Light curve and spectral evolution of the Type IIb SN 2011fu

Abstract

We present the low-resolution spectroscopic and UBVRI broad-band photometric investigations of the Type IIb supernova 2011fu, discovered in UGC 01626. The photometric follow-up of this event has been initiated a few days after the explosion and covers a period of about 175 days. The early-phase light curve shows a rise followed by steep decay in all bands and shares properties very similar to that seen in case of SN 1993J, with a possible detection of the adiabatic cooling phase. Modelling of the quasi-bolometric light curve suggests that the progenitor had an extended ($\sim 1 \times 10^{13}$ cm), low-mass ($\sim 0.1$ $M_\odot$) H-rich envelope on top of a dense, compact ($\sim 2 \times 10^{11}$ cm), more massive ($\sim$ 1.1 $M_\odot$) He-rich core. The nickel mass synthesized during the explosion was found to be $\sim$ 0.21 $M_\odot$, slightly larger than seen in case of other Type IIb SNe. The spectral modelling performed with SYNOW suggests that the early-phase line velocities for H and Fe\,{\sc ii} features were $\sim 16000$ km s$^{-1}$ and $\sim 14000$ km s$^{-1}$, respectively. Then the velocities declined up to day +40 and became nearly constant at later epochs.