RCW 86: A Type Ia Supernova in a Wind-Blown Bubble
  We report results from a multi-wavelength analysis of the Galactic SNR RCW
86, the proposed remnant of the supernova of 185 A.D. We report new infrared
observations from {\it Spitzer} and {\it WISE}, where the entire shell is
detected at 24 and 22 $\mu$m. We fit the infrared flux ratios with models of
collisionally heated ambient dust, finding post-shock gas densities in the
non-radiative shocks of 2.4 and 2.0 cm$^{-3}$ in the SW and NW portions of the
remnant, respectively. The Balmer-dominated shocks around the periphery of the
shell, large amount of iron in the X-ray emitting ejecta, and lack of a compact
remnant support a Type Ia origin for this remnant. From hydrodynamic
simulations, the observed characteristics of RCW 86 are successfully reproduced
by an off-center explosion in a low-density cavity carved by the progenitor
system. This would make RCW 86 the first known case of a Type Ia supernova in a
wind-blown bubble. The fast shocks ($> 3000$ km s$^{-1}$) observed in the NE
are propagating in the low-density bubble, where the shock is just beginning to
encounter the shell, while the slower shocks elsewhere have already encountered
the bubble wall. The diffuse nature of the synchrotron emission in the SW and
NW is due to electrons that were accelerated early in the lifetime of the
remnant, when the shock was still in the bubble. Electrons in a bubble could
produce gamma-rays by inverse-Compton scattering. The wind-blown bubble
scenario requires a single-degenerate progenitor, which should leave behind a
companion star.
