Deceleration by Dynamical Friction in a Gaseous Medium

The drag force experienced by a gravitational body moving in a straight-line trajectory through a homogeneous isothermal gaseous medium of given sound speed is investigated numerically. For perturbers with constant velocity, linear theory describes successfully the temporal evolution and magnitude of the force. The result obtained recently by E. Ostriker—that for Mach numbers = 1-2 the force is stronger in a gaseous medium than in a collisionless medium, as described by the standard Chandrasekhar formula—is confirmed. The corresponding minimum impact radius r min for a body described with a Plummer model with core radius R soft is r min / R soft ≈ 2.25. When < 1, the drag force is strongly suppressed, which is consistent with Ostriker's results but in disagreement with the Chandrasekhar formula. However, when the perturber is decelerated by its own wake to < 1, the effective drag force remains initially somewhat larger than the value in the case of constant velocity because it takes some time to get rid of the wake that was generated during its supersonic history.