Almost Gone: SN~2008S and NGC~300 2008OT-1 are Fainter than their Progenitors

TL;DR

This study investigates the faintness and nature of transients SN 2008S and NGC 300 2008OT-1, analyzing late-time observations to determine if they are obscured survivors or genuine supernovae, with implications for understanding stellar death.

Contribution

The paper provides detailed late-time multi-wavelength observations and models to evaluate whether these transients are obscured surviving stars or actual supernovae, challenging previous classifications.

Findings

Both objects are over 15 times fainter than their progenitors in mid-IR.

No optical variability has been observed since 2010.

Dust models can hide surviving stars, but supernova explanations remain plausible.

Abstract

We present late-time Hubble and Spitzer Space Telescope 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 the 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. Explaining these transients as supernovae explosions, such as the electron capture supernovae believed to be associated with extreme AGB stars, seems an equally viable solution. 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% slower than estimated from emission line widths while the transient was still optically bright. Continued SST monitoring and 10-20 micron observations with JWST can resolve any remaining ambiguities.