  We present VLT-FORS spectropolarimetric observations of the type II supernova
(SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to
120d after explosion. We correct for the interstellar polarization by
postulating that the SN polarization is naught near the rest wavelength of the
strongest lines - this is later confirmed by our modeling. SN2012aw exhibits
intrinsic polarization, with strong variations across lines, and with a
magnitude 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 for
other type II SNe. A suitable rotation of Stokes vectors places the bulk of the
polarization in q, suggesting the ejecta of SN2012aw is predominantly
axisymmetric. Using an upgraded version of our 2D polarized radiative transfer
code, we model the wavelength- and time-dependent polarization of SN2012aw. The
key observables may be explained by the presence of a confined region of
enhanced 56Ni at ~4000km/s, which boosts the electron density in a cone having
an opening angle of ~50deg and an observer's inclination of ~70deg to the axis
of symmetry. With this fixed asymmetry in time, the observed evolution of the
SN2012aw 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 for
ejecta with a large asymmetry. An axisymmetric ejecta with a latitude-dependent
explosion energy can also yield similar polarization signatures as an
asymmetric 56Ni distribution. Despite these uncertainties, SN2012aw provides
additional evidence for the generic asymmetry of type II SN ejecta, of which
VLT-FORS spectropolarimetric observations are a decisive and exquisite probe.
