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 ∼0.1M_⊙ of ⁵⁶Ni. 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 × 10⁵¹ 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 ⁵⁶Ni at high velocities as deduced from early-time models, suggests that the explosion was asymmetric, especially in the inner part.