Synthetic line and continuum linear-polarization signatures of axisymmetric Type II supernova ejecta

We present synthetic single-line and continuum linear-polarization signatures due to electron scattering in axially symmetric Type II supernovae (SNe) which we calculate using a Monte Carlo and a long characteristic radiative-transfer code. Aspherical ejecta are produced by prescribing a latitudinal scaling or stretching of SN ejecta inputs obtained from recent 1D non-local thermodynamic equilibrium (non-LTE) time-dependent calculations. We study polarization signatures as a function of inclination, shape factor, wavelength, line identity and post-explosion time. At early times, cancellation and optical-depth effects make the polarization intrinsically low, even causing complicated sign reversals with inclination or continuum wavelength, and across line profiles. While the line polarization is positive (negative) for an oblate (prolate) morphology at the peak and in the red wing, the continuum polarization may be of any sign. These complex polarization variations are produced not just by the asymmetric distribution of scatterers but also of the flux. Our early-time signatures are in contradiction with predictions for a centrally illuminated aspherical nebula, although this becomes a better approximation at nebular times. For a fixed asymmetry, our synthetic continuum polarization is generally low, may evolve non-monotonically during the plateau phase, but it systematically rises as the ejecta become optically thin and turn nebular. Thus changes in polarization over time do not necessarily imply a change in the asymmetry of the ejecta. The SN structure (e.g. density and ionization) critically influences the level of polarization. Importantly, a low polarization (< 0.5 per cent) at early times does not necessarily imply a low degree of asymmetry as usually assumed. Asphericity influences line-profile morphology and the luminosity, and thus may compromise the accuracy of SN characteristics inferred from these.