The outer gravitational potential of an inhomogeneous torus with an elliptical cross-section

Abstract Toroidal/ring structures are a common feature in a wide variety of astrophysical objects, including dusty tori in active galactic nuclei (AGNs), rings in galaxies, protoplanetary disks, and others. The matter distribution in such structures is not homogeneous and can be flattened by self-gravity or become elongated in the vertical direction, as is the case with obscuring tori in AGNs. This led us to consider the more general case of the gravitational potential of an inhomogeneous torus with an elliptical cross-section. We begin by showing that the outer potential of a homogeneous elliptical torus can be effectively approximated with less than 1% error by the potentials of two infinitely thin rings with a minor correction term. These two rings have masses each equal to half the total mass of the torus. The most notable feature is that each such infinitely thin ring is positioned at precisely the halfway point between the center and the focus of the elliptical cross-section, regardless of the torus’ other parameters. The result, which holds for both oblate and prolate geometries, allows us to find a new expression to handle the outer potential of an inhomogeneous torus with an elliptical cross-section. The confocal density distribution is a special case. We have found that the outer potential of such a torus is only weakly dependent on the density distribution law. Consequently, even for the confocal inhomogeneous case, the outer potential is well represented by two infinitely thin rings. This approach simplifies problems of dynamics and allows for the analysis of the results of N-body simulations for the systems consisting of toroidal structures. For completeness, we have derived the expressions for the components of the external force exerted by a homogeneous torus with an elliptical cross-section, both for the exact form of the potential and for our approximation by two infinitely thin rings. Comparison of the two shows that our model fits the true trend of the force well.