SN 2008gz – most likely a normal Type IIP event

Abstract

We present BVRI photometric and low-resolution spectroscopic investigation of the Type II core-collapse supernova (SN) 2008gz, which occurred in a star-forming arm and within a halflight radius (solar metallicity region) of the nearby spiral galaxy NGC 3672. The SN event was detected late, and a detailed investigation of its light curves and spectra spanning 200 d suggest that it is an event of Type IIP similar to the archetypal SNe 2004et and 1999em. However, in contrast to other events of its class, SN 2008gz exhibits a rarely observed V magnitude drop of 1.5 over the period of a month during the plateau to nebular phase. Using an AV of 0.21 mag as a lower limit and a distance of 25.5 Mpc, we estimate a synthesized ⁵⁶Ni mass of 0.05 ± 0.01Mʘ, a mid-plateau MV of −16.6 ± 0.2mag and a total radiant energy of ∼10⁴⁹ erg. The photospheric velocity is observed to be higher than observed for SN 2004et at similar epochs, indicating that the explosion energy was comparable to or higher than that of SN 2004et. A similar trend was also seen for the expansion velocity of H envelopes. By comparing the properties of SN 2008gz with other well-studied events, as well as by using a recent simulation of pre-SN models by Dessart, Livne & Waldman, we infer an explosion energy range of 2–3 × 10⁵¹ erg, and this coupled with the observed width of the forbidden [O I] 6300–6364Å line at 275 d after the explosion gives an upper limit for the main-sequence (non-rotating, solar metallicity) progenitor mass of 17Mʘ. Our narrow-band Hα observation, taken nearly 560 d 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.