Spectropolarimetry of the Type Ia SN 2019ein rules out significant\n global asphericity of the ejecta

Detailed spectropolarimetric studies may hold the key to probing the\nexplosion mechanisms and the progenitor scenarios of Type Ia supernovae (SNe\nIa). We present multi-epoch spectropolarimetry and imaging polarimetry of SN\n2019ein, an SN Ia showing high expansion velocities at early phases. The\nspectropolarimetry sequence spans from $\\sim -11$ to $+$10 days relative to\npeak brightness in the $B$-band. We find that the level of the continuum\npolarization of SN 2019ein, after subtracting estimated interstellar\npolarization, is in the range $0.0-0.3\\%$, typical for SNe Ia. The polarization\nposition angle remains roughly constant before and after the SN light-curve\npeak, implying that the inner regions share the same axisymmetry as the outer\nlayers. We observe high polarization ($\\sim 1\\%$) across both the Si II\n$\\lambda6355$ and Ca II near-infrared triplet features. These two lines also\ndisplay complex polarization modulations. The spectropolarimetric properties of\nSN 2019ein rule out a significant departure from spherical symmetry of the\nejecta for up to a month after the explosion. These observations disfavour\nmerger-induced and double-detonation models for SN 2019ein. The imaging\npolarimetry shows weak evidence for a modest increase in polarization after\n$\\sim 20$ days since the $B$-band maximum. If this rise is real and is observed\nin other SNe Ia at similar phases, we may have seen, for the first time, an\naspherical interior similar to what has been previously observed for SNe IIP.\nFuture polarization observations of SNe Ia extending to post-peak epochs will\nhelp to examine the inner structure of the explosion.\n