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 R-s moving at constant velocity V-0 through a background homogeneous medium of adiabatic gas. Since the potential is non-diverging, there is no gas removal due to accretion. When R-s is larger than the Bondi radius R-B, the perturbation is linear at every point and the drag force is well fitted by the time-dependent Ostriker's formula with r(min) = 2.25R(s), where r(min) is the minimum impact parameter in the Coulomb logarithm. In the deep nonlinear supersonic regime (R-s << R-B), the minimum radius is no longer related to R-s but to R-B. We find r(min) = 3.3M(-2.5)R(B) for Mach numbers of the perturber between 1.5 and 4, although r(min) = 2M(-2)R(B) = 2GM/V-0(2) also provides a good fit at M > 2. As a consequence, the drag force does not depend sensitively on the nonlinearity parameter A, defined as R-B/R-s, for A values larger than a certain critical value A(cr). We show that our generalized Ostriker's formula for the drag force is more accurate than the formula suggested by Kim and Kim.