Oxygen Recombination in the nebular phase of supernovae 1998bw and 2002ap

TL;DR

This study investigates whether oxygen recombination alone can power the O I 7774 Å line in late-time spectra of supernovae 1998bw and 2002ap, using atomic data calculations and nebular emission modeling.

Contribution

The paper provides the first detailed modeling of oxygen recombination line formation in supernova nebulae, including atomic data calculations and nebular spectrum modeling for two specific supernovae.

Findings

Oxygen recombination explains late-time O I 7774 Å emission at very late epochs.

SN 2002ap's nebular emission is well modeled by a one-dimensional shell.

SN 1998bw requires a two-dimensional model for accurate representation.

Abstract

Late-time spectra of stripped-envelope CC-SNe are dominated by strong [O {\sc i}] $\lambda\lambda$6300,6363 emission, caused by thermal electron excitation of forbidden [O {\sc i}] transitions. The permitted O {\sc i} 7774 \AA\ line is also often observed. This line cannot result from thermal electron excitation of the oxygen ground state. In this work tests are performed to verify whether the line can be powered by oxygen recombination alone, using the examples of two of the best studied type Ic SNe, 1998bw and 2002ap. Temperature-dependent effective recombination coefficients for neutral oxygen are calculated using available atomic data. Missing atomic data are computed in a temperature range typical for SN nebulae. Core ejecta models for SNe 1998bw and 2002ap are obtained from modelling their nebular emission spectra so that oxygen recombination line formation is computed consistently with oxygen forbidden line emission. While SN 2002ap can be explained well by a one dimensional shell model, this seems not to be possible for SN 1998bw, for which a two dimensional model is found. At very late epochs the formation of the O {\sc i} 7774 \AA\ line can be explained by recombination radiation for both SNe, but at earlier epochs strong absorption is present which may determine the strength of this line even at $\sim$ 200 days.