The origin of the late-time luminosity of supernova 2011dh

Due to the small amount of hydrogen (≤0.1 M ⊙) remaining on the surface of their progenitors, SNe IIb are sensitive probes of the mass-loss processes of massive stars toward the ends of their lives, including the role of binarity. We report late-time Hubble Space Telescope observations of SN 2011dh in M51, and a brief period of rebrightening and plateau in the photometric light curve, from 1.8 to 6.2 yr after the explosion. These observations exclude the role of circumstellar interaction, however, a slow rotating magnetar, a significant quantity of radioactive elements, or a light echo could be responsible for the late-time luminosity observed at t > 1000 days. If the late-time light curve is powered by the decay of radioactive elements, SN 2011dh is required to have produced ~2.6 × 10−3 M ⊙ of 44Ti, which is significantly in excess of the amount inferred from earlier nebular spectra of SN 2011dh itself or measured in the Cas A SN remnant. The evolution of the brightness and the color of the late-time light curve also supports the role of a light echo originating from dust with a preferred geometry of a disk of extent ~1.8 to ~2.7 pc from the SN, consistent with a wind-blown bubble. Accounting for the long-term photometric evolution due to a light echo, the flux contribution from a surviving binary companion at ultraviolet wavelengths can be isolated and corresponds to a star of ~9–10 M ⊙.