The grids are centred on the centre of mass of the system (star + planets + gas). This is for a correct computation of the angular momentum flux through the interfaces between the two grids.
The frame can be
corotating. In the first case, it is inertial.
This frame being centred on the center of mass, no Indirect term need to be computed.
At every time-step, the procedure
BarycenterConservation (in file
Gframeforce.c) ensures that the centre of mass remains at r=0, and v=0. This is done by translation of the planetary system in the phase space. If there is no planet, the star is hang at (0,0).