Nebular spectra of pair-instability supernovae

Abstract

If very massive stars (M 100 M ⊙ ) can form and avoid too strong mass loss during their evolution, they are predicted to explode as pair-instability supernovae (PISNe). One critical test for candidate events is whether their nucleosynthesis yields and internal ejecta structure, being revealed through nebular-phase spectra at t 1 yr, match those of model predictions. Here we compute theoretical spectra based on model PISN ejecta at 1-3 years post-explosion to allow quantitative comparison with observations. The high column densities of PISNe lead to complete gamma-ray trapping for t 2 years which, combined with fulfilled conditions of steady state, leads to bolometric supernova luminosities matching the 56 Co decay. Most of the gamma-rays are absorbed by the deep-lying iron and silicon/sulphur layers. The ionization balance shows a predominantly neutral gas state, which leads to emission lines of Fe I, Si I, and S I. For low-mass PISNe the metal core expands slowly enough to produce a forest of distinct lines, whereas high-mass PISNe expand faster and produce more featureless spectra. Line blocking is complete below ∼5000Å for several years, and the model spectra are red. The strongest line is typically [Ca II] λλ7291, 7323, one of few lines from ionized species. We compare our models with proposed PISN candidates SN 2007bi and PTF12dam, finding discrepancies for several key observables and thus no support for a PISN interpretation. We discuss distinct spectral features predicted by the models, and the possibility of detecting pair-instability explosions among non-superluminous supernovae.

Long-Duration Superluminous Supernovae at Late Times

“…With the multi-epoch coverage of SN 2015bn (and LSQ14an) we see significantly more rapid decline than 56 Co decay (see also (Nicholl et al 2016b) and Inserra et al, in prep.). This cannot be achieved in PISN models, which have complete gamma-trapping for at least 500d (Jerkstrand et al 2016). Some transfer to the nearinfrared may occur, but the NIR observations here indicate that this is a weak process.…”

“…While there is some debate whether broad-lined Type Ic SNe are powered by 56 Ni or a central engine, existing 56 Ni models are successful enough to maintain this as the standard picture. Thus, "scaled-up" 56 Ni powered explosion models must still be pursued for SLSNe, even though the PISN variant of these have not compared well to observations so far (Dessart et al 2013;Jerkstrand et al 2016). …”

Hydrogen-poor Superluminous Supernovae with Late-time Hα Emission: Three Events From the Intermediate Palomar Transient Factory

“…Calculated optical spectra at the nebular phase, based on pair-instability supernova (PISN) models, seem to show a relatively weak [O I] 6300 doublet (Jerkstrand et al 2016). At face value, this could be considered to be supporting evidence for a PISN or PPISN model for these events.…”

“…First, the ejecta of our three events may have lower oxygen masses, and perhaps also lower progenitor masses than that of SN2015bn and SN2007bi. The recent modeling of the nebular spectra of SN2015bn and SN2007bi by Jerkstrand et al (2016) …”

Host-Galaxy Properties of 32 Low-Redshift Superluminous Supernovae From the Palomar Transient Factory

“…Among the SLSNe I, we denote a small number of events (three) as belonging to the subclass of long-lived "R"-types, which show exponentially declining late-time light curves consistent with radioactive decay and which have been suggested to be examples of PISNe (Gal-Yam et al 2009;Gal-Yam 2012), although this interpretation is contested by other authors (e.g., Dessart et al 2012;Nicholl et al 2013;Jerkstrand et al 2016). We will generally refer to them as Type "I-R." With only three events, we do not have sufficient sample size to perform a statistically robust comparison between the host properties of these events and the more rapidly declining SNeI, but as we observe no strong distinction between the host properties of these events and other SLSNe I in our sample (and other authors have reported similar results; e.g., Lunnan et al 2014;Leloudas et al 2015), we will generally treat all SLSNe I together in our analysis regardless of their light-curve properties.…”