Spectropolarimetry of the Type Ic Supernova SN 2002ap in M74: More Evidence for Asymmetric Core Collapse

Abstract

High-quality spectropolarimetry (range 417-860 nm; spectral resolution 1.27 nm with 0.265 nm pixel-1) of SN 2002ap was obtained with the ESO Very Large Telescope Melipal (+FORS1) at three epochs that correspond to -6, -2, +1, and +3 days for a V maximum of 2002 February 9. The polarization spectra show three distinct broad (~100 nm) features at ~400, 550, and 750 nm that evolve in shape, amplitude, and orientation in the Q-U plane. The continuum polarization grows from nearly zero to ~0.5%. The 750 nm feature is polarized at a level ≳1%. We identify the 550 and 750 nm features as Na I D and O I 777.4 nm moving at about 20,000 km s-1. The blue feature may be Fe II. The position angle of the continuum is ~83°, and that of the O I line is ~133°. This indicates that the 56Ni that provides the continuum luminosity is distributed with a systematically different geometry than that of the fast-moving outer oxygen structure. The lack of significant polarization prior to optical maximum indicates that the photosphere was not extremely highly distorted at early times but became more aspherical toward optical maximum. The structure is consistent with the impact of a bipolar flow from the core that has severely distorted, but is stopped within, the outer envelope of a carbon/oxygen core. The difference in position angle of the oxygen feature and the continuum demands a symmetry breaking that may be difficult to provide in a single-star model. This difference may be a clue to the binary nature of the progenitor. We conclude that the features that characterize SN 2002ap, specifically its high velocity, can be accounted for in an asymmetric model with a larger ejecta mass than SN 1994I such that the photosphere remains longer in higher velocity material. The characteristics of "hypernovae" may be the result of orientation effects in a mildly inhomogeneous set of progenitors, rather than requiring an excessive total energy or luminosity. In the analysis of asymmetric events with spherically symmetric models, it is probably advisable to refer to "isotropic equivalent" energy, luminosity, ejected mass, and nickel mass.