Spectroscopic Observations of SN 2012fr: A Luminous Normal Type Ia
  Supernova with Early High Velocity Features and Late Velocity Plateau
  We present 65 optical spectra of the Type Ia supernova SN 2012fr, of which 33
were obtained before maximum light. At early times SN 2012fr shows clear
evidence of a high-velocity feature (HVF) in the Si II 6355 line which can be
cleanly decoupled from the lower velocity "photospheric" component. This Si II
6355 HVF fades by phase -5; subsequently, the photospheric component exhibits a
very narrow velocity width and remains at a nearly constant velocity of
v~12,000 km/s until at least 5 weeks after maximum brightness. The Ca II
infrared (IR) triplet exhibits similar evidence for both a photospheric
component at v~12,000 km/s with narrow line width and long velocity plateau, as
well as a high-velocity component beginning at v~31,000 km/s two weeks before
maximum. SN 2012fr resides on the border between the "shallow silicon" and
"core-normal" subclasses in the Branch et al. (2009) classification scheme, and
on the border between normal and "high-velocity" SNe Ia in the Wang et al.
(2009a) system. Though it is a clear member of the "low velocity gradient"
(LVG; Benetii et al., 2005) group of SNe Ia and exhibits a very slow
light-curve decline, it shows key dissimilarities with the overluminous SN
1991T or SN 1999aa subclasses of SNe Ia. SN 2012fr represents a well-observed
SN Ia at the luminous end of the normal SN Ia distribution, and a key
transitional event between nominal spectroscopic subclasses of SNe Ia.
