The Aspherical Properties of the Energetic Type Ic SN 2002ap as Inferred from Its Nebular Spectra

The nebular spectra of the broad-lined, Type Ic SN 2002ap are studied by means of synthetic spectra. Two different modeling techniques are employed. In one technique, the SN ejecta are treated as a single zone, while in the other a density and abundance distribution in velocity is used from an explosion model. In both cases, heating caused by γ-ray and positron deposition is computed (in the latter case using a Monte Carlo technique to describe the propagation of γ-rays and positrons), as is cooling via forbidden-line emission. The results are compared, and although general agreement is found, the stratified models are shown to reproduce the observed line profiles much more accurately than the single-zone model. The explosion produced ~0.1 M ☉ of 56 Ni. The distribution in velocity of the various elements is in agreement with that obtained from the early-time models, which indicated an ejected mass of ~2.5 M ☉ with a kinetic energy of 4 × 10 51 erg. Nebular spectroscopy confirms that most of the ejected mass (~1.2 M ☉ ) was oxygen. The presence of an oxygen-rich inner core, combined with that of 56 Ni at high velocities as deduced from early-time models, suggests that the explosion was asymmetric, especially in the inner part.