The Nature of the Si II 6150A, Ca II HK, Ca II IR-triplet, and other Spectral Features in Supernova Type Ia Spectra

TL;DR

This paper investigates the true nature of spectral features in Type Ia Supernova spectra, revealing that many are emission-dominated and that traditional absorption-based interpretations are often misleading, using synthetic models and a novel 'knock-out' technique.

Contribution

It introduces a new method to analyze spectral features, showing that many are emission-dominated and that line overlaps significantly affect measurements, challenging traditional absorption-based interpretations.

Findings

Many spectral features are emission dominated, not absorption.

Apparent absorption troughs often result from emission feature overlaps.

Line velocities and strengths are affected by spectral line blending.

Abstract

Spectra of Type Ia Supernovae (SN Ia) show both continuum-like and absorption-like line features. The pseudo equivalent width (pEW) and Doppler shift of absorption-line-like features, such as Si II 6150A, Ca II HK 3750A, and the Ca II IR-triplet 8150A quantities that are often associated with the optical depth and the velocity of a shell of interacting material, are important tools for interpreting and classifying the SN Ia spectra. In this paper, we examine the nature of spectral features in SN Ia spectra using W7 model spectra and a technique that we call "knock-out" spectra. We show that the P-Cyg profiles of Si II 6150A and many other features are largely emission dominated, rather than absorption dominated, and that the concepts of "absorption line" and "continuum" are therefore not adequate to describe the nature of spectral features in SNe Ia. Apparent absorption features, like Si II 6150A, are frequently just coincidental troughs between two (or more) uncorrelated emission features. In this situation, the pEW measured between these emission peaks is little related to the true strength of the presumed absorption feature. Furthermore, using the same knock-out technique, we demonstrate how spectral features overlap each other at different times after the explosion. This overlap distorts individual line profiles and affects measured absorption-line velocities. With synthetic SN Ia ejecta stratifications that are tuned to match specific observations, the method presented in this paper can in principle be used to quantify the effects of specific line opacities on measured line velocity, line strength, and line blending, and improve the interpretation and informative value of observed SN Ia spectra.