# Dynamical Friction of a Circular-Orbit Perturber in a Gaseous Medium - Astrophysics

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Abstract: We investigate the gravitational wake due to, and dynamical friction on, aperturber moving on a circular orbit in a uniform gaseous medium using asemi-analytic method. This work is a straightforward extension of Ostriker(1999) who studied the case of a straight-line trajectory. The circular orbitcauses the bending of the wake in the background medium along the orbit,forming a long trailing tail. The wake distribution is thus asymmetric, givingrise to the drag forces in both opposite (azimuthal) and lateral (radial)directions to the motion of the perturber, although the latter does notcontribute to orbital decay much. For subsonic motion, the density wake with aweak tail is simply a curved version of that in Ostriker and does not exhibitthe front-back symmetry. The resulting drag force in the opposite direction isremarkably similar to the finite-time, linear-trajectory counterpart. On theother hand, a supersonic perturber is able to overtake its own wake, possiblymultiple times, and develops a very pronounced tail. The supersonic tailsurrounds the perturber in a trailing spiral fashion, enhancing the perturbeddensity at the back as well as far front of the perturber. We provide thefitting formulae for the drag forces as functions of the Mach number, whoseazimuthal part is surprisingly in good agreement with the Ostriker-s formula,provided Vp t=2 Rp, where Vp and Rp are the velocity and orbital radius of theperturber, respectively.

Author: ** Hyosun Kim (1), Woong-Tae Kim (1) ((1) Department of Physics and Astronomy, FPRD, Seoul National University, Seoul, Korea)**

Source: https://arxiv.org/