SN 2019ein: a Type Ia supernova likely originated from a sub-Chandrasekhar-mass explosion

We present extensive optical photometric and spectroscopic observations for the nearby Type Ia supernova (SN Ia) 2019ein, spanning the phases from ~3 to ~330 d after the explosion. This SN Ia is characterized by extremely fast expansion at early times, with initial velocities of Si II and Ca II being above ~25 000-30 000 km s^-1. After experiencing an unusually rapid velocity decay, the ejecta velocity dropped to ~13 000 km s^-1 around maximum light. Photometrically, SN 2019ein has a moderate post-peak decline rate (Δm₁₅(B) = 1.35 ± 0.01 mag), while being fainter than normal SNe Ia by about 40 per cent (with $M^{\rm max}_{B} \approx -18.71 \pm 0.15$ mag). The nickel mass synthesized in the explosion is estimated to be 0.27-0.31 M_⊙ from the bolometric light curve. Given such a low nickel mass and a relatively high photospheric velocity, we propose that SN 2019ein likely had a sub-Chandrasekhar-mass white dwarf (WD) progenitor, M_WD ≲ 1.22 M_⊙. In this case, the explosion could have been triggered by a double-detonation mechanism, for which 1D and 2D models with WD mass M_WD ≈ 1 M_⊙ and a helium shell of 0.01 M_⊙ can reasonably produce the observed bolometric light curve and spectra. The predicted asymmetry as a result of double detonation is also favoured by the redshifted Fe II and Ni II lines observed in the nebular-phase spectrum. Possible diversity in origin of high velocity SNe Ia is also discussed.