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 from ~−11 to +109 d with respect to the B-band maximum, which occurred on JD 245 6132.89 ± 0.19, with an apparent magnitude of m^max_B = 14.38 ± 0.02. The absolute magnitudes at maximum in B and V bands are M^max_B = −19.52±0.15 and M^max_V = −19.42±0.15, respectively. SN 2012dn is marginally luminous compared to normal Type Ia supernovae. The peak bolometric luminosity of log L ^max_bol = 43.27±0.06 erg s⁻¹ suggests that 0.82 ± 0.12 M☉ of ⁵⁶Ni was synthesized in the explosion. The decline rate Δm₁₅(B)_true = 0.92 ± 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 II 6580 Å line, indicating the presence of unburned materials. The velocity evolution of C II line is peculiar. Except for the very early phase (~−13 d), the C II line velocity is lower than the velocity estimated using the Si II line. During the pre-maximum and close to the maximum phase, to reproduce observed shape of the spectra, the synthetic spectrum code 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.