Properties and Spatial Distribution of Dust Emission in the Crab Nebula
  Recent infrared (IR) observations of freshly-formed dust in supernova
remnants (SNRs) have yielded significantly lower dust masses than predicted by
theoretical models and measured from high redshift observations. The Crab
Nebula's pulsar wind is thought to be sweeping up freshly-formed supernova (SN)
dust along with the ejected gas. The evidence for this dust was found in the
form of an IR excess in the integrated spectrum of the Crab and in extinction
against the synchrotron nebula that revealed the presence of dust in the
filament cores. We present the first spatially resolved emission spectra of
dust in the Crab Nebula acquired with the Infrared Spectrograph aboard the
Spitzer Space Telescope. The IR spectra are dominated by synchrotron emission
and show forbidden line emission from from S, Si, Ne, Ar, O, Fe, and Ni. We
derived a synchrotron spectral map from the 3.6 and 4.5 microns images, and
subtracted this contribution from our data to produce a map of the residual
continuum emission from dust. The dust emission appears to be concentrated
along the ejecta filaments and is well described by an amorphous carbon or
silicate grain compositions. We find a dust temperature of 55+/- 4 K for
silicates and 60 +/- 7 K for carbon grains. The total estimated dust mass is
0.0012-0.012 solar masses, well below the theoretical dust yield predicted for
a core-collapse supernova. Our grain heating model implies that the dust grain
radii are relatively small, unlike what is expected for dust grains formed in a
Type IIP SN.
