# Understanding nebular spectra of Type Ia supernovae

## TL;DR

This paper uses radiative transfer simulations to study how micro-clumping affects nebular spectra of Type Ia supernovae, revealing its role in ionization and cooling line emissions, with implications for explosion models.

## Contribution

It introduces the effect of micro-clumping in nebular phase simulations of Type Ia supernovae, highlighting its impact on ionization and spectral line diagnostics.

## Key findings

- Clumping reduces ionization and emission from certain ions.
- [Ca II] lines become dominant cooling lines with increased clumping.
- Sub-Chandrasekhar models lack [Ni II] diagnostic features.

## Abstract

In this study, we present one-dimensional, non-local-thermodynamic-equilibrium, radiative transfer simulations (using CMFGEN) in which we introduce micro-clumping at nebular times into two Type Ia supernova ejecta models. We use one sub-Chandrasekhar (sub-M$_{\rm Ch}$) ejecta with 1.02 M$_\odot$ and one M$_{\rm Ch}$ ejecta model with 1.40 M$_\odot$. We introduce clumping factors $f=$0.33,0.25, and 0.10 which are constant throughout the ejecta and compared to the unclumped $f=1.0$ case. We find that clumping is a natural mechanism to reduce the ionization of the ejecta, reducing emission from [Fe III], [Ar III], and [S III] by a factor of a few. For decreasing values of the clumping factor $f$, the [Ca II] $\lambda\lambda$7291,7324 doublet became a dominant cooling line for our M$_{\rm Ch}$ model but still weak in our sub-M$_{\rm Ch}$ model. Strong [Ca II] $\lambda\lambda$7291,7324 indicates non-thermal heating in that region and may constrain explosion modelling. Due to the low abundance of stable nickel, our sub-M$_{\rm Ch}$ model never showed the [Ni II] 1.939 micron diagnostic feature for all clumping values.