SN 2019ein: New Insights into the Similarities and Diversity among High-velocity Type Ia Supernovae

We present optical observations of the Type Ia supernova (SN) 2019ein, starting two days after the estimated explosion date. The spectra and light curves show that SN 2019ein belongs to a high-velocity (HV) and broad-line group with a relatively rapid decline in the light curves (Δm₁₅(B) = 1.36 ± 0.02 mag) and a short rise time (15.37 ± 0.55 days). The Si II λ6355 velocity, associated with a photospheric component but not with a detached high-velocity feature, reached ∼20,000 km s^-1 12 days before the B-band maximum. The line velocity, however, decreased very rapidly and smoothly toward maximum light, to ∼13,000 km s^-1, which is relatively low among HV SNe. This indicates that the speed of the spectral evolution of HV SNe Ia is correlated with not only the velocity at maximum light, but also the light-curve decline rate, as is 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 an 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 Δm₁₅(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 Δm₁₅) and the extent of the O-Ne-C burning layer provides an important constraint on the explosion mechanism(s) of HV and NV SNe.