Comparative Analysis of SN 2012dn Optical Spectra: Days -14 to +114
  SN 2012dn is a super-Chandrasekhar mass candidate in a purportedly normal
spiral (SAcd) galaxy, and poses a challenge for theories of type Ia supernova
diversity. Here we utilize the fast and highly parameterized spectrum synthesis
tool, SYNAPPS, to estimate relative expansion velocities of species inferred
from optical spectra obtained with six facilities. As with previous studies of
normal SN Ia, we find that both unburned carbon and intermediate mass elements
are spatially coincident within the ejecta near and below 14,000 km/s. Although
the upper limit on SN 2012dn's peak luminosity is comparable to some of the
most luminous normal SN Ia, we find a progenitor mass exceeding ~1.6 Msun is
not strongly favored by leading merger models since these models do not
accurately predict spectroscopic observations of SN 2012dn and more normal
events. In addition, a comparison of light curves and host-galaxy masses for a
sample of literature and Palomar Transient Factory SN Ia reveals a diverse
distribution of SN Ia subtypes where carbon-rich material remains unburned in
some instances. Such events include SN 1991T, 1997br, and 1999aa where trace
signatures of C III at optical wavelengths are presumably detected.
