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

TL;DR

This study analyzes the nebular spectra of SN 2002ap using synthetic models to reveal its aspherical explosion characteristics and element distribution, confirming asymmetry and ejecta composition.

Contribution

It introduces stratified nebular spectral modeling for SN 2002ap, demonstrating improved accuracy over single-zone models and providing insights into explosion asymmetry.

Findings

Stratified models better reproduce observed line profiles.

Ejected mass of ~2.5 Msun with ~0.1 Msun of 56Ni.

Explosion was asymmetric with an oxygen-rich inner core.

Abstract

The nebular spectra of the broad-lined, SN 1998bw-like Type Ic SN 2002ap are studied by means of synthetic spectra. Two different modelling 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 gamma-ray and positron deposition is computed (in the latter case using a Monte Carlo technique to describe the propagation of gamma-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 Msun 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 Msun with a kinetic energy of 4 x 10^{51} erg. Nebular spectroscopy confirms that most of the ejected mass (~ 1.2 Msun) 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.