An Intermediate Type Ia Supernova Between Normal And Super-Chandrasekhar
  In this paper, we report observations of a peculiar Type Ia supernova
iPTF13asv (a.k.a., SN2013cv) from the onset of the explosion to months after
its peak. The early-phase spectra of iPTF13asv show absence of iron absorption,
indicating that synthesized iron elements are confined to low-velocity regions
of the ejecta, which, in turn, implies a stratified ejecta structure along the
line of sight. Our analysis of iPTF13asv's light curves and spectra shows that
it is an intermediate case between normal and super-Chandrasekhar events. On
the one hand, its light curve shape (B-band $\Delta m_{15}=1.03\pm0.01$) and
overall spectral features resemble those of normal Type Ia supernovae. On the
other hand, similar to super-Chandrasekhar events, it shows large peak optical
and UV luminosity ($M_B=-19.84\,\rm{mag}$, $M_{uvm2}=-15.5\,\rm{mag}$) a
relatively low but almost constant \ion{Si}{2} velocities of about
$10,000\,\rm{km}\,\rm{s}^{-1}$, and persistent carbon absorption in the
spectra. We estimate a $^{56}$Ni mass of $0.81^{+0.10}_{-0.18}M_\odot$ and a
total ejecta mass of $1.59^{+0.45}_{-0.12}M_\odot$. The large ejecta mass of
iPTF13asv and its stratified ejecta structure together seemingly favor a
double-degenerate origin.
