Multi-layered Spectral Formation in SNe Ia Around Maximum Light

TL;DR

This study uses radiative transfer modeling to analyze the spectral line formation in Type Ia supernovae around maximum light, focusing on the silicon feature and its relation to supernova brightness.

Contribution

It identifies the key ions influencing spectral features and demonstrates the complex nature of the true continuum compared to blackbody approximations.

Findings

Fe III and Fe II dominate flux transfer

Si II and S II form in the same physical region

Blackbody fits differ significantly from the true continuum

Abstract

We use the radiative transfer code PHOENIX to study the line formation of the wavelength region 5000-7000 Angstroms. This is the region where the SNe Ia defining Si II feature occurs. This region is important since the ratio of the two nearby silicon lines has been shown to correlate with the absolute blue magnitude. We use a grid of LTE synthetic spectral models to investigate the formation of line features in the spectra of SNe Ia. By isolating the main contributors to the spectral formation we show that the ions that drive the spectral ratio are Fe III, Fe II, Si II, and S II. While the first two strongly dominate the flux transfer, the latter two form in the same physical region inside of the supernova. We also show that the naive blackbody that one would derive from a fit to the observed spectrum is far different than the true underlying continuum.