List of Past Astronomy Colloquia : 01-Jan-2003 to 01-Jun-2003

Date:   Wednesday 12-Feb-2003
Speaker:   Alice Quillen (U. Rochester)
Title:  Prospecting for Structure in the Milky Way disk

In the solar neighborhood only a few dozen rotations around the Galactic Center have taken place since the earliest disk stars formed. In fact the stellar velocity distribution reveals a surprising amount of currently unexplained structure, none of which was predicted from previous diffusion based scattering models. I will discuss theoretical approaches based on resonances with spiral and barred pattern modes which I have developed to explain and predict structure in the velocity distribution. I will introduce ways to probe Galactic structure based on current data sets such as 2MASS and large scale CO surveys, and the forthcoming surveys RAVE and GAIA.

Date:   Wednesday 19-Feb-2003
Speaker:   CANCELLED DUE TO SNOW: Paul Feldman (Johns Hopkins)
Title:  A Spectroscopic Tour of the Solar System with the Far Ultraviolet Spectroscopic Explorer

The spectral band, resolution, and sensitivity of the Far Ultraviolet Spectroscopic Explorer satellite, launched in June 1999, provide a unique capability to study the principal atoms, ions and molecules in a variety of planetary environments. The wavelength region from 905 to 1187 A, which is rich in emissions of H2 and CO as well as many ionic species, is observed at a spectral resolution of 0.3 A or less. This talk will describe recent observations and results on the atmospheres of Earth and Mars, polar aurora on Jupiter and Saturn, the Io plasma torus, and cometary comae.

Date:   Wednesday 26-Feb-2003
Speaker:   Paul Goldsmith (Cornell)
Title:  Inappropriate Probes of Dense Molecular Clouds

In this talk I discuss some unexpected results obtained using the world's smallest and largest radio telescopes. They deserve the above appellation because they involve the surprising absence of O2,a key expected molecular species, and the unanticipated presence of a large abundance of an atomic species, HI, in ``molecular'' regions.

The first part of the talk concerns molecular oxygen (O2), studied with the Submillimeter Wave Astronomy Satellite (SWAS), which has been in operation since December 1998. We have not detected this species at a level of a few times 10-7 relative to H2 - several orders of magnitude below the prediction of standard gas phase chemistry. This suggests that other factors must be important, possibly turbulent diffusion or depletion of gas--phase oxygen onto grain surfaces, or both. A tentative detection of O2 in the Rho Oph A molecular outflow suggests that conditions there may reset the chemical clock, and briefly allow abundances expected from standard chemical models to obtain.

The second portion of the talk concerns recent Arecibo observations of HI in cold quiescent clouds. This atomic gas is manifest by very narrow absorption features, referred to as HI Narrow Line Self Absorption, or HINSA. When the absorption profiles are analyzed, and HINSA and molecular maps compared, it is evident that there is considerable atomic hydrogen at temperatures between 10 and 20 K, well--mixed with the molecular material. In some clouds, the fractional abundance of HI is consistent with theoretical models based on cosmic ray destruction of H2, but in others there is a factor up to 100 excess atomic gas, again suggesting that other processes are at work which significantly change the composition of ``molecular'' clouds, make atomic H the third most abundant constituent after H2 and He, and which we need to appreciate in order to understand how low mass star formation operates in these regions.

Date:   Wednesday 05-Mar-2003
Speaker:   Caleb Scharf (Columbia)
Title:  Complex, Complex Clusters

Clusters of galaxies have been used as probes of cosmology and astrophysics since Fritz Zwicky discovered the presence of dark matter in these systems. It has often been a necessity, and generally a matter of convenience to treat these systems in a very idealized way. I will discuss how recent data and investigations have all but overturned this view, and are leading to a renaissance in the study of the astrophysics and origins of clusters.

Speaker:   Chuck Bennett (GSFC)
Title:  First Results From the Wilkinson Microwave Anisotropy Probe (WMAP)

The first findings from a year of WMAP satellite operations provide a detailed full sky map of the cosmic microwave background radiation. The anisotropy in this map, combined with polarization information, carry a wealth of cosmological information, including the age of the universe, the epoch when the first stars formed, and the overall composition of normal matter, dark matter, and dark energy. The results have tantalizing implications for the period of inflationary expansion in the very first moments of time. These and other aspects of the mission will be discussed.

Date:   Wednesday 19-Mar-2003
Speaker:   Duane Pontius (Birmingham-Southern College)
Title:  Electromagnetic coupling between Jupiter and its magnetospheric plasma

Except in very distant regions, Jupiter's magnetosphere is tightly coupled to its ionosphere. This is most immediately apparent in that the plasma moves around the planet at very nearly Jupiter's rotational rate, much faster than Keplerian speed. Dynamical processes within the magnetosphere lead to departures from perfect corotation, and in response the ionosphere exerts electromagnetic torques to reestablish corotation. The currents that communicate this coupling drive aurora that can be observed and studied, providing direct images of these processes. This talk will discuss several illustrative examples of these processes, driven by mass loading, radial transport, and the very fascinating case of the Io current wing.

Date:   Wednesday 26-Mar-2003
Speaker:   Spring Break!!
Title:  No Colloquium Today

Date:   Wednesday 02-Apr-2003
Speaker:   David Nesvorny (SwRI)
Title:  Irregular Satellites of the Jovian Planets

The irregular moons of the jovian planets are a puzzling part of the inventory of the Solar System. Unlike regular satellites, the irregular moons revolve around planets at large distances in tilted and eccentric orbits. Their origin, which is intimately linked with the origin of the planets themselves, has yet to be explained. In this talk, we review the orbital and collisional evolution of the irregular satellites from times after their formation to the present epoch. The purpose of this study was to find out which features of the observed irregular moons can be attributed to this evolution, and which features are signatures of the formation process.

To begin with, we show that the satellite orbits that are highly inclined to the ecliptic (i.e., inclinations near 90 degrees) are unstable due to a dynamical phenomenon that increases the orbit's eccentricity so that satellites either escape from the Hill sphere, collide with massive inner moons, or impact the parent planet. Similarly, prograde satellite orbits with large semimajor axes are unstable due to the effect of a resonance which locks the orbit's apocenter to the apparent motion of the Sun around the parent planet. We use these results to explain the sizes and tilts of the orbits of the known irregular moons and show that the orbits of the fifty known irregular moons (as of 8/16/2002) are stable over long time spans.

In the second part of this talk, we discuss the importance of collisions between irregular satellites. In particular, we show that: (i) the large irregular moons must have collisionally eliminated many small irregular moons, thus transforming their population into the one currently observed; (ii) observed groups of satellites could have been formed by relatively recent catastrophic collisions between irregular moons; and (iii) the surface of Saturn's moon Phoebe must have been heavily cratered by a large primordial population of saturnian irregular moons. We therefore propose that most of the craters that will be observed on Phoebe's surface during Cassini's close flyby in June 2004 were generated by impactors from the primordial population of irregular satellites at Saturn.

We then continue by describing the two previously unknown dynamical groups of tightly clustered orbits that have been recently discovered within the jovian population of irregular satellites. It is possible that these new clusters are remnants of two large, collisionally disrupted moons. Future physical studies of these bodies will offer an unique opportunity to learn about the interior composition of the parent moons. Finally, we show that several irregular moons, namely Jupiter's Pasiphae, Sinope, and S/2001 J10 and Saturn's S/2000 S5, S/2000 S6, and S/2000 S3 have orbits characterized by peculiar resonances. The orbits of some of these moons apparently evolved by dissipative effects in the past and became captured within a tiny resonant volume. The origin and magnitude of these dissipative effects in the primordial circumplanetary environment have yet to be determined.

We conclude this presentation by a brief discussion of the origin of the irregular moons.

Date:   Wednesday 09-Apr-2003
Speaker:   Omer Blaes (UCSB)
Title:  Radiatively Driven Instabilities in Accretion Disks and Stars

Radiative diffusion is often a significant source of damping of acoustic waves in astrophysical fluids. However, in a stratified medium with a background radiative flux, acoustic waves can be driven unstable by periodic radiative forcing. This is the basic physical mechanism behind a disparate variety of instabilities that have been discussed in the literature, including hydrodynamic "strange modes" in stars and so-called "photon bubble modes" in radiation pressure supported accretion flows onto black holes and neutron stars. I will discuss the basic physics of these instabilities, and also present results of simulations of their nonlinear evolution. Finally, I will discuss how these instabilities may manifest themselves observationally in a variety of astrophysical contexts.

Date:   Wednesday 16-Apr-2003
Speaker:   Scott Tremaine (Princeton)
Title:  The Demography of Massive Black Holes

If accretion onto massive black holes is the power source for quasars, then many nearby galaxies should contain "dead quasars"-massive black holes at their centers that do not shine because they are starved for fuel. I will review the accumulating evidence that almost all nearby galaxies contain black holes of 106- 109 solar masses at their centers. I will also discuss how the local distribution of black-hole masses is related to the history of the quasar population.

Date:   Wednesday 23-Apr-2003
Speaker:   Peter Meszaros (Penn State)
Title:  Gamma-Ray Bursts and their Ultra-high Energy Signatures

I will discuss the physics of the 'standard' model of GRB, and after a brief review of the success of the model in explaining the MeV gamma-rays and predicting X-ray, optical and radio afterglows, I concentrate on a discussion of the expected GeV to ZeV energy photon, cosmic ray and neutrino signatures.

Date:   Wednesday 30-Apr-2003
Speaker:   Jean-Luc Margot (Caltech)
Title:  Accurate Measurements of Planetary Rotations: New Advances in Planetary Geophysics

We have implemented a new Earth-based radar technique (Holin, 1992) to determine the rotational dynamics of terrestrial planets with unprecedented accuracy. The mechanisms affecting rotation provide crucial information about interior structure, mechanical properties, and interactions at the core-mantle and atmosphere-surface boundaries. Long-term monitoring of planetary rotations is expected to reveal excitations on a variety of timescales and significant insights into the underlying geophysical processes. The main scientific goal at Mercury is to characterize the size and state of the core in a manner described by Peale (1976). This investigation in conjunction with the MESSENGER mission may have a profound impact on theories of planetary magnetic field generation and planetary thermal evolution. At Venus the main goals is to measure the secular and seasonal variations in spin rate, with fundamental implications for the climate, atmospheric dynamics, and rotational history of the planet. I will present the Mercury data obtained so far and future plans for Mercury, Venus, and Mars.

Date:   Wednesday 7-May-2003
Speaker:   Harry Varvoglis (Aristotle U. Thessaloniki, Greece)
Title:  Motion in the Asteroid Belt: Kirkwood Gaps, Stable Chaos and Local Integrals of Motion

Even two hundred years after Newton's death, astronomers were thinking of the Solar system as being absolutely quasi-periodic. Today, however, we know that there exist large regions of chaotic motion in the asteroid belt, where the motion of asteroids changes in a non-periodic manner. These changes result in an increase of the eccentricity of their orbits. There exist three main mechanisms, through which the eccentricity is increased chaotically. These mechanisms are directly related to the number of local integrals of motion and the topology of the phase space:

* stable chaos, with typical time-scale larger than 1,000 Myrs,
* diffusion, with typical time-scale between 100 - 1,000 Myrs, and
* escape along the unstable manifold of a periodic orbit, with typical time-scale between 1 - 10 Myr, a mechanism related to the creation of the main Kirkwood gaps.

Date:   Wednesday 14-May-2003
Speaker:   Robert Rood (U. Virginia)
Title:  The Role of Binaries and Stellar Collisions in Shaping the Populations of Globular Clusters

I would like to say none. Life is complicated enough with single stars. Unfortunately life is complicated. I will begin by describing a technique which makes it possible to determine the binary fraction in clusters. If binaries exist in cluster centers (and they do in abundance) their interaction cross-sections are large which makes them a dominant player in the dynamical evolution of the cluster.

Binary evolution can produce types of star that would not exist in a population of single stars. For example, low mass x-ray binaries have been known to exist in clusters for three decades. I will discuss the rather more mundane bilue straggler stars (BSS). These exist in all properly observed cluster centers. As simple models of BSS suggest we have found BSS produced both by the merger of primordial binaries (i.e. those present since the birth of the cluster) and as the result of stellar collisions. On the other hand, we are finding many puzzles.

I will also briefly mention the possible role of binary evolution in producing the hottest horizontal branch stars and how data such as my collaborators are gathering can constrain the properties of cluster black holes.

Date:   Wednesday 21-Feb-2018
Speaker:   Dr. Nora Troja (UMD/GSFC)
Title:  Neutron Star Mergers in the New Era of Multi-Messenger Astrophysics

The second run of advanced LIGO led to the breakthrough discovery of the first gravitational wave signal from a neutron star merger, GW170817, coincident with a short duration gamma-ray burst (GRB), at a distance of 40 Mpc. The discovery of GW170817 and its electromagnetic counterparts marked the beginning of a new era of multi-messenger astrophysics, in which photons and gravitational waves provide complementary views of the same source. We now have direct evidence that neutron star mergers are the progenitors of short duration gamma-ray bursts, and the dominant sites for the production of r-process elements. On-going observations of the GRB afterglow are completing the picture by probing the structure and geometry of the relativistic outflow launched after the merger. I will describe the discovery of this unique source, its evolution over the last six months, and present the first results of our multi-messenger analysis. Finally, I will discuss how this event changed our view of neutron star mergers, and why similar cases might have been missed in the past.

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