SN 2008gz − most likely a normal type IIP event

Abstract

We present BV RI photometric and low-resolution spectroscopic investigation of a type II core-collapse supernova (SN) 2008gz, which occurred in a star forming arm and within a half-light radius (solar metallicity region) of a nearby spiral galaxy NGC 3672. The SN event was detected late, and a detailed investigation of its light curves and spectra spanning 200 days suggest that it is an event of type IIP similar to archetypal SNe 2004et and 1999em. However, in contrast to other events of its class, the SN 2008gz exhibits rarely observed V magnitude drop of 1.5 over the period of a month during plateau to nebular phase. Using 0.21 mag of AV as a lower limit and a distance of 25.5 Mpc, we estimate synthesized 56Ni mass of 0.05 ± 0.01M⊙ and a mid-plateau MV of −16.6 ± 0.2 mag. The photospheric velocity is observed to be higher than that was observed for SN 2004et at similar epochs, indicating explosion energy was comparable to or higher than SN 2004et. Similar trend was also seen for the expansion velocity of H-envelopes. By comparing its properties with other well studied events as well as by using a recent simulation of pre-SN models of Dessart, Livne & Waldman (2010), we infer an explosion energy range of 2−3×1051 erg and this coupled with the observed width of the forbidden [O i] 6300-6364°A line at 275 days after the explosion gives an upper limit for the main-sequence (non-rotating, solar metallicity) progenitor mass of 17M⊙. Our narrow-band Hα observation, taken nearly 560 days after the explosion and the presence of an emission kink at zero velocity in the Doppler corrected spectra of SN indicate that the event took place in a low luminosity star forming Hii region.