On Spectral Line Profiles in Type Ia Supernova Spectra

TL;DR

This paper analyzes the spectral line profiles in Type Ia supernova spectra, revealing that the blue component of the Ca H&K feature is mainly caused by Si II 3858, affecting interpretations of supernova properties and cosmological measurements.

Contribution

The study clarifies the origin of the blue component in Ca H&K features, showing it is predominantly due to Si II 3858, and discusses implications for supernova analysis.

Findings

Blue component caused by Si II 3858 in most SNe Ia

Strength of Si II 3858 varies with light-curve shape

Velocity from single-Gaussian fit correlates with light-curve shape

Abstract

We present a detailed analysis of spectral line profiles in Type Ia supernova (SN Ia) spectra. We focus on the feature at ~3500 - 4000 A, which is commonly thought to be caused by blueshifted absorption of Ca H&K. Unlike some other spectral features in SN Ia spectra, this feature often has two overlapping (blue and red) components. It is accepted that the red component comes from photospheric calcium. However, it has been proposed that the blue component is caused by either high-velocity calcium (from either abundance or density enhancements above the photosphere of the SN) or Si II 3858. By looking at multiple data sets and model spectra, we conclude that the blue component of the Ca H&K feature is caused by Si II 3858 for most SNe Ia. The strength of the Si II 3858 feature varies strongly with the light-curve shape of a SN. As a result, the velocity measured from a single-Gaussian fit to the full line profile correlates with light-curve shape. The velocity of the Ca H&K component of the profile does not correlate with light-curve shape, contrary to previous claims. We detail the pitfalls of assuming that the blue component of the Ca H&K feature is caused by calcium, with implications for our understanding of SN Ia progenitors, explosions, and cosmology.