## List of Past Astronomy Colloquia : 01-Sep-1996 to 31-Dec-1996

Date:   Wednesday 8-May-96
Speaker:   Joe Burns (Cornell U.)
Title:  A Renaissance in Celestial Mechanics

When the space age began, the solar system was viewed as static. This talk will show why and how that view changed. Major advances have been made in understanding the solar system's structure and its cause owing to provocative data provided by spacecraft and ground-based observatories, improved and widely accessible computers that have allowed extraordinarily long orbit integrations, and new paradigms from non-linear dynamics. Precise tracking of spacecraft/celestial bodies has measured the solar system's dimensions and has significantly constrained geophysical parameters of various planets.

No longer is the solar system thought to be a fixed, deterministic entity. Dissipation (by tides, drags and anelasticity) is now known to have profoundly modified the solar system: rotations damp into pure spins while orbits evolve significantly, occasionally leading to the loss of past objects through collisions. Objects that drift into resonant orbits may become trapped at these positions, sometimes protecting themselves, but may also suffer substantial jumps in orbital eccentricity and inclinations, which allow orbits to cross at high speeds. Many orbits were--and are--chaotic. In total, dissipation, close-interactions and chaos have reconfigured the solar system, ejecting many objects to interstellar space, causing some to collide and transmuting yet others from "comets" to "asteroids".

Rotations have also evolved over the eons: Mercury is locked in a 3:2 spin-orbit resonance while Venus' spin does not appear to be tidally damped; Mars undergoes substantial obliquity oscillations that may have markedly influenced its climatic history; Saturn's satellite Hyperion tumbles chaotically and other irregular moons may have, with important thermal consequences. The rotations of asteroids are intimately tied to the collisional histories of the minor planets; some spin extraordinarily slowly, very few others wobble like comet Halley.

Planetary rings exhibit a bewildering array of dynamical phenomena: bending and spiral density waves at resonances; satellite perturbations, including shepherding and clumping; angular momentum transfer through collisions; spokes; and electromagnetic interactions.

Date:   Wednesday 11-Sep-96
Speaker:   Michael F. A'Hearn
Title:  Comets Hyakutake and Hale-Bopp: Why Do We Care?

These two comets present a very rare opportunity to make measurements that are impossible on most comets. These include direct measurements of the nuclei and their physical and dynamical properties, chemical studies including discovery of many new species that may have already placed new constraints on the formation conditions in the early solar system, and the creation of an entire new sub-field - X-Ray studies of comets.

Date:   Wednesday 18-Sep-96
Speaker:   Mark Dickinson (STScI)
Title:  "Galaxies and Galaxy Clusters at Large Redshift"

Date:   Wednesday 25-Sep-96
Speaker:   Prof. Ramesh Narayan (Harvard University)
Title:  "Accretion Flows Around Black Holes"

Date:   Wednesday 02-Oct-96
Speaker:   Dr. Jean Swank (NASA-Goddard)
Title:  Observing accreting compact stars with RXTE

A major goal of RXTE was to investigate the fastest timing signals from neutron stars and black holes. New phenomena were discovered very quickly. In the early observations of some low mass X-ray binaries that are burst sources and "atoll" like in their spectral and previously known temporal properties, quasiperiodic oscillations at frequencies around a kilohertz were found. In some bursts oscillations are seen that are candidates for being the rotation period of the neutron star. In the "Z" source low mass X-ray binary, Sco X-1, similar high frequencies are seen, with marked correlations with other properties. While interpretation of the high frequency power spectra in terms of the disk flow, photon bubble oscillations, neutron star surface waves, and the neutron star rotation are still being argued, it is clear that the dynamical time scale of neutron stars is an active regime. Several black hole candidates have been observed as targets of opportunity when they became very bright. The flux sometimes oscillates as the matter spirals in. The range of luminosities, several orders of magnitude, at which these systems are observed, take them through different physical states in a pattern not yet understood.

Date:   Wednesday 09-Oct-96
Speaker:   Prof. Roger Chevalier (University of Virginia)
Title:  The Diversity of Supernovae: SN 1987A and SN 1993J

Recent radio and X-ray observations of SN 1987A give evidence for supernova interaction with an H II region created in the swept up dense wind from a previous evolutionary phase. The spectrum of the ejecta can be explained with radioactive power input and does not show any signs of power from a central compact source. The presence of the H II region will delay the arrival of the shock front at the dense ring, when bright optical/uv emission is expected. In the case of SN 1993J in M81, dense gas was present close to the explosion, giving rise to radiative phenomena at an early phase.

Date:   Thursday 24-Oct-96 (NOTE SPECIAL DAY)
Speaker:   Dr. Robert Williams (Space Telescope Science Institute)
Title:  "Recent Science Results from HST"

Some of the more recent scientific results from HST will be discussed, including high spatial resolution images of emission regions, color magnitude diagrams of stars in local group galaxies, and work on the Deep Field. The characteristics and some preliminary conclusions from the HDF will be given, including an update on the plans for a similar observational campaign in the southern Continuous Viewing Zone of HST.

Date:   Wednesday 30-Oct-96
Speaker:   Dr. Kenneth Johnston (Scientific Director, U.S. Naval Observatory)
Title:  Optical Interferometry At the Beginning of the Twentyfirst Century

Astronomers have always sought methods to improve the angular resolution of their observations of celestial objects. The technology of optical interferometry will soon offer improved images of stars and in addition precisely measure their positions, motions and diameters. Various programs underway to apply this technology will be described with emphasis given to the Navy Prototype Optical Interferometer.

Date:   Wednesday 06-Nov-96
Speaker:   Dr. Pawan Kumar (Institute for Advanced Study, Princeton)
Title:  "Orbital evolution of an unusual pulsar binary in the SMC and evidence for asymmetric supernova"

Recent observations of a radio pulsar in the Small Magellanic Cloud (SMC), which is in a close eccentric orbit with a massive B-star companion, indicate that the system's orbital period is decreasing on a timescale of $\approx 5 \times10^{5}$ years. This is a factor of about 10$^3$ smaller than the timescale given by the standard theory of tidal dissipation in radiative stars. Observations also provide strong evidence that the B-star is rotating at nearly its break up speed. I will show that these observations imply that the rotation of the B-star must be retrograde (in a direction opposite to the orbtal motion). Tidal torques spin down the surface of the star more efficiently leading to appreciable differential rotation. I will describe how this can enhance the dissipation of tidal energy by a factor of 10$^3$ and thus accounting for the rapid evolution of the orbit.

Date:   Wednesday 13-Nov-96
Speaker:   Dr. Charles Bennett (GSFC)
Title:  "The Microwave Anisotropy Probe (MAP) Mission"

Date:   Wednesday 20-Nov-96
Speaker:   Prof. Peter L. Biermann (Max-Planck-Institut fur Radioastronomie)
Title:  "The propagation of the highest energy particles in cosmic rays through the universe: Analogies to the interaction of the solar wind and the interstellar medium"

The origin of cosmic rays is one of the oldest enigmas of modern particle astrophysics. We expect any particle beyond about $3 \, 10^{18}$ eV to originate from extragalactic space, since its gyrating motion in galactic magnetic fields cannot be contained in our Galaxy. The detection of a significant number of events beyond $10^{20}$ eV deepens the mystery. The interaction with the ubiquitous microwave background should cut off the observed particle flux near $5 \, 10^{19}$ eV.

I will review the present state of the theory on the origin and propagation of cosmic rays through the interstellar medium, including energies up to $\sim 3 \, 10^{18}$ eV, and include a brief discussion of the abundances in cosmic rays and their spallation contribution to interstellar medium abundances.

I will argue that the highest energy cosmic ray particles may be protons originating in the hot spots and jets of powerful radio galaxies, where they may have source energies of up to a few times $10^{21}$ eV. Highest energy cosmic ray have arrival directions which correlate with the cosmological locus of nearby galaxies, the supergalactic plane. This has now been shown on the basis of the data sets from two major experiments, the Haverah Park array, and the AGASA array. However, the correlation is best with the geometric sheet plane, and not with the distribution of galaxies and radio galaxies, which actually make up this plane observationally. An analogy of the interaction between the solar wind and the interstellar medium, applied to the formation of structure in the universe may help to resolve this question.

Date:   Wednesday 04-Dec-96
Speaker:   Dr. Chryssa Kouveliotou (NASA-Marshall)
Title:  The bursting pulsar GRO J1744-28

I will discuss the discovery of the transient source GRO J1744-28 and its properties. GRO J1744-28 is a low-mass X-ray binary, which is unique in showing both X-ray bursts and pulsations. I will describe the properties of the pulsar (spin, orbital and spectral parameters) and of the bursts (durations, fluxes/fluences, rates, and spectral evolution). I will also describe a unique property of this source, namely the 'interaction' between the bursts and the pulsations. This interaction is exhibited as a sudden phase-shift of the pulsations during each burst, which decays slowly after the burst during persistent emission. This phenomenon, together with other evidence, relates the bursts to an accretion instability.

Date:   Wednesday 11-Dec-96
Speaker:   Prof. Jeremiah Ostriker (Princeton University)
Title:  Gravitational Lensing and the Nature of Dark Matter

Gravitational lensing of cosmologically distant objects provides direct information on the gravitational potential fluctuations along lines of sight to those objects. This information has two advantages over most cosmological information based on galaxy observations. First, it provides a fair sample since the lines of sight randomly sample the universe towards distant objects, and second we are looking at the mass density directly (via the induced potential fluctuations), so no questions of "bias" arise. But, despite these advantages, lensing has been little used, since it is difficult to compute directly. A report will be given of recent simulations which have sufficient resolution to allow a direct computation of the expected number of gravitational lenses. This work already allows one to eliminate some models (such as the standard Cold Dark Matter scenario) and may help point the way to the correct cosmological theory.