The evolution of continuum polarization in Type II supernovae as a diagnostic of ejecta morphology

Linear polarization of the optical continuum of type II supernovae (SNe), together with its temporal evolution, is a promising source of information on the large-scale geometry of their ejecta. To help tap this information we have undertaken 2D polarized radiative transfer calculations to map out the possible landscape of type II SN continuum polarization (Pcont) from 20 to 300d after explosion. Our simulations are based on crafted 2D, axisymmetric ejecta constructed from 1D nonlocal thermodynamic equilibrium time-dependent radiative transfer calculations for a red-supergiant star explosion. Following the approach used in our previous work on SN2012aw, we consider a variety of bipolar explosions in which spherical symmetry is broken by the presence, within ~30deg of the poles, of material with a higher kinetic energy (up to a factor of two) and higher 56Ni abundance (up to a factor of about five, with allowance for 56Ni at high velocity). Our set of eight 2D ejecta configurations produces considerable diversity in Pcont (~7000A), although its maximum of 1-4% occurs systematically around the transition to the nebular phase. Before and after that transition, Pcont may be null, constant, rising, or decreasing, which results from the complex geometry of the depth-dependent density and ionization as well as from optical depth effects. Our modest angle-dependent explosion energy can yield Pcont of 0.5-1% at early times. Residual optical-depth effects can yield an angle-dependent SN brightness and constant polarization at nebular times. Observed values of Pcont tend to be lower than obtained here, suggesting more complicated geometries with competing large-scale structures causing polarization cancellation. Extreme asymmetries seem to be excluded.