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

We present spectropolarimetry of SN 2009ip throughout the evolution of its\n2012 explosion. During the initial 2012a phase, when the source spectrum\nexhibits broad P-Cygni lines, we measure a V-band polarization of P~0.9% at a\nposition angle of ~166 deg, indicating substantial asphericity for the 2012a\noutflow. Near the subsequent peak of the 2012b phase, when the spectrum shows\nsigns of intense interaction with circumstellar material (CSM), we measure\nP~1.7% at a position angle of 72 deg, indicating a separate physical component\nof polarization, which has a higher degree of asphericity than the 2012a\noutflow and an orthogonal axis of symmetry on the sky. Around 30 days past\npeak, which is coincident with a bump in the declining light curve, we measure\nP~0.7% and significant variations in P across some broad lines, particularly\nHeI/NaI. By 60 days past peak the source appears to be approaching a low value\nof interstellar polarization (P<0.2%). The results are consistent with a\nscenario in which a potentially bipolar explosion during 2012a impacts a\ntoroidal distribution of CSM, thereby causing the 2012b brightening.\nOrthogonality between the 2012a and 2012b geometries is inconsistent with the\nhypothesis that the 2012a event launched the CSM that was hit 40 days later by\nmaterial from the 2012b event. Rather, the aspherical CSM probably has a\nseparate origin associated with the outbursts of the stellar progenitor during\nprior years. Previous calculations that assumed spherical symmetry for the CSM\nhave underestimated the required explosion energy, as the results suggest that\n<10% of the fast SN ejecta participated in strong CSM interaction during the\n2012b phase. In light of the spectropolarimetric results, a kinetic energy of\n1e51 erg for the ejecta is difficult to avoid, supporting the interpretation\nthat the 2012 outburst was the result of a core-collapse supernova explosion\n(abridged)\n