Constraining the type 1a supernova progenitor: The search for hydrogen in nebular spectra

Despite intense scrutiny, the progenitor system(s) that give rise to Type Ia supernovae remains unknown. The favored theory invokes a carbon-oxygen white dwarf accreting hydrogen-rich material from a close companion until a thermonuclear runaway ensues that incinerates the white dwarf However, simulations resulting from this single-degenerate, binary channel demand the presence of low-velocity Ha emission in spectra taken during the late nebular phase, since a portion of the companion's envelope becomes entrained in the ejecta. This hydrogen has never been detected. Here we present first results from a campaign to obtain deep, nebular-phase spectroscopy of nearby Type la supernovae, and include multi-epoch observations of two events: SN 2005am (slightly subluminous) and SN 2005cf (normally bright). No Hot emission is detected in the spectra of either object. An upper limit of 0.01 M-circle dot of solar abundance material in the ejecta is established from the models of Mattila et al. (2005) which, when coupled with the mass-stripping simulations of Marietta et al. (2000), effectively rules out progenitor systems for these supernovae with secondaries close enough to the white dwarf to be experiencing Roche lobe overflow at the time of explosion.