed to produce enough luminosity in Mg I] 4571, Mg I 1.50 μm, and O I recombination lines. The high Ca II NIR/[Ca II] 7300 ratio of ∼ 2 indicates a high electron density of ne 10 cm−3, also supportive of clumping. We review the constraints from the nebular spectral modelling in the context of the competing physical mechanisms which have been proposed to produce superluminous supernovae.