Constraining the Type Ia Supernova Progenitor: The Search for Hydrogen in Nebular Spectra

TL;DR

This study searches for hydrogen in nebular spectra of Type Ia supernovae to constrain their progenitor systems, finding no hydrogen emission and thus ruling out certain single-degenerate models involving close companions.

Contribution

The paper provides deep nebular-phase spectra of nearby Type Ia supernovae and sets stringent upper limits on hydrogen mass, challenging existing progenitor models involving Roche lobe overflow.

Findings

No H-alpha emission detected in spectra.

Upper limit of 0.01 solar masses of hydrogen.

Rules out certain single-degenerate progenitor scenarios.

Abstract

Despite intense scrutiny, the progenitor system(s) that gives 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 H-alpha 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, but has only rarely been sought. Here we present results from a campaign to obtain deep, nebular-phase spectroscopy of nearby Type Ia supernovae, and include multi-epoch observations of two events: SN 2005am (slightly subluminous) and SN 2005cf (normally bright). No H-alpha emission is detected in the spectra of either object. An upper limit of 0.01 M_Sun of solar abundance material in the ejecta is established from the models of Mattila et al. which, when coupled with the mass-stripping simulations of Marietta et al. and Meng et al. 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. Alternative explanations for the absence of H-alpha emission, along with suggestions for future investigations necessary to confidently exclude them as possibilities, are critically evaluated.