Program: GN-2020B-Q-124

Type Ia supernovae (SNe Ia) are utilized throughout astronomy as extragalactic distance indicators and cosmological probes, yet we lack a consensus on their progenitor systems. Two recent discoveries have provided new insights into these important stellar explosions, both involving the intrinsically faint fast-declining class of thermonuclear supernovae. The first major development is the discovery of bimodal emission lines of elements synthesized in the explosion (Co and Fe). The bimodality suggests two clumps of ejecta moving in opposite directions (i.e., the Doppler effect), which implies the collision or merger of two white dwarfs as the source of the explosion. However, this theory conflicts with spectropolarmetric observations of young SNe Ia which constrain explosion apshericity to $\lesssim 10\%$. The second major development is the detection of weak H$\alpha$ emission in two underluminous SNe Ia exploding in passive elliptical galaxies, deviating from classical SNe Ia with circumstellar material (SNe Ia-CSM). Typical SNe Ia-CSM are rare ($\sim 1\%$ of the population), only occur in spiral/irregular host-galaxies (i.e., with active star formation), and are overlumious SN Ia explosions. Both of the advancements in the literature have serious implications for the progenitors of SNe Ia which remain elusive even after years of study. We propose to follow the evolution of a few ($2-4$) underluminous, fast-declining SNe Ia as they evolve out of the photospheric phase and into the nebular phase (i.e., become optically thin with time). This provides insights into the location and distribution of elements synthesized in the explosion (e.g., Co and Fe) and constrains the location of any H-rich material, if it is present. In this semester, we can accomplish both of these objectives with 5 nights on SNIFS/UH88 while the SNe Ia evolve through the photospheric phase and 18 hours on GMOS/Gemini to obtain deep, high-quality nebular spectroscopy. Over the next 5 semesters, we will observe $10-20$ fast-declining, underluminous SNe Ia and build upon the existing literature sample (8 bimodal events). Assuming the bimodality of a given SN Ia is only dependent on viewing angle, we can estimate what fraction of SNe Ia have \textit{intrinsic} bimodality. This is far more instructive than the inverse argument invoked by studies in the literature which use the observed number of bimodal SNe Ia to infer a critical viewing angle needed to observe bimodality. Similarly, the fraction of underluminous SNe Ia with H$\alpha$ emission is poorly constrained (10 objects in the literature) which our sample will at least double. At the end of our multi-semester observing campaign, we can place statistical constraints on 1) the fraction of SNe Ia that have bimodal signatures and if that fraction is consistent with viewing angle effects, and 2) what fraction of underluminous, fast-declining SNe Ia have H$\alpha$ emission and if these events are the edge of a continuous distribution of SNe Ia-CSM or a unique class of thermonuclear explosions all their own. Answering these unresolved questions will provide a better knowledge of explosion physics and assist cosmologists in disentangling subtle systematics in SNe Ia cosmological data.