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Constraining the Type Ia Supernova Progenitor: The Search for Hydrogen in Nebular Spectra
Despite intense scrutiny, the progenitor system(s) that gives rise toType Ia supernovae remains unknown. The favored theory invokes acarbon-oxygen white dwarf accreting hydrogen-rich material from a closecompanion until a thermonuclear runaway ensues that incinerates thewhite dwarf. However, simulations resulting from this single-degenerate,binary channel demand the presence of low-velocity Hα emission inspectra taken during the late nebular phase, since a portion of thecompanion's envelope becomes entrained in the ejecta. This hydrogen hasnever been detected, but has only rarely been sought. Here we presentresults from a campaign to obtain deep, nebular-phase spectroscopy ofnearby Type Ia supernovae, and include multiepoch observations of twoevents: SN 2005am (slightly subluminous) and SN 2005cf (normallybright). No Hα emission is detected in the spectra of eitherobject. An upper limit of 0.01 Msolar of solar abundancematerial in the ejecta is established from the models of Mattila et al.,which, when coupled with the mass-stripping simulations of Marietta etal. and Meng et al., effectively rules out progenitor systems for thesesupernovae with secondaries close enough to the white dwarf to beexperiencing Roche lobe overflow at the time of explosion. Alternativeexplanations for the absence of Hα emission, along withsuggestions for future investigations necessary to confidently excludethem as possibilities, are critically evaluated.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. Additional observations were obtained at the GeminiObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under a cooperative agreement with the NSFon behalf of the Gemini partnership: the National Science Foundation(United States), the Particle Physics and Astronomy Research Council(United Kingdom), the National Research Council (Canada), CONICYT(Chile), the Australian Research Council (Australia), CNPq (Brazil), andCONICET (Argentina).
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