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

ABSTRACT 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, MWD ≲ 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 MWD ≈ 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.