Narrow absorption lines from intervening material in supernovae

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Abstract
Type Ia supernovae (SNe Ia) are thermonuclear runaways of some white dwarfs in binary systems. They have been extensively studied, yet their progenitor and explosion mechanisms remain poorly understood. We studied 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 environmental characteristics. 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 relation with intrinsic properties that is independent of the environment. In fact, there is substantial evidence of 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 ( R V ). Performing a clustering of the SNe Ia, we recovered an old population of SNe with a low extinction and normal ejecta velocity, while the young population can indeed be subdivided into a group of highly extincted high-velocity SNe Ia with much stronger blueshifted sodium absorption and another group of low-extincted normal-velocity objects with little sodium absorption. We interpret this relation of intervening material with intrinsic properties as evidence of the young SN Ia population (occurring in young and star-forming environments) having asymmetric radiation that interacts with nearby material and observables that depend on the viewing angle. Finally, we show that the cosmological mass step is consistent with these populations.

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