Supernova SN 2012dn: a spectroscopic clone of SN 2006gz

Abstract

We present optical and UV analysis of the luminous type Ia supernova SN 2012dn covering the period $\sim -$11 to +109 days with respect to the $B$ band maximum, that occurred on JD 2456132.89 $\pm$ 0.19, with an apparent magnitude of $m_{B}^\text{max}$ = 14.38 $\pm$ 0.02. The absolute magnitudes at maximum in $B$ and $V$ bands are $M_{B}^\text{max} = -19.52 \pm 0.15$ and $M_{V}^\text{max} = -19.42 \pm 0.15$, respectively. SN 2012dn is marginally luminous compared to normal type Ia supernovae. The peak bolometric luminosity of $\log L_\text{bol}^\text{max} = 43.27 \pm 0.06$ erg s$^{-1}$ suggests that $0.82 \pm 0.12$ M$_\odot$ of $^{56}$Ni was synthesized in the explosion. The decline rate $\Delta m_{15}(B)_\text{true}= 0.92 \pm 0.04$ mag is lower than that of normal type Ia supernovae, and similar to the luminous SN 1991T. However, the photometric and spectroscopic behaviour of SN 2012dn is different from that of SN 1991T. Early phase light curves in $R$ and $I$ bands are very broad. The $I$ band peak has a plateau-like appearance similar to the super-Chandra SN 2009dc. Pre-maximum spectra show clear evidence of C\,{\sc ii} 6580 \AA\, line, indicating the presence of unburned materials. The velocity evolution of C\,{\ sc ii} line is peculiar. Except for the very early phase ($\sim-$13 d), the C\,{\sc ii} line velocity is lower than the velocity estimated using the Si\,{\sc ii} line. During the pre-maximum and close to maximum phase, to reproduce observed shape of the spectra, the synthetic spectrum code {\sc syn++} needs significantly higher blackbody temperature than those required for normal type Ia events. The photospheric velocity evolution and other spectral properties are similar to those of the carbon-rich SN 2006gz.