A Compact Degenerate Primary-Star Progenitor of SN 2011fe
  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.
