A Compact Degenerate Primary-Star Progenitor of SN 2011fe

TL;DR

This study uses early non-detection data of SN 2011fe to tightly constrain the progenitor star's size, ruling out non-degenerate stars and supporting a degenerate primary such as a white dwarf or neutron star.

Contribution

It provides the most stringent upper limit on the primary star radius of SN 2011fe using early non-detection data, refining progenitor models.

Findings

Primary star radius R_p <~ 0.02 R_sun

Excludes hydrogen-burning stars and giants as progenitors

Restricts secondary star radius to R_c <~ 0.1 R_sun

Abstract

While a white dwarf is, from a theoretical perspective, the most plausible primary star in Type Ia supernova (SN Ia), many other candidates have not been formally ruled out. Shock energy deposited in the envelope of any exploding primary contributes to the early SN brightness and, since this radiation energy is degraded by expansion after the explosion, the diffusive luminosity depends on the initial primary radius. We present a new non-detection limit of the nearby SN Ia 2011fe, obtained what appears to be just 4 hours after explosion, allowing us to directly constrain the initial primary radius, R_p. Coupled with the non-detection of a quiescent X-ray counterpart and the inferred synthesized Ni mass, we show that R_p <~ 0.02 R_sun (a factor of 5 smaller than previously inferred), that the average density of the primary must be rho_p > 10,000 gm cm^{-3}, and that the effective temperature must be less than a few x 10^5 K. This rules out hydrogen burning main sequence stars and giants. Constructing the helium-burning main sequence and carbon-burning main sequence, we find such objects are also excluded. By process of elimination, we find that only degeneracy-supported compact objects---WDs and neutron stars---are viable as the primary star of SN 2011fe. With few caveats, we also restrict the companion (secondary) star radius to R_ c <~ 0.1 R_sun, excluding Roche-Lobe overflowing red giant and main-sequence companions to high significance.