A Galactic dust devil: far-infrared observations of the Tornado supernova remnant candidate

We present complicated dust structures within multiple regions of the candidate supernova remnant (SNR) the 'Tornado' (G357.7-0.1) using observations with Spitzer and Herschel. We use point process mapping, PPMAP, to investigate the distribution of dust in the Tornado at a resolution of 8 arcsec, compared to the native telescope beams of 5-36 arcsec. We find complex dust structures at multiple temperatures within both the head and the tail of the Tornado, ranging from 15 to 60 K. Cool dust in the head forms a shell, with some overlap with the radio emission, which envelopes warm dust at the X-ray peak. Akin to the terrestrial sandy whirlwinds known as 'dust devils', we find a large mass of dust contained within the Tornado. We derive a total dust mass for the Tornado head of 16.7 $\rm M_{\odot }$, assuming a dust absorption coefficient of κ₃₀₀ = 0.56 $\rm m^2\, kg^{-1}$, which can be explained by interstellar material swept up by a SNR expanding in a dense region. The X-ray, infrared, and radio emission from the Tornado head indicate that this is a SNR. The origin of the tail is more unclear, although we propose that there is an X-ray binary embedded in the SNR, the outflow from which drives into the SNR shell. This interaction forms the helical tail structure in a similar manner to that of the SNR W50 and microquasar SS 433.