The recently discovered bright type II supernova OGLE14-073 evolved very slowly. The light curve rose to maximum for 90 days from discovery and then declined at a rate compatible with the radioactive decay of 56Co. In a paper led by A. Kozyreva (Tel Aviv University), we show that a pair-instability supernova is a plausible mechanism for this event. We calculate explosion models and light curves with the radiation hydrodynamics code STELLA starting from two progenitor stars with an initial mass of 150 solar masses and low metallicity (Z=0.001). We obtain satisfactory fits to OGLE14-073 broadband light curves by including additional 56Ni in the centre of the models and mixing hydrogen down into the inner layers of the ejecta to a radial mass coordinate of 10 solar masses. The extra 56Ni required points to a slightly more massive progenitor star. The mixing of hydrogen could be due to large-scale mixing during the explosion. We also present synthetic spectra for our models simulated with the Monte Carlo radiative transfer code ARTIS. The synthetic spectra reproduce the main features of the observed spectra of OGLE14-073. We conclude that OGLE14-073 is one of the most promising candidates for a pair-instability explosion.

Synthetic spectrum for one of the models of Kozyreva et al. 2018 (P150-bf, shown in red) 7 days before peak brightness. For comparison the observed spectrum of OGLE14-073 is shown in black. The labels indicate the species that cause the most prominent spectral features.

Paper published in Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 3, Pages 3106–3114, full text available at https://arxiv.org/abs/1804.05791