  We present optical observations of type Ia supernova (SN) 2019ein, starting
at 2 days after the estimated explosion date. The spectra and the light curves
show that SN 2019ein belongs to the High-Velocity (HV) and Bload Line groups
with relatively rapid decline in the light curves (Delta m15(B) = 1.36 +- 0.02
mag) and the short rise time (15.37 +- 0.55 days). The Si II 6355 velocity,
associated with a photospheric component but not with a detached high-velocity
feature, reached ~ 20,000 km s-1 at 12 days before the B-band maximum. The line
velocity however decreased very rapidly and smoothly toward the maximum light,
where it was ~ 13,000 km s-1 as relatively low among HV SNe. This indicates
that the speed of the spectral evolution of HV SNe Ia is correlated not only to
the velocity at the maximum light, but also to the light curve decline rate
like the case for Normal-Velocity (NV) SNe Ia. Spectral synthesis modeling
shows that the outermost layer at > 17,000 km s-1 is well described by the
O-Ne-C burning layer extending to at least 25,000 km s-1, and there is no
unburnt carbon below 30,000 km s-1; these properties are largely consistent
with the delayed detonation scenario, and are shared with the prototypical HV
SN 2002bo despite the large difference in Delta m15(B). This structure is
strikingly different from that derived for the well-studied NV SN 2011fe. We
suggest that the relation between the mass of 56Ni (or Delta m15) and the
extent of the O-Ne-C burning layer provides an important constraint on the
explosion mechanism(s) of HV and NV SNe.

High Energy Astrophysical Phenomena