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We present the discovery and the photometric and spectroscopic study ofH-rich Type II supernova (SN) KSP-SN-2016kf (SN2017it) observed in the KMTNetSupernova Program in the outskirts of a small irregular galaxy at$z\simeq0.043$ within a day from the explosion. Our high-cadence, multi-color($BVI$) light curves of the SN show that it has a very long rise time($t_\text{rise}\simeq 20$ days in $V$ band), a moderately luminous peak($M_V\simeq -$17.6 mag), a notably luminous and flat plateau ($M_V\simeq -$17.4mag and decay slope $s\simeq0.53$ mag per 100 days), and an exceptionallybright radioactive tail. Using the color-dependent bolometric correction to thelight curves, we estimate the $^{56}$Ni mass powering the observed radioactivetail to be $0.10\pm0.01$ M$_\odot$, making it a H-rich Type II SN with one ofthe largest $^{56}$Ni masses observed to date. The results of our hydrodynamicsimulations of the light curves constrain the mass and radius of the progenitorat the explosion to be $\sim$15 M$_\odot$ (evolved from a star with an initialmass of $\sim$ 18.8 M$_\odot$) and $\sim1040$ R$_\odot$, respectively, with theSN explosion energy of $\sim 1.3\times 10^{51}$ erg s$^{-1}$. The above-averagemass of the KSP-SN-2016kf progenitor, together with its low metallicity $Z/Z_\odot \simeq0.1-0.4$ obtained from spectroscopic analysis, is indicative ofa link between the explosion of high-mass red supergiants and theirlow-metallicity environment. The early part of the observed light curves showsthe 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 andlow-metallicity environment of KSP-SN-2016kf on our understanding of the originof Type II SNe.

KSP-SN-2016kf: a long-rising H-rich Type II Supernova with unusually high $^{56}$Ni mass discovered in the KMTNet Supernova Program