The nature of PISN candidates: clues from nebular spectra

Abstract

A group of super-luminous supernovae (SL-SNe) characterised by broad light curves have been suggested to be Pair Instability SNe (PISNe). Nebular spectra computed using PISN models have failed to reproduce the broad emission lines observed in these SNe, casting doubts on their true nature. Here, models of both PISNe and the explosion following the collapse of the core of a very massive star (100 M ⊙ ) are used to compute nebular spectra, which are compared to the spectrum of the prototypical PISN candidate, SN 2007bi. PISN models are confirmed to produce synthetic spectra showing narrow emission lines, resulting from the confinement of 56 Ni to the lowest velocities ( ∼ < 2000 km s −1 ) and in clear disagreement with the spectrum of SN 2007bi. Spectra more closely resembling SN 2007bi are obtained if the PISN models are fully mixed in abundance. Massive core-collapse models produce enough 56 Ni to power the light curve of PISN candidates, but their spectra are also not adequate. The nebular spectrum of SN 2007bi can be successfully reproduced if the inner region is artificially filled with oxygen-rich, low-velocity ejecta. This most likely requires a grossly aspherical explosion. A major difference between PISN and massive collapse models is that the former emit much more strongly in the NIR. It is concluded that: a) current PISN candidates, in particular SN 2007bi, are more likely the result of the collapse and explosion of massive stars below the PI limit; b) significant asymmetry is required to reproduce the late-time spectrum of SN 2007bi.

SN 2016iet: The Pulsational or Pair Instability Explosion of a Low-metallicity Massive CO Core Embedded in a Dense Hydrogen-poor Circumstellar Medium

“…The nebular spectra of SN 2007bi exhibit strong features of Fe-group elements, which we do not observe in SN 2016iet. In SN 2016iet we observe much narrower lines than in SN 2007bi, closer to the velocities of ∼ 2000 − 4000 km s −1 expected from a PISN (Woosley 2017;Mazzali et al 2019). We also plot a PISN model of a 100 M He core from Mazzali et al (2019).…”

“…However, Nicholl et al (2013) suggested that SN 2007bi was powered by a magnetar central engine with an ejecta mass of only 7 M . The spectra, metallicity, and observational features of SN 2007bi has been shown to be inconsistent with a PISN (Jerkstrand et al 2016;Dessart et al 2013;Mazzali et al 2019). To date, there have been no definitive cases of PISN explosions.…”

PTF11rka: an interacting supernova at the crossroads of stripped-envelope and H-poor superluminous stellar core collapses

“…However, the early-time spectra of PTF11rka resemble those of the SLSN 2007bi. In recent work, Moriya, Mazzali & Tanaka (2019b) and Mazzali et al (2019) showed that SN 2007bi is consistent with the explosion of a ∼ 40 M CO core of a massive star (of initial mass probably 60-80 M ). The explosion was not particularly energetic, given the large mass (E k ≈ 4 × 10 52 erg, E k /M ej ≈ 1), which makes the light curve broad.…”

“…The unusual light curve of PTF11rka (which was almost as luminous as a GRB-SN; see Fig. 2) and its early-time spectra, characterised by rather sharp, narrow, unblended absorption lines, are reminiscent of SN 2007bi, which was thought to be a pair-instability SN candidate (Gal-Yam et al 2009), although this was later challenged (Jerk-strand, Smartt & Heger 2016;Moriya, Mazzali & Tanaka 2019b;Mazzali et al 2019). These features were reproduced by Moriya, Mazzali & Tanaka (2019b) by adopting a sharp cut in the ejecta distribution with velocity, at 13,000 km s −1 .…”

“…They can be ejected from the inner stellar core, but in the nebular phase they radiate mostly in the near-infrared, such that only if information in this wavelength band is available can one reliably quantify their production (e.g." Mazzali et al 2010Mazzali et al , 2015Mazzali et al , 2019. As near-infrared information is not available for PTF11rka we test two options: a high-mass solution, where as much as 2 M is Si and S in a ratio of 3:1, and a low-mass solution, where Si and S combined account for only 0.4 M , with the same ratio.…”