Multi-epoch spectropolarimetry of SN 2009ip: direct evidence for aspherical circumstellar material

We present spectropolarimetry of SN 2009ip throughout the evolution of its 2012 explosion. During the 2012a phase, when the spectrum exhibits broad P-Cygni lines, we measure a V-band polarization of P a parts per thousand 0.9 per cent at a position angle of theta a parts per thousand 166A degrees, indicating substantial asphericity for the 2012a outflow. Near the subsequent peak of the 2012b phase, when the spectrum shows signs of intense interaction with circumstellar material (CSM), we measure P a parts per thousand 1.7 per cent and theta a parts per thousand 72A degrees, indicating a separate component of polarization during 2012b, which exhibits a higher degree of asphericity than 2012a and an orthogonal axis of symmetry on the sky. Around 30 d past peak, coincident with a substantial bump in the declining light curve, we measure P a parts per thousand 0.7 per cent and another significant shift in theta. At this point, broad photospheric lines have again become prominent and exhibit significant variations in P relative to the continuum, particularly He i/Na iD. By 60 d past peak, the continuum polarization has dropped below 0.2 per cent, probably declining towards a low value of interstellar polarization. The results are consistent with a scenario in which a prolate (possibly bipolar) explosion launched during the 2012a phase impacts an oblate (toroidal) distribution of CSM in 2012b. Previous calculations that assumed spherical symmetry for the CSM have substantially underestimated the required explosion energy, since only a small fraction of the SN ejecta appears to have participated in strong CSM interaction. A kinetic energy of similar to 10(51) erg is difficult to avoid, supporting the interpretation that the 2012 outburst of SN 2009ip was the result of a core-collapse explosion.