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SN 2008es is one of the rare cases of a Type II superluminous supernova(SLSN) showing no relatively narrow features in its early-time spectra, andtherefore its powering mechanism is under debate between circumstellarinteraction (CSI) and magnetar spin-down. Late-time data are required forbetter constraints. We present optical and near-infrared (NIR) photometryobtained from Gemini, Keck, and Palomar Observatories from 192 to 554 daysafter explosion. Only broad H$\alpha$ emission is detected in a Gemini spectrumat 288 days. The line profile exhibits red-wing attenuation relative to theearly-time spectrum. In addition to the cooling SN photosphere, a NIR excesswith blackbody temperature $\sim1500$ K and radius $\sim10^{16}$ cm isobserved. This evidence supports dust condensation in the cool dense shellbeing responsible for the spectral evolution and NIR excess. We favour CSI,with $\sim2$--3 $\textrm{M}_\odot$ of circumstellar material (CSM) and$\sim$10--20 $\textrm{M}_\odot$ of ejecta, as the powering mechanism, whichstill dominates at our late-time epochs. Both models of uniform density andsteady wind fit the data equally well, with an effective CSM radius $\sim10^{15}$ cm, supporting the efficient conversion of shock energy to radiationby CSI. A low amount ($\lesssim 0.4$ $\textrm{M}_\odot$) of $^{56}$Ni ispossible but cannot be verified yet, since the light curve is dominated by CSI.The magnetar spin-down powering mechanism cannot be ruled out, but is lessfavoured because it overpredicts the late-time fluxes and may be inconsistentwith the presence of dust.

The Type II Superluminous SN 2008es at Late Times: Near-Infrared Excess and Circumstellar Interaction