Orbits in Extended Mass Distributions: General Results and the Spirographic Approximation

Abstract

This paper explores orbits in extended mass distributions and develops an analytic approximation scheme based on epicycloids (spirograph patterns). We focus on the Hernquist potential which provides a good model for many astrophysical systems, including elliptical galaxies, dark matter halos, and young embedded star clusters. For a given potential, one can readily calculate orbital solutions as a function of energy and angular momentum using numerical methods. In contrast, this paper presents a number of analytic results for the Hernquist potential and proves a series of general constraints showing that orbits have similar properties for any extended mass distribution (including, e.g., the NFW profile). We discuss circular orbits, radial orbits, zero energy orbits, different definitions of eccentricity, analogs of Kepler's law, the definition of orbital elements, and the relation of these orbits to spirograph patterns (epicycloids). Over much of parameter space the orbits can be adequately described (with accuracy better than 10%) using the parametric equations of epicycloids, thereby providing an analytic description of the orbits. As an application of this formal development, we find a solution for the orbit of the Large Magellanic Cloud in the potential of our Galaxy.

arXiv comment40 pages including 9 figures; accepted to The Astrophysical Journal

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• Planets in Other Universes: Habitability constraints on density fluctuations and galactic structure
• Universes without the Weak Force: Astrophysical Processes with Stable Neutrons
• Geometrical view of quantum entanglement
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• Baryonic Collapse within Dark Matter Halos and the Formation of Gaseous Galactic Disks