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We present VLT-FORS spectropolarimetric observations of the type II supernova(SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to120d after explosion. We correct for the interstellar polarization bypostulating that the SN polarization is naught near the rest wavelength of thestrongest lines - this is later confirmed by our modeling. SN2012aw exhibitsintrinsic polarization, with strong variations across lines, and with amagnitude that grows in the 7000A line-free region from 0.1% at 16d up to 1.2%at 120d. This behavior is qualitatively similar to observations gathered forother type II SNe. A suitable rotation of Stokes vectors places the bulk of thepolarization in q, suggesting the ejecta of SN2012aw is predominantlyaxisymmetric. Using an upgraded version of our 2D polarized radiative transfercode, we model the wavelength- and time-dependent polarization of SN2012aw. Thekey observables may be explained by the presence of a confined region ofenhanced 56Ni at ~4000km/s, which boosts the electron density in a cone havingan opening angle of ~50deg and an observer's inclination of ~70deg to the axisof symmetry. With this fixed asymmetry in time, the observed evolution of theSN2012aw polarization arises from the evolution of the ejecta optical depth,ionization, and the relative importance of multiple versus single scattering.However, the polarization signatures exhibit numerous degeneracies.Cancellation effects at early times imply that a low polarization may occur forejecta with a large asymmetry. An axisymmetric ejecta with a latitude-dependentexplosion energy can also yield similar polarization signatures as anasymmetric 56Ni distribution. Despite these uncertainties, SN2012aw providesadditional evidence for the generic asymmetry of type II SN ejecta, of whichVLT-FORS spectropolarimetric observations are a decisive and exquisite probe.

Multiepoch VLT-FORS spectro-polarimetric observations of supernova 2012aw reveal an asymmetric explosion