Comparative Analysis of Peculiar Type Ia 1991bg-like Supernovae Spectra

TL;DR

This study analyzes the spectra of 1991bg-like Type Ia supernovae, revealing their homogeneity and spectral features, and discusses their classification within the broader supernova taxonomy.

Contribution

It provides a detailed spectroscopic comparison of 1991bg-like supernovae using synthetic spectra, highlighting their common features and subgroup distribution.

Findings

1991bg-like supernovae are spectroscopically homogeneous

Fe II dominates many blue spectral features

Spectroscopic subgroup distribution of Type Ia supernovae is discussed

Abstract

Spectroscopic analyses of Type Ia supernovae have shown there exist four spectroscopic groups---cools, broad line, shallow silicon, and core normal---defined by the widths of the Si II features at 5972 Angstroms and 6355 Angstroms. 1991bg-likes are classified as "cools". Cools are dim, undergo a rapid decline in luminosity, and produce significantly less nickel than normal Type Ia supernovae. They also have an unusually deep and wide trough in their spectra around 4200 Angstroms and a relatively strong Si II absorption attributed to the line at 5972 Angstroms. We examine the spectra of supernova (SN) 1991bg and the cools SN 1997cn, SN 1999by, and SN 2005bl using the highly parameterized synthetic spectrum code SYNOW, and find general agreement with similar spectroscopic studies. Our analysis reveals that this group of supernovae is fairly homogeneous, with many of the blue spectral features well fit by Fe II. The nature of the spectroscopic commonalities and the variations in the class are discussed. Finally, we examine intermediates such as SN 2004eo and discuss the spectroscopic subgroup distribution of Type Ia supernovae.