Optical and Ultraviolet Observations of the Narrow-Lined Type Ia SN 2012fr in NGC 1365

Abstract

Extensive optical and ultraviolet (UV) observations of the type Ia supernova (SN Ia) 2012fr are presented in this paper. It has a relatively high luminosity, with an absolute $B$-band peak magnitude of about $-19.5$ mag and a smaller post-maximum decline rate than normal SNe Ia [e.g., $\Delta m _{15}$($B$) $= 0.85 \pm 0.05$ mag]. Based on the UV and optical light curves, we derived that a $^{56}$Ni mass of about 0.88 solar masses was synthesized in the explosion. The earlier spectra are characterized by noticeable high-velocity features of \ion{Si}{2} $\lambda$6355 and \ion{Ca}{2} with velocities in the range of $\sim22,000$--$25,000$ km s$^{-1}$. At around the maximum light, these spectral features are dominated by the photospheric components which are noticeably narrower than normal SNe Ia. The post-maximum velocity of the photosphere remains almost constant at $\sim$12,000 km s$^{-1}$ for about one month, reminiscent of the behavior of some luminous SNe Ia like SN 1991T. We propose that SN 2012fr may represent a subset of the SN 1991T-like SNe Ia viewed in a direction with a clumpy or shell-like structure of ejecta, in terms of a significant level of polarization reported in Maund et al. (2013).