Properties of Hot, Dense Matter

We have developed a liquid droplet nuclear model, known as the "LLPR EOS", that allows calculation of matter up to nuclear densities. Comparison with laboratory experiments constrains the nuclear parameters of the model, chiefly the incompressibility parameter, the symmetry energies and the level densities. We are exploring the extrapolation of the equation of state to higher densities, including the possible existence of strange matter in the form of hyperons, quarks and/or a kaon condensate. We are pursuing the role the surface energy plays in the specific heat of nuclei in order to resolve the long-standing discrepancy between experiment and liquid-drop type models of nuclei.

Simulations of Supernova Explosions

We are engaged in numerical simulations of the gravitational collapse of massive stellar cores and the generation and the propagation of the shock wave produced when the core rebounds at nuclear density. Modelling is done with a general relativistic hydrodynamics code, including neutrino transport, in both 1 and 3 dimensions. The ultimate aims are to understand the supernova process, predict the neutrino emission, and infer their dependencies on properties of the nuclear force.

The Composition, Structure and Evolution of Neutron Stars

At birth, a neutron star has a large number of trapped neutrinos which will diffuse out of the star in about 10-15 seconds. As they leave, the composition and structure of the star changes. If exotic matter like quarks, a kaon or pion condensate, or hyperons ever exists in a neutron star, it probably appears only after most of the trapped neutrinos are gone. The appearance of this exotic component always softens the equation of state, and the possibility exists that the star collapses to a black hole at this time. This could explain why no neutron star has yet been seen in the remnant of SN 1987A, even though one certainly existed when the neutrinos detected on Feb 27, 1987 were observed.

Neutrino Emission from Proto(n)-Neutron Stars

Detailed calculations of the neutrino emission from supernovae and from newly formed nucleon stars and the expected signals in terrestrial detectors could permit the observational determination of the supernova mechanism, parameters of the nucleon-nucleon force, and the existence of strange matter at super-nuclear densities. Initial simulations with our existing hydrostatic cooling models are in progress, but with upgraded neutrino transport and tabular neutrino opacities and equations of state. Hydrodynamic simulations are eventually contemplated.

Long-Term Cooling of Neutron Stars

Neutron stars continue to cool following their deleptonization. An important question is whether or not the star cools rapidly, due to the direct URCA process or to the presence of exotic matter. If it does, the thermal X-rays from the stellar surface may become unobservable after only 10-200 years when the surface subsequently cools and the star achieves near isothermality. Otherwise, the X-rays will be observable for up to 100,000 years. Although some observations of thermal emission from neutron stars are claimed, it is controversial whehter or not this emission is actually from the stellar surface or from another source. Studies are being undertaken of neutron star atmospheres to clarify the link between theory and observations of these objects.

Mergers of Compact Objects in Binary Systems

Stony Brook is part of a NASA High Performance Computing Grand Challenge to perform numerical simulations of the inspiral and merger of neutron star-neutron star or neutron star-black hole binaries with neutrino transport and with various equations of state are studied. Predictions of gravitational wave emission are carried out; these mergers are potentially observable with the proposed LIGO orbiting gravitational wave observatory and are about the strongest sources that can be detected. We anticipate being able to determine if merging compact binaries are the sources of the ubiquitous gamma-ray bursters. The nucleosynthesis of matter ejected from these mergers is also studied for its possible role in the r-process and galactic evolution.

A Nearby, Isolated Old Neutron Star

Stony Brook astronomers ( Discovery of a Nearby Isolated Neutron Star , F.M. Walter, S.J. Wolk & R. Neuhauser, Nature, in press 1996, with Figure 1 and Figure 2 ) recently discovered a nearby isolated neutron star that is a unique object because no other neutron stars that are neither pulsars or situated in binary systems are known. In addition, its low extinction and location in the direction of the R CrA molecular cloud limits the distance to D<120 pc. It may give us the cleanest view so far of the surface of a neutron star without complications from non-thermal emission in a magnetosphere. This, coupled with a known upper limit to its distance, severely limits its radius and the equation of state of matter above nuclear density. Hubble observations Walter, et al. (1997) and theoretical arguments Lattimer & Prakash (1997) imply the star's radius is smaller than about 9 km. Such a small radius is only consistent with a large degree of softening of the equation of state at high densities. Additionally, the small amount of crustal moment of inertia is incompatible with many models of pulsar glitches.