Stellar Dynamics Near Black Holes

Institute For Advanced Study, Princeton NJ

Investigators

Abstract

Dr. Tremaine will develop new methods to study stellar systems around black holes at the centers of galaxies, which are much more massive than any individual star. Changes in the stellar orbits can result in X-ray flares as tidally-disrupted stars fall into the black hole, and cause compact objects such as stars or smaller black holes to spiral slowly into the black hole. If a binary star gets too near the black hole, one of its members can be shot out of the galaxy as a hypervelocity star while the other becomes tightly bound to the black hole. The new method will average over the fast motion of each star in its orbit, and represent the tug on each star of all the others with a basis-function expansion of the secular Hamiltonian, representing each star by its orbital elements or action-angle variables. In a further simplification, the angular momentum of each star usually changes much faster than its energy; so the thermodynamic equilibrium of a disk of stars can be explored under the constraint that the orbital energies are frozen. Preliminary work suggests that a disk of stars around a black hole may undergo a 'phase transition' to become lopsided. This new method will be significantly faster than competing approaches, and offer deeper physical insight. A graduate student will be trained by participating in the research. This work will give deeper insight into the way that the nuclei of galaxies develop around their central black holes, which is one of the fundamental problems of galaxy formation. Dr. Tremaine is co-author of the heavily-cited graduate-level text 'Galactic Dynamics'; a second edition has just been published. This work is jointly funded by the NSF Division of Astronomical Sciences and NASA's Astrophysics Theory Program.