2013ApJ...762L...5S

A generic prediction of the single-degenerate model for Type Ia supernovae (SNe Ia) is that a significant amount of material will be stripped from the donor star (∼0.5 M_☉ for a giant donor and ∼0.15 M_☉for a main-sequence donor) by the supernova ejecta. This material, excited by gamma-rays from radioactive decay, would then produce relatively narrow ( ≲ 1000 km/s) emission features observable once the supernova enters the nebular phase. Such emission has never been detected, which already provides strong constraints on Type Ia progenitor models. In this Letter, we report the deepest limit yet on the presence of Hα emission originating from the stripped hydrogen in the nebular spectrum of an SN Ia obtained using a high signal-to-noise spectrum of the nearby normal SN Ia 2011fe 274 days after B-band maximum light with the Large Binocular Telescope's Multi-Object Double Spectrograph. We put a conservative upper limit on the Hα flux of 3.14x10^–17 erg/s/cm2, which corresponds to a luminosity of 1.57x10³⁵ erg/s. By scaling models from the literature, our flux limit translates into an upper limit of ≲ 0.001 M_☉ of stripped material. This is an order of magnitude stronger than previous limits. SN 2011fe was a typical SN Ia, special only in its proximity, and we argue that lack of hydrogen emission in its nebular spectrum adds yet another strong constraint on the single-degenerate class of models for SNe Ia.