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Dynamical Friction in a Gas: The Supersonic Case

Any gravitating mass traversing a relatively sparse gas experiences a retarding force created by its disturbance of the surrounding medium. In a previous contribution (Lee & Stahler 2011), we determined this dynamical friction force when the object's velocity was subsonic. We now extend our analysis to the supersonic regime. As before, we consider small perturbations created in the gas far from the gravitating object, and thereby obtain the net influx of linear momentum over a large, bounding surface. Various terms in the perturbation series formally diverge, necessitating an approximate treatment of the flow streamlines. Nevertheless, we are able to derive exactly the force itself. As in the subsonic case, we find that F=Mdot*V, where Mdot is the rate of mass accretion onto the object and V its instantaneous velocity with respect to distant background gas. Our force law holds even when the object is porous (e.g., a galaxy) or is actually expelling mass in a wind. Quantitatively, the force in the supersonic regime is less than that derived analytically by previous researchers, and is also less than was found in numerical simulations through the mid 1990s. We urge simulators to revisit the problem using modern numerical techniques. Assuming our result to be correct, it is applicable to many fields of astrophysics, ranging from exoplanet studies to galactic dynamics.