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 $\sim 3$ days to $\sim 330$ days 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. After experiencing an unusually rapid velocity decay, the ejecta velocity dropped to ~ 13,000 km/s around maximum light. Photometrically, SN 2019ein has a moderate post-peak decline rate ($\hat{I}”m_{15}(B)=1.35\pm 0.01$ mag), while being fainter than normal SNe Ia by about 40% (with $M_B^{\max }\approx -18.71\pm 0.15$ mag). The nickel mass synthesized in the explosion is estimated to be 0.27--0.31 $M_{\odot }$ 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, $M_{\mathrm{WD}}\lesssim 1.22M_{\odot }$. In this case, the explosion could have been triggered by a double-detonation mechanism, for which 1- and 2-dimensional models with WD mass $M_{\mathrm{WD}}\approx 1M_{\odot }$ and a helium shell of 0.01 $M_{\odot }$ can reasonably produce the observed bolometric light curve and spectra. The predicted asymmetry as a result of double detonation is also favored 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.

34 pages, 16 figures, accepted for publication in MNRAS