Abstract
We present VLT–FORS spectropolarimetric observations of the type II supernova (SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to 120 d after explosion. We corrected for 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. SN 2012aw exhibits intrinsic polarization, with strong variations across lines, and with a magnitude that grows in the 7000 Å line-free region from 0.1% at 16 d up to 1.2% at 120 d. 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 SN 2012aw is predominantly axisymmetric. Using an upgraded version of our 2D polarized radiative transfer code, we modeled the wavelength- and time-dependent polarization of SN 2012aw. The key observables may be explained by the presence of a confined region of enhanced 56Ni at ~4000 km s−1, which boosts the electron density in a cone having an opening angle of ~50 deg and an observer’s inclination of ~70 deg to the axis of symmetry. With this fixed asymmetry in time, the observed evolution of the SN 2012aw 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 low polarization may even occur for ejecta with a large asymmetry. An axisymmetric ejecta with a latitudinal-dependent explosion energy can also yield similar polarization signatures as asymmetry in the 56Ni distribution. In spite of these uncertainties, SN 2012aw provides additional evidence for the generic asymmetry of type II SN ejecta, of which VLT–FORS spectropolarimetric observations are a decisive and exquisite probe.
Highlights
Photometry and spectroscopy provide critical information on type II supernova (SN)
We have presented Very Large Telescope (VLT)–FORS spectropolarimetric observations of the type II SN 2012aw spanning seven epochs of the photospheric phase until the onset of the drop-off from the plateau
SN 2012aw presents polarization characteristics that are qualitatively and quantitatively similar to what has been observed in type II SNe so far, such as the emblematic SN 2004dj (Leonard et al 2006)
Summary
Photometry and spectroscopy provide critical information on type II supernova (SN). They characterize, for example, the timescale over which the energy stored in the ejecta is released, the color of the escaping radiation, the level of ionization of the spectrum formation region, and the composition of the ejecta. For a fixed ejecta asymmetry, the continuum polarization is expected to reach a maximum when the core is revealed This is understood as stemming from the progressive growth of the extent of the spectrum formation region during the photospheric phase and the drop in envelope optical depth (favoring single instead of multiple scatterings). This agreement suggests that a spherical ejecta model yields a satisfactory match to the photometric and spectroscopic observations of SN 2012aw. With this adopted density scaling, and at a given radius r, opacities and emissivities scale with B2(1 + A cos β), while the electron-scattering opacity scales with B(1 + A cos β)
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