  It has been widely accepted that Type Ia supernovae (SNe Ia) are
thermonuclear explosions of a CO white dwarf. However, the natures of the
progenitor system(s) and explosion mechanism(s) are still unclarified. Thanks
to the recent development of transient observations, they are now frequently
discovered shortly after the explosion, followed by rapid spectroscopic
observations. In this study, by modeling very early-phase spectra of SNe Ia, we
try to constrain the explosion models of SNe Ia. By using the Monte Carlo
radiation transfer code, TARDIS, we estimate the properties of their outermost
ejecta. We find that the photospheric velocity of normal-velocity supernovae
(NV SNe) in the first week is $\sim$15000 km s$^{-1}$. The outer velocity, to
which the carbon burning extends, spans the range between $\sim$20000 and 25000
km s$^{-1}$. The ejecta density of NV SNe also shows a large diversity. For
high-velocity supernovae (HV SNe) and 1999aa-like SNe, the photospheric
velocity is higher, $\sim$20000 km s$^{-1}$. They are different in the
photospheric density, with HV SNe having higher density than 1999aa-like SNe.
For all these types, we show that the outermost composition is closely related
to the outermost ejecta density; the carbon burning layer and the unburnt
carbon layer are found in the higher-density and lower-density objects,
respectively. This finding suggests that there might be two sequences, the
high-density and carbon-poor group (HV SNe and some NV SNe) and the low-density
and carbon-rich group (1999aa-like and other NV SNe), which may be associated
with different progenitor channels.
