2016MNRAS.460.1645A

We present late-time Hubble and Spitzer Space Telescope (SST) imaging of SN 2008S and NGC 300 2008OT-1, the prototypes of a common class of stellar transients whose true nature is debated. Both objects are still fading and are now >15 times fainter than their progenitors in the mid-IR and are undetected in the optical and near-IR. Data from the Large Binocular Telescope and Magellan show that neither source has been variable in the optical since fading in 2010. We present models of surviving sources obscured by dusty shells or winds and find that extreme dust models are needed for surviving stars to be successfully hidden by dust, which suggests that these transients may be genuine, but low-energy, supernova explosions. Though SN 2008S is not detected in Chandra X-Ray Observatory data taken in 2012, the flux limits allow the fading IR source to be powered solely by the shock interaction of ejecta with the circumstellar medium if the shock velocity at the time of the observation was >=20 per cent slower than estimated from emission line widths while the transient was still optically bright. Continued SST monitoring and 10-20 µm observations with James Webb Space Telescope can resolve any remaining ambiguities.