SN 2017ivv: two years of evolution of a transitional Type II supernova

Abstract

We present the photometric and spectroscopic evolution of the Type II supernova (SN II) SN 2017ivv (also known as ASASSN-17qp). Located in an extremely faint galaxy (M${}_r=-10.3$ mag), SN 2017ivv shows an unprecedented evolution during the two years of observations. At early times, the light curve shows a fast rise ($\sim 6-8$ days) to a peak of ${\mathrm{M}}_g^{\max }=-17.84$ mag, followed by a very rapid decline of $7.94\pm 0.48$ mag per 100 days in the $V-$band. The extensive photometric coverage at late phases shows that the radioactive tail has two slopes, one steeper than that expected from the decay of ${}^{56}$Co (between 100 and 350 days), and another slower (after 450 days), probably produced by an additional energy source. From the bolometric light curve, we estimated that the amount of ejected ${}^{56}$Ni is $\sim 0.059\pm 0.003$ M$\odot$. The nebular spectra of SN 2017ivv show a remarkable transformation that allows the evolution to be split into three phases: (1) H$\hat{I}\pm$ strong phase ($<200$ days); (2) H$\hat{I}\pm$ weak phase (between 200 and 350 days); and (3) H$\hat{I}\pm$ broad phase ($>500$ days). We find that the nebular analysis favours a binary progenitor and an asymmetric explosion. Finally, comparing the nebular spectra of SN 2017ivv to models suggests a progenitor with a zero-age main-sequence mass of 15 -- 17 \Msun.

Accepted for publication in MNRAS