The influence of dust growth on the observational properties of circumplanetary discs

Dust growth is often indirectly inferred observationally in star-forming environments, is theoretically predicted to produce millimetre-sized particles in circumstellar discs, and has also acted on the predecessors of the terrestrial meteoritic record. For those reasons, it is believed that young gas giants under formation in protoplanetary discs that have putative circumplanetary discs (CPDs) surrounding them, such as PDS 70c, should contain millimetre-sized particles. We modelled the spectra of a set of CPDs, which we obtained from radiation hydrodynamic simulations at varying Rosseland opacities, κ_R. The κ_R from the hydrodynamic simulations are matched with consistent opacity sets of an interstellar-medium-like composition, but grown to larger sizes. Our high κ_R hydro data nominally corresponds to 10 µm-sized particles, and our low κ_R cases correspond to millimetre-sized particles. We investigated the resulting broad spectral features at first, while keeping the overall optical depth in the planetary envelope constant. Dust growth to size distributions dominated by millimetre particles generally results in broad, featureless spectra with black-body like slopes in the far-infrared, while size distributions dominated by small dust develop steeper slopes in the far-infrared and maintain some features stemming from individual minerals. We find that significant dust growth from microns to millimetres can explain the broad features of the PDS 70c data, when upscaling the dust masses from our simulations by one hundred times. Furthermore, our results indicate that the spectral range of 30–500 µm is an ideal hunting ground for broadband features arising from the CPD, but that longer wavelengths observed with ALMA can also be used for massive CPDs.