  We report results from Spitzer observations of SNR 0509-68.7, also known as
N103B, a young Type Ia supernova remnant in the Large Magellanic Cloud that
shows interaction with a dense medium in its western hemisphere. Our images
show that N103B has strong IR emission from warm dust in the post-shock
environment. The post-shock gas density we derive, 45 cm$^{-3}$, is much higher
than in other Type Ia remnants in the LMC, though a lack of spatial resolution
may bias measurements towards regions of higher than average density. This
density is similar to that in Kepler's SNR, a Type Ia interacting with a
circumstellar medium. Optical images show H$\alpha$ emission along the entire
periphery of the western portion of the shock, with [O III] and [S II] lines
emitted from a few dense clumps of material where the shock has become
radiative. The dust is silicate in nature, though standard silicate dust models
fail to reproduce the "18 $\mu$m" silicate feature that peaks instead at 17.3
$\mu$m. We propose that the dense material is circumstellar material lost from
the progenitor system, as with Kepler. If the CSM interpretation is correct,
this remnant would become the second member, along with Kepler, of a class of
Type Ia remnants characterized by interaction with a dense CSM hundreds of
years post-explosion. A lack of N enhancement eliminates symbiotic AGB
progenitors. The white dwarf companion must have been relatively unevolved at
the time of the explosion.
