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 narrow Na I D absorption in a large sample of 981 SNe Ia to connect line strength and velocity with host environments and intrinsic SN properties. Using KS/FF tests and Gaussian Mixture clustering on photometric, spectroscopic, and environmental data, the authors identify two environmental SN Ia populations (old and young) and further split the young group into two subpopulations (young-receding and young-approaching) with distinct Nai D characteristics and extinction properties. The results suggest a link between explosion asymmetry, nearby material, and viewing angle, consistent with a sub-Chandrasekhar DDet-like channel for the young population and ISM-based absorbing material, with implications for the cosmological mass-step and SN standardisation. Overall, the work argues for multiple SN Ia channels whose local environments and intrinsic explosion properties jointly govern intervening material signatures, and cautions that cosmological calibrations should account for population heterogeneity rather than applying a single, global correction.

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.

Figures (23)

• Figure 1: SNooPy$s_{BV}$ vs SiFTO$s$ light-curve width parameters for SNe Ia in our sample with their corresponding histograms in the panels.
• Figure 2: SNooPy$R_V$ vs $EBV$ parameters for SNe Ia in our sample with their corresponding histograms in the panels. The point colours indicate the SiFTO colour $\mathcal{C}$. Crosses indicate SNe with NIR used in the fits.
• Figure 3: Velocity gradient vs velocity at maximum for Siii (6355Å) and their corresponding histograms. The point colours indicate the nebular velocity when available.
• Figure 4: Cumulative distributions of Nai D EW divided according to the median of the nebular velocity $v_{neb}$.
• Figure 5: Cumulative distributions of Nai D EW divided according to the median of the $R_V$ (top) and the $v_{max}$ (bottom) for the old and young subsamples.