  G54.1+0.3 is a young pulsar wind nebula (PWN), closely resembling the Crab,
for which no thermal shell emission has been detected in X-rays. Recent Spitzer
observations revealed an infrared (IR) shell containing a dozen point sources
arranged in a ring-like structure, previously proposed to be young stellar
objects. An extended knot of emission located in the NW part of the shell
appears to be aligned with the pulsar's X-ray jet, suggesting a possible
interaction with the shell material. Surprisingly, the IRS spectrum of the knot
resembles the spectrum of freshly formed dust in Cas A, and is dominated by an
unidentified dust emission feature at 21 microns. The spectra of the shell also
contain various emission lines and show that some are significantly broadened,
suggesting that they originate in rapidly expanding supernova (SN) ejecta. We
present the first evidence that the PWN is driving shocks into expanding SN
ejecta and we propose an alternative explanation for the origin of the IR
emission in which the shell is composed entirely of SN ejecta. In this
scenario, the freshly formed SN dust is being heated by early-type stars
belonging to a cluster in which the SN exploded. Simple dust models show that
this interpretation can give rise to the observed shell emission and the IR
point sources.
