Nebular Hα Limits for Fast Declining SNe Ia

One clear observational prediction of the single-degenerate progenitor scenario as the origin of type Ia supernovae (SNe) is the presence of relatively narrow (≈1000 km s^-1) Hα emission at nebular phases, although this feature is rarely seen. We present a compilation of nebular phase Hα limits for SN Ia in the literature and demonstrate that this heterogenous sample has been biased toward SN Ia with relatively high luminosities and slow decline rates, as parameterized by Δm ₁₅(B), the difference in B-band magnitude between maximum light and 15 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 Δm ₁₅(B) > 1.3 mag (including members of the transitional and SN1991bg-like subclasses) and measure nondetection limits of L _Hα < 0.85-9.9 × 10³⁶ erg s^-1, which we confirmed by implanting simulated Hα emission into our data. Based on the latest models of swept-up material stripped from a nondegenerate companion star, these L _Hα values correspond to hydrogen mass limits of M _H lsim; 1-3 × 10^-4 M _o, which are roughly three orders of magnitude below that expected for the systems modeled, although we note that no simulations of Hα nebular emission in such weak explosions have yet been performed. Despite the recent detection of strong Hα in ASASSN-18tb (SN 2018fhw; Δm ₁₅(B) = 2.0 mag), we see no evidence that fast-declining systems are more likely to have late time Hα emission, although a larger sample is needed to confirm this result.