Prof. Saul A Rappaport

Professor Rappaport's current research interests are centered on theoretical studies of the formation, evolution, and population synthesis of binary systems containing collapsed stars—white dwarfs, neutron stars, and black holes. Recent focus has been on the development of a number of comprehensive population synthesis codes to study the following types of binary systems:

(i) cataclysmic variables and supersoft X-ray sources with white dwarf accretors;
(ii) low- and intermediate mass X-ray binaries with neutron star accretors;
(iii) recycled binary millisecond pulsars, and
(iv) planetary nebulae which form in interacting binaries.

The formation and evolution of binaries in categories (i), (ii) and (iii) above in globular clusters are significantly different than in the galactic plane due to the possibility of exchange collisions in clusters; both types of scenarios are being investigated. For planetary nebulae formed in interacting binary systems, the wind of the asymptotic giant branch star may be shaped by either the binary companion or a possible collimated fast outflow from the companion. These are being studied with smooth particle hydrodynamic simulations.

Such population synthesis calculations yield theoretical distributions of the properties of the systems of interest, including the masses of the compact object and its companion, the orbital period, and the mass transfer rate at the current epoch. By comparing the theoretical distributions to the observational results obtained by X-ray, optical, and radio astronomers, one can hope to learn about the validity of the various input parameters utilized in the calculations. For example, the assumptions about stellar birth rates, orbital period distribution, and mass ratios of primordial binaries can be tested. Moreover, the method of handling the common envelope phase of the binary evolution (likely involved in the production of most collapsed stars in binary systems), and the distribution of 'kick' velocities imparted to the neutron star during its birth in a supernova explosion, are also testable.

The observational side of Prof. Rappaport's research emphasizes studies of X-ray pulsars. These involve precise determinations of orbital parameters, orbital decay rates, apsidal motion, and the constituent masses, including the neutron stars. His group is particularly interested in what implications the observational results have for binary star evolution. Recent work includes the first determination of the orbit of the neutron star in the well-known classical Be-star system X Per. The combination of the wide orbital separation (about 2 AU) and the small orbital eccentricity (0.11) has significant implications for the 'kick' velocities imparted to neutron stars at their birth. This discovery bolsters previous claims by some researchers that the natal kick distribution is bimodal, with a substantial fraction of neutron stars receiving at most a small kick.