The role of wind asphericity in dynamical friction

ABSTRACT Dynamical friction (DF) may affect the dynamics of stars moving through dense media. This is the case for stars and compact objects (COs) crossing active galactic nuclei (AGNs) discs, stellar clusters, and common envelopes (CE), driving stellar migration. DF may decelerate the moving stellar object and may also, under certain conditions, produce an acceleration. In this paper, we study the DF and its effects in the interaction between a star and the ambient gaseous medium through a set of two-dimensional, hydrodynamical numerical simulations using a wind tunnel configuration. Three different stellar wind configurations are considered: isotropic, polar, and equatorial. We confirm that the DF can decelerate and accelerate the star and find the critical value of the normalized velocity ($u_c$) that marks the transition between these regimes, for the three wind profiles. The value of $u_c$ for the isotropic wind differs slightly from that obtained in the thin shell approximation; for an aspherical wind, it may either be larger or smaller. Aspherical winds with small u values produce larger accelerations than isotropic winds, while at high u values, they lead to greater deceleration than the isotropic case. The time-scale for DF to substantially affect the velocity of a stellar object is calculated. It is shown to be relevant in AGN discs and CEs.