One year of monitoring of the Type IIb supernova SN 2011dh

Optical UBVRI photometry and low-resolution spectroscopy of the Type IIb supernova SN 2011dh in M51 are presented, covering the first year after the explosion. The light curve and spectral evolution are discussed. The early phase light-curve evolution of SN 2011dh is very similar to SN 1993J and SN 2008ax. In the late phase, however, SN 2011dh declines faster than SN 1993J. The late phase decline in the B band is steeper than in the R and I bands, indicating the possibility of dust formation. With a peak V-band absolute magnitude of M V = −17.123 ± 0.18 mag, SN 2011dh is a marginally faint type IIb event. The reddening corrected colour curves of SN 2011dh are found to be redder than other well-studied Type IIb supernovae. The bolometric light curve indicates ∼0.09 M of 56Ni is synthesized during the explosion. The He i lines were detected in the spectra during the rise to maximum. The nebular spectra of SN 2011dh show a box-shaped emission in the red wing of the [O i] 6300−6363 Å feature, that is attributed to Hα emission from a shock-excited circumstellar material. The analysis of nebular spectra indicates that ∼0.2 M of oxygen was ejected during the explosion. Further, the [Ca ii]/[O i] line ratio in the nebular phase is ∼0.7, indicating a progenitor with a main-sequence mass of 10-15 M.