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 .