Narrow absorption lines from intervening material in supernovae. IV. Type Ia supernovae: Na I D line strength relating to external material and intrinsic properties

TL;DR

This study analyzes the relationship between sodium absorption lines and properties of Type Ia supernovae, revealing distinct environmental and intrinsic populations and their implications for supernova progenitors and cosmology.

Contribution

It identifies two supernova populations with different sodium absorption and intrinsic properties, linking environmental factors to supernova characteristics and viewing angle effects.

Findings

Younger SNe Ia have stronger sodium absorption.

Two distinct supernova populations are identified based on environment and intrinsic properties.

The mass-step in cosmology is consistent with these populations.

Abstract

Type Ia supernovae (SNe Ia) are thermonuclear runaways in certain white dwarfs in binary systems. They have been extensively studied, yet their progenitor and explosion mechanisms remain poorly understood. We study a large sample of SNe Ia comparing the narrow interstellar absorption features in their spectra with various photometric and spectroscopic supernova properties, as well as with environmental characteristics. We find that the sodium absorption is significantly stronger in younger, more star-forming and more centrally located SNe Ia, as expected. However, we also show that there is a strong dependence on intrinsic properties that is independent of the environment. In fact, we find strong evidence for two environmental SN Ia populations, an old and a young one, with the young population showing significantly different distributions of sodium strength when divided according to the Si II ejecta velocity, nebular velocity, extinction, E(B-V), and reddening curve, RV. Performing a clustering of the SNe Ia, we recover an old population of SNe with low extinction and normal ejecta velocity, while we confirm that the young population can be subdivided into a group of highly-extincted, high-velocity SNe Ia with much stronger blueshifted sodium absorption, and another of low-extincted, normal-velocity objects with little sodium absorption. We interpret this relation of intervening material with intrinsic properties as evidence for the young SN Ia population, occurring in young and star-forming environments, to have asymmetric radiation that interacts with nearby material, and whose observables depend thus on the viewing angle. Finally, we show that the cosmological mass-step is consistent with these populations.