Infrared Spitzer Survey of SNRs in the Magellanic Clouds and Shock Models

Astronomy and Astrophysics – Astronomy

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Scientific paper

The Spitzer Space Telescope carried out an imaging survey of supernova remnants (SNRs) in the Magellanic Clouds (MCs) with the Multiband Imaging Photometer (MIPS) and the Infrared Array Camera (IRAC), in a collaborative effort involving a dozen of researchers. Thermal dust emission was detected in most X-ray bright SNRs in the 24 micron MIPS band, and also at 70 microns in the brightest remnants. This emission is produced by the collisionally-heated dust in SNR blast waves, because there is generally a very good match between IR emission and the swept-up X-ray emitting gas. No dust emission can be clearly associated with supernova ejecta. The pulsar wind nebula in 0540-69.3 was detected in all IRAC bands and with MIPS at 24 microns. No emission was found in the IRAC bands for most SNRs, except for several older SNRs and the young SNR N103B where radiative shocks are present.I will describe efforts of our collaboration to model thermal dust emission in MC SNRs, focusing on a simple plane-parallel nonradiative shock model and its use in interpreting Spitzer observations. As dust grains enter the shock, they are subjected to destruction by sputtering in collisions with energetic ions, while both electrons and ions heat dust grains. Thermally fluctuating, nanometer-sized grains are destroyed first, leaving only much larger grains further downstream from the shock. This explains lack of observed emission in the IRAC bands which is usually produced by hot nanometer-sized grains. The measured MIPS 70/24 micron flux ratios can be explained only by models which include destruction of grains by sputtering. The measured dust/gas mass ratios are lower than expected by a factor of several. Thermal dust spectra can be used to infer densities in the X-ray emitting gas; this is demonstrated for the Balmer-dominated Type Ia SNRs in the LMC.

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