The Type II superluminous SN 2008es at late times: near-infrared excess and circumstellar interaction

SN 2008es is one of the rare cases of a Type II superluminous supernova (SLSN), showing no narrow features in its early-time spectra, and therefore, its powering mechanism is under debate between circumstellar interaction (CSI) and magnetar spin-down. Late-time data are required for better constraints. We present optical and near-infrared (NIR) photometry obtained from Gemini, Keck, and Palomar Observatories from 192 to 554 d after explosion. Only broad H alpha emission is detected in a Gemini spectrum at 288 d, The line profile exhibits red-wing attenuation relative to the early-time spectrum. In addition to the cooling SN photosphere, an NIR excess with blackbody temperature similar to 1500 K and radius similar to 10(16) cm is observed. This evidence supports dust condensation in the cool dense shell being responsible for the spectral evolution and MR excess, We favour CSI, with similar to 2-3 M-circle dot of circumstellar material (CSM) and similar to 10-20 M-circle dot of ejecta, as the powering mechanism, which still dominates at our late time epochs. Roth models of uniform density and steady wind fit the data equally well, with an effective CSM radius similar to 10(15) cm, supporting the efficient conversion of shock energy to radiation by CSI. A low amount (less than or similar to 0.4 M-circle dot)) of Ni-56 is possible but cannot be verified yet, since the light curve is dominated by CS1. The magnetar spin-down powering mechanism cannot he ruled out, but is less favoured because it overpredicts the late-time fluxes and may be inconsistent with the presence of dust.