Observability of forming planets and their circumplanetary discs - III. Polarized scattered light in near-infrared

There is growing amount of very high resolution polarized scattered light images of circumstellar discs. Nascent giant planets are surrounded by their own circumplanetary discs that may scatter and polarize both the planetary and stellar light. Here, we investigate whether we could detect circumplanetary discs with the same technique and what can we learn from such detections. Here, we created scattered light mock observations at 1.245 microns (J band) for instruments like SPHERE and GPI, for various planetary masses (0.3, 1.0, 5.0, and 10.0 $\rm {\mathrm{ M}_{Jup}}$), disc inclinations (90, 60, 30, and 0 deg), and planet position angles (0, 45, and 90 deg). We found that the detection of a circumplanetary disc at 50 au from the star is significantly favoured if the planet is massive (${\ge} 5 \,\rm {\mathrm{ M}_{Jup}}$) and the system is nearly face-on (≤30°). In these cases, the accretion shock front on the surface of the circumplanetary discs is strong and bright enough to help the visibility of this subdisc. Its detection is hindered by the neighbouring circumstellar disc that also provides a strong polarized flux. However, the comparison between the PI and the Q_ϕ maps is a viable tool to pinpoint the presence of the circumplanetary disc within the circumstellar disc, as the two discs are behaving differently on those images.