The gravitational drag force on an extended object moving in a gas
  Using axisymmetrical numerical simulations, we revisit the gravitational drag
felt by a gravitational Plummer sphere with mass M and core radius Rs, moving
at constant velocity V0 through a background homogeneous medium of adiabatic
gas. Since the potential is non-diverging, there is no gas removal due to
accretion. When Rs is larger than the Bondi radius RB, the perturbation is
linear at every point and the drag force is well fitted by the time-dependent
Ostriker's formula with r_{min}= 2.25Rs, where r_{min} is the minimum impact
parameter in the Coulomb logarithm. In the deep nonlinear supersonic regime
(Rs<< RB), the minimum radius is no longer related with Rs but with RB. We find
r_min=3.3mach^{-2.5}RB, for Mach numbers of the perturber between $1.5$ and
$4$, although r_{min} = 2\mach^{-2}RB=2GM/V0^{2} also provides a good fit at
mach>2. As a consequence, the drag force does not depend sensitively on the
nonlinearity parameter RB/Rs, for RB/Rs-values larger than a certain critical
value. We show that our generalized Ostriker's formula for the drag force is
more accurate than the formula suggested by Kim & Kim (2009).
