  In the core-accretion model the nominal runaway gas-accretion phase brings
most planets to multiple Jupiter masses. However, known giant planets are
predominantly Jupiter-mass bodies. Obtaining longer timescales for gas
accretion may require using realistic equations of states, or accounting for
the dynamics of the circumplanetary disk (CPD) in low-viscosity regime, or
both. Here we explore the second way using global, three-dimensional isothermal
hydrodynamical simulations with 8 levels of nested grids around the planet. In
our simulations the vertical inflow from the circumstellar disk (CSD) to the
CPD determines the shape of the CPD and its accretion rate. Even without
prescribed viscosity Jupiter's mass-doubling time is $\sim 10^4$ years,
assuming the planet at 5.2 AU and a Minimum Mass Solar Nebula. However, we show
that this high accretion rate is due to resolution-dependent numerical
viscosity. Furthermore, we consider the scenario of a layered CSD, viscous only
in its surface layer, and an inviscid CPD. We identify two planet-accretion
mechanisms that are independent of the viscosity in the CPD: (i) the polar
inflow -- defined as a part of the vertical inflow with a centrifugal radius
smaller than 2 Jupiter-radii and (ii) the torque exerted by the star on the
CPD. In the limit of zero effective viscosity, these two mechanisms would
produce an accretion rate 40 times smaller than in the simulation.
