Probing Interstellar Dust with Infrared Echoes from the Cas A Supernova
  We present the analysis of an IRS 5-38 {\mu}m spectrum and MIPS photometric
measurements of an infrared echo near the Cassiopeia A supernova remnant
observed with the Spitzer Space Telescope. We have modeled the recorded echo
accounting for PAHs, quantum-heated carbon and silicate grains, as well as
thermal carbon and silicate particles. Using the fact that optical light echo
spectroscopy has established that Cas A originated from a type IIb supernova
explosion showing an optical spectrum remarkably similar to the prototypical
type IIb SN 1993J, we use the latter to construct template data input for our
simulations. We are then able to reproduce the recorded infrared echo spectrum
by combining the emission of dust heated by the UV burst produced at the shock
breakout after the core-collapse and dust heated by optical light emitted near
the visual maximum of the supernova light curve, where the UV burst and optical
light curve characteristics are based on SN 1993J. We find a mean density of
\sim680 H cm^{-3} for the echo region, with a size of a few light years across.
We also find evidence of dust processing in the form of a lack of small PAHs
with less than \sim300 carbon atoms, consistent with a scenario of PAHs
destruction by the UV burst via photodissociation at the estimated distance of
the echo region from Cas A. Furthermore, our simulations suggest that the weak
11 {\mu}m features of our recorded infrared echo spectrum are consistent with a
strong dehydrogenated state of the PAHs. This exploratory study highlights the
potential of investigating dust processing in the interstellar medium through
infrared echoes.
