KSP-SN-2016kf: A Long-rising H-rich Type II Supernova with Unusually High ⁵⁶Ni Mass Discovered in the KMTNet Supernova Program

Abstract

We present the discovery and the photometric and spectroscopic study of H-rich Type II supernova (SN) KSP-SN-2016kf (SN2017it) observed in the KMTNet Supernova Program in the outskirts of a small irregular galaxy at z 0.043 within a day from the explosion. Our high-cadence, multi-color (BV I ) light curves of the SN show that it has a very long rise time (t rise 20 days in V band), a moderately luminous peak (M V −17.6 mag), a notably luminous and flat plateau (M V −17.4 mag and decay slope s 0.53 mag per 100 days), and an exceptionally bright radioactive tail. Using the color-dependent bolometric correction to the light curves, we estimate the 56 Ni mass powering the observed radioactive tail to be 0.10 ± 0.01 M , making it a H-rich Type II SN with one of the largest 56 Ni masses observed to date. The results of our hydrodynamic simulations of the light curves constrain the mass and radius of the progenitor at the explosion to be ∼15 M (evolved from a star with an initial mass of ∼ 18.8 M ) and ∼ 1040 R , respectively, with the SN explosion energy of ∼ 1.3 × 10 51 erg s −1 . The above-average mass of the KSP-SN-2016kf progenitor, together with its low metallicity Z/Z 0.1 − 0.4 obtained from spectroscopic analysis, is indicative of a link between the explosion of high-mass red supergiants and their low-metallicity environment. The early part of the observed light curves shows the presence of excess emission above what is predicted in model calculations, suggesting there is interaction between the ejecta and circumstellar material. We further discuss the implications of the high progenitor initial mass and low-metallicity environment of KSP-SN-2016kf on our understanding of the origin of Type II SNe.

Infant-phase reddening by surface Fe-peak elements in a normal type Ia supernova

“…If He-shell DDet is the origin of surface Fe-peak elements in SN 2018aoz, it would imply that detonations of He-shells as thin as ∼ 0.01 M can successfully initiate normal Type Ia SNe, consistent with the predictions of our simulations and other recent simulations 7,48 . In our best-fit He-shell DDet model for SN 2018aoz, 12.9% of the outer 1% of the ejecta by mass is composed of 56 Ni, 52 Fe, and 48 Cr. Although this is comparable to the amount of radioactive material needed to reproduce the infant-phase excess emission in SN 2018aoz (see section "Distribution of Fe-peak Elements in the Extreme Outer Ejecta"), the Heshell DDet model under-predicts the emission 0.5 days (Supplementary Section 7.1), indicating that either the current He-shell DDet models do not fully capture radioactive heating at early times or another source of emission is also required (e.g., ejecta collision with a companion 49 ).…”

“…Such a selective suppression of flux can only be from line absorption. Fe-peak and associated nucleosynthetic elements, including 56 Ni, 52 Fe, 48 Cr, 44 Ti, and their decay products, are the elements expected in Type Ia SNe that can significantly suppress flux in the vicinity of 5000 Å. While H and He lines can blanket 4000 Å41,42 , they are not generally expected in Type Ia SNe and most of the B-band flux would not be significantly affected.…”

Dwarf Galaxy Discoveries from the KMTNet Supernova Program. II. The NGC 3585 Group and Its Dynamical State*

“…We are conducting a systematic study of dwarf galaxies in nearby galaxy groups using data from the KMTNet (Korea Microlensing Telescope Network) Supernova Program (KSP; Moon et al 2016). KSP is a program to search for supernovae and optical transients (He et al 2016;Antoniadis et al 2017;Brown et al 2018;Lee et al 2019;Afsariardchi et al 2019) using three 1.6-m telescopes located at the Cerro Tololo Inter-American Observatory (CTIO, Chile), the South African Astronomical Observatory (SAAO, South Africa), and the Siding Spring Observatory (SSO, Australia). When several hundred KSP images in each field are stacked, the program provides deep BV I images reaching a sensitivity of about 28 mag arcsec −2 within a 1.5 acrsec radius aperture.…”

The Nickel Mass Distribution of Stripped-Envelope Supernovae: Implications for Additional Power Sources

“…In this case, attributing the full tail luminosity to 56 Ni would lead to an overestimate of the true values. Within this context, it is notable that while simulations of neutrino-driven core-collapse SNe (Sukhbold et al 2016;Ertl et al 2019) are able to self-consistently produce the range of M Ni values observed in H-rich Type II SN (∼0.004-0.13 M ; Müller et al 2017, Afsariardchi et al 2019, they are unable to produce M Ni values as high as those derived for the upper ∼30% of our SESN sample (see Figures 7 and 8). It was for this reason that Ertl et al (2019) invoked magnetars to explain the observed light curves of SESNe.…”