TITLE: "The Innermost Stable Circular Orbit in Compact Binaries"

SPEAKER: Professor Thomas Baumgarte,

TIME: Thursday, October 12, 2000, starting at 4:00 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

Solving the Kepler problem shows that a Newtonian point mass binary can be brought into arbitrarily close circular orbits. Neutron stars and black holes, however, are extended, relativistic objects. Both finite size and relativistic effects make very close orbits unstable, so that there exists an innermost stable circular orbit (ISCO). The slow and adiabatic inspiral of neutron star and black hole binaries therefore terminates at the ISCO and changes into a rapid plunge and merger. The ISCO leaves a characteristic signature in the emitted gravitational wave signal and is of great interest for gravitational wave astronomy. I will illustrate the physics of the ISCO in a simple model problem, and will then review different techniques which have been employed to locate the ISCO in compact binaries. I will discuss different assumptions and approximations, and will speculate on how differences in the results may be explained and resolved.