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

TL;DR

This study presents late-time optical and NIR observations of superluminous supernova SN 2008es, revealing dust formation and favoring circumstellar interaction as the main powering mechanism over magnetar spin-down.

Contribution

It provides the first late-time NIR data for SN 2008es, supporting dust condensation and circumstellar interaction as key factors in its spectral evolution.

Findings

Detection of broad Hα emission at 288 days

Observation of NIR excess with ~1500 K blackbody temperature

Evidence supporting dust formation in the cool dense shell

Abstract

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, 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 days after explosion. Only broad H$\alpha$ emission is detected in a Gemini spectrum at 288 days. The line profile exhibits red-wing attenuation relative to the early-time spectrum. In addition to the cooling SN photosphere, a NIR excess with blackbody temperature $\sim1500$ K and radius $\sim10^{16}$ cm is observed. This evidence supports dust condensation in the cool dense shell being 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, which still dominates at our late-time epochs. Both models of uniform density and steady wind fit the data equally well, with an effective CSM radius $\sim 10^{15}$ cm, supporting the efficient conversion of shock energy to radiation by CSI. A low amount ($\lesssim 0.4$ $\textrm{M}_\odot$) of $^{56}$Ni is possible 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 less favoured because it overpredicts the late-time fluxes and may be inconsistent with the presence of dust.