Nebular-Phase Spectra of Nearby Type Ia Supernovae

TL;DR

This study presents late-time spectra of nearby Type Ia supernovae, analyzing their physical properties, potential explosion asymmetries, and hydrogen content, to better understand their explosion mechanisms and evolution.

Contribution

It provides new nebular-phase spectra of nearby SNe Ia, explores correlations between early and late spectral features, and constrains hydrogen presence in these supernovae.

Findings

Possible correlation between early and late spectral features suggesting asymmetry.

Limits on hydrogen mass in SNe Ia from nondetections of Hα emission.

Not all SNe Ia show the expected redward shift of iron lines at late times.

Abstract

We present late-time spectra of eight Type Ia supernovae (SNe Ia) obtained at $>200$ days after peak brightness using the Gemini South and Keck telescopes. All of the SNe Ia in our sample were nearby, well separated from their host galaxy's light, and have early-time photometry and spectroscopy from the Las Cumbres Observatory (LCO). Parameters are derived from the light curves and spectra such as peak brightness, decline rate, photospheric velocity, and the widths and velocities of the forbidden nebular emission lines. We discuss the physical interpretations of these parameters for the individual SNe Ia and the sample in general, including comparisons to well-observed SNe Ia from the literature. There are possible correlations between early-time and late-time spectral features that may indicate an asymmetric explosion, so we discuss our sample of SNe within the context of models for an offset ignition and/or white dwarf collisions. A subset of our late-time spectra are uncontaminated by host emission, and we statistically evaluate our nondetections of H$\alpha$ emission to limit the amount of hydrogen in these systems. Finally, we consider the late-time evolution of the iron emission lines, finding that not all of our SNe follow the established trend of a redward migration at $>200$ days after maximum brightness.