SOFIA mid-infrared observations of Supernova 1987A in 2016 --- forward shocks and possible dust re-formation in the post-shocked region?

Abstract: The equatorial ring of Supernova (SN) 1987A has been exposed to forward shocks from the SN blast wave, and it has been suggested that these forward shocks have been causing on-going destruction of dust in the ring. We obtained SOFIA FORCAST 11.1, 19.7 and 31.5 micron photometry of SN\,1987A in 2016. Compared with Spitzer measurements 10 years earlier, the 31.5 micron flux has significantly increased. The excess at 31.5 micron appears to be related to the Herschel 70 micron excess, which was detected 5 years earlier. The dust mass needed to account for the the 31.5--70 micron excess is 3--7x10^-4 Msun, more than ten times larger than the ring dust mass (1x10^-5 Msun) estimate from the data 10-years earlier. We argue that dust grains are re-formed or grown in the post-shock regions in the ring after forward shocks have destroyed pre-existing dust grains in the ring and released refractory elements into gas. In the post-shock region, atoms can stick to surviving dust grains, and the dust mass may have increased (grain growth), or dust grains might have condensed directly from the gas. An alternative possibility is that the outer part of the expanding ejecta dust might have been heated by X-ray emission from the circumstellar ring. The future development of this excess could reveal whether grains are reformed in the post-shocked region of the ring or eject dust is heated by X-ray.