# Nebular H$\alpha$ Limits for Fast Declining Type Ia Supernovae

## TL;DR

This study investigates the presence of H-alpha emission in nebular phases of Type Ia supernovae, finding no strong correlation with decline rates and setting stringent hydrogen mass limits, challenging some progenitor models.

## Contribution

The paper provides new nebular spectra for fast-declining SNe Ia and establishes tighter hydrogen mass limits, expanding the parameter space explored for supernova progenitor scenarios.

## Key findings

- No significant H-alpha emission detected in fast-declining SNe Ia.
- Hydrogen mass limits are about three orders of magnitude below model expectations.
- No correlation found between decline rate and late-time H-alpha emission.

## Abstract

One clear observational prediction of the single degenerate progenitor scenario as the origin of type Ia supernovae (SNe) is the presence of relatively narrow ($\approx$1000 km s$^{-1}$) H$\alpha$ emission at nebular phases, although this feature is rarely seen. We present a compilation of nebular phase H$\alpha$ limits for SN Ia in the literature and demonstrate that this heterogenous sample has been biased towards SN Ia with relatively high luminosities and slow decline rates, as parameterized by $\Delta$m$_{15}(B)$, the difference in $B$-band magnitude between maximum light and fifteen days afterward. Motivated by the need to explore the full parameter space of SN~Ia and their subtypes, we present two new and six previously published nebular spectra of SN Ia with $\Delta$m$_{15}(B)$$ > $1.3 mag (including members of the transitional and SN1991bg-like subclasses) and measure nondetection limits of $L_{H\alpha}$$~<~$0.85--9.9$\times$10$^{36}$ ergs s$^{-1}$, which we confirmed by implanting simulated H$\alpha$ emission into our data. Based on the lastest models of swept-up material stripped from a nondegenerate companion star, these $L_{H\alpha}$ values correspond to hydrogen mass limits of $M_H$$~\lesssim~$1-3$\times$10$^{-4}$ $M_{\odot}$, roughly three orders of magnitude below that expected for the systems modeled, although we note that no simulations of H$\alpha$ nebular emission in such weak explosions have yet been performed. Despite the recent detection of strong H$\alpha$ in ASASSN-18tb (SN 2018fhw; $\Delta$m$_{15}(B)$ = 2.0 mag), we see no evidence that fast declining systems are more likely to have late time H$\alpha$ emission, although a larger sample is needed to confirm this result.