  Several detections of wide-orbit planet-mass/sub-stellar companions around
young solar-like stars were reported in the last decade. The origin of those
possible planets is still unclear but accretion tracers and VLT/SPHERE
observations indicate that they are surrounded by circumplanetary material or
even a circumplanetary disk. We want to investigate if the gas component of
disks around wide-orbit companions is detectable with current and future
(sub)mm telescopes and what constraints such gas observations can provide on
the nature of the circumplanetary material and on the mass of the companion. We
applied the radiation thermo-chemical disk code ProDiMo to model the dust and
gas component of passive circumplanetary disks and produced realistic synthetic
observables. We considered different companion properties, disk parameters and
radiative environments and compared the resulting synthetic observables to
telescope sensitivities and to existing dust observations. The main criterion
for a successful detection is the size of the circumplanetary disk. At a
distance of about 150 pc, a circumplanetary disk with an outer radius of about
10 au is detectable with ALMA in about 6 hours in optically thick CO lines.
Other aspects such as the companion's luminosity, disk inclination and
background radiation fields are also relevant and should be considered to
optimize the observing strategy for detection experiments. For most of the
known wide-orbit planet-mass companions, their maximum theoretical disk size of
one third of the Hill radius would be sufficient to allow detection of CO
lines. It is therefore feasible to detect their gas disks and constrain the
mass of the companion through the kinematic signature. Even in the case of
non-detections such observations will provide stringent constraints on disk
size and gas mass, information crucial for formation theories.
