Dynamical friction of bodies orbiting in a gaseous sphere

The dynamical friction experienced by a body moving in a gaseous medium is different from the friction in the case of a collisionless stellar system. Here we consider the orbital evolution of a gravitational perturber inside a gaseous sphere using three-dimensional simulations, ignoring however self -gravity. The results are analysed in terms of a 'local' formula with the a ssociated Coulomb logarithm taken as a free parameter. For forced circular orbits, the asymptotic value of the component of the drag force in the dire ction of the velocity is a slowly varying function of the Mach number in th e range 1.0-1.6. The dynamical friction time-scale for free decay orbits is typically only half as long as in the case of a collisionless background, which is in agreement with E. C. Ostriker's recent analytic result. The orb ital decay rate is rather insensitive to the past history of the perturber. It is shown that, similarly to the case of stellar systems, orbits are not subject to any significant circularization. However, the dynamical frictio n time-scales are found to increase with increasing orbital eccentricity fo r the Plummer model, whilst no strong dependence on the initial eccentricit y is found for the isothermal sphere.