Probing interstellar dust with infrared echoes from the Cas A supernova

We present the analysis of an Infrared Spectrograph 5-38 μm spectrum and Multiband Imaging Photometer for Spitzer photometric measurements of an infrared echo near the Cassiopeia A (Cas A) supernova (SN) remnant observed with the Spitzer Space Telescope. We have modeled the recorded echo accounting for polycyclic aromatic hydrocarbons (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 TypeIIbSN explosion showing an optical spectrum remarkably similar to the prototypical TypeIIb SN1993J, 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 SN light curve, where the UV burst and optical light curve characteristics are based on SN1993J. We find a mean density of 680Hcm^-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 300 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 μm 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.