2017ApJ...843...95M

We present a physically motivated model involving the different processes affecting supernova dust grains as they are incorporated into the thermalized medium within young massive star clusters. The model is used to explain the near- to mid-infrared (NIR-MIR) excess found in such clusters and usually modeled as a blackbody with temperature ∼(400–1000) K. In our approach, dust grains are efficiently produced in the clumpy ejecta of core-collapse supernovae, shattered into small pieces ( 0.05µm) as they are incorporated into the hot and dense ISM, heated via frequent collisions with electrons and the absorption of energetic photons. Grains with small sizes can more easily acquire the high temperatures (∼1000 K) required to produce an NIR-MIR excess with respect to the emission of foreground PAHs and starlight. However, the extreme conditions inside young massive clusters make it difficult for these small grains to have a persistent manifestation at NIR-MIR wavelengths as they are destroyed by efficient thermal sputtering. Nevertheless, the chances for a persistent manifestation are increased by taking into account that small grains become increasingly transparent to their impinging ions as their sizes decrease. For an individual SN event, we find that the NIR-MIR excess lasts longer if the time required to incorporate all the grains into the thermalized medium is also longer, and, in some cases, comparable to the characteristic interval between supernova explosions. Our models can successfully explain the near-infrared excesses found in the star clusters observed in M33 assuming a low heating efficiency and mass loading. In this scenario, the presence of the NIR-MIR excess is an indication of efficient dust production in SNe and its subsequent destruction.