  Circumplanetary discs (CPDs) form around young gas giants and are thought to
be the sites of moon formation as well as an intermediate reservoir of gas that
feeds the growth of the gas giant. How the physical properties of such CPDs are
affected by the planetary mass and the overall opacity is relatively poorly
understood. In order to clarify this, we use the global radiation hydrodynamics
code FARGOCA, with a grid structure that allows resolving the planetary
gravitational potential sufficiently well for a CPD to form. We then study the
gas flows and density/temperature structures that emerge as a function of
planet mass, opacity and potential depth. Our results indicate interesting
structure formation for Jupiter-mass planets at low opacities, which we
subsequently analyse in detail. Using an opacity level that is 100 times lower
than that of ISM dust, our Jupiter-mass protoplanet features an envelope that
is sufficiently cold for a CPD to form, and a free-fall region separating the
CPD and the circumstellar disc emerges. Interestingly, this free-fall region
appears to be a result of supersonic erosion of outer envelope material, as
opposed to the static structure formation that one would expect at low
opacities. Our analysis reveals that the planetary spiral arms seem to pose a
significant pressure barrier that needs to be overcome through radiative
cooling in order for gas to free-fall onto the CPD. The circulation inside the
CPD is near-keplerian and modified by the presence of CPD spiral arms. For high
opacities we recover results from the literature, finding an essentially
featureless hot envelope. With this work, we demonstrate the first simulation
and analysis of a complete detachment process of a protoplanet from its parent
disc in a 3D radiation hydrodynamics setting.
