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We present seven epochs of spectropolarimetry of the Type IIb supernova (SN)2011dh in M51, spanning 86 days of its evolution. The first epoch was obtained9 days after the explosion, when the photosphere was still in the depletedhydrogen layer of the stripped-envelope progenitor. Continuum polarization issecurely detected at the level of P~0.5% through day 14 and appears to diminishby day 30, which is different from the prevailing trends suggested by studiesof other core-collapse SNe. Time-variable modulations in P and position angleare detected across P-Cygni line features. H-alpha and HeI polarization peakafter 30 days and exhibit position angles roughly aligned with the earliercontinuum, while OI and CaII appear to be geometrically distinct. We discussseveral possibilities to explain the evolution of the continuum and linepolarization, including the potential effects of a tidally deformed progenitorstar, aspherical radioactive heating by fast-rising plumes of Ni-56 from thecore, oblique shock breakout, or scattering by circumstellar material. Whilethese possibilities are plausible and guided by theoretical expectations, theyare not unique solutions to the data. The construction of more detailedhydrodynamic and radiative-transfer models that incorporate complex asphericalgeometries will be required to further elucidate the nature of the polarizedradiation from SN 2011dh and other Type IIb supernovae.

Spectropolarimetry of SN 2011dh in M51: geometric insights on a Type IIb supernova progenitor and explosion