List of Past Astronomy Colloquia : 01-Jan-2000 to 01-Jun-2000

Date:   Wednesday 2-Feb-00
Speaker:   Dr. Romeel Dave (Princeton University)
Title:  "The Evolution of the Intergalactic Medium"

I will discuss the physics of the evolving IGM as traced by (HI) Lyman alpha forest absorbers seen in the spectra of distant quasars. Hydrodynamic simulations suggest that, at all redshifts, these absorbers primarily arise from highly photoionized gas tracing large-scale structure, and are governed by the well-understood physics of gravitational instability, photoionization heating, and adiabatic cooling. Artificial spectra drawn from these simulations are able to reproduce many of the observed features of the Ly-alpha absorber population at all redshifts. I will present comparisons to Keck/HIRES spectra at redshifts 2.5

Date:   Wednesday 9-Feb-00
Speaker:   Dr. Derek Richardson (University of Washington)
Title:  "Adventures with Rubble Piles: The Evolution of Fragile Planetesimals"

There is increasing evidence that many km-sized bodies in the Solar System are piles of rubble bound together by gravity. The spectacular breakup of Comet D/Shoemaker-Levy 9, the low bulk density of Asteroid 253 Mathilde, the large craters on Mathilde and other small bodies such as the martian moon Phobos, the presence of crater chains on the Galilean moons and even our own Moon, and the spin-period distribution of small asteroids, can all be explained if small bodies in the Solar System ("planetesimals") are predominantly fragile rubble piles. I will present this evidence and demonstrate the implications for the dynamical and collisional evolution of planetesimals through a variety of numerical simulations. I will show how Comet D/Shoemaker-Levy 9 may have disrupted, how tidal disruption and distortion of Earth-crossing asteroids may produce crater chains on the Moon, and what happens when rubble piles collide. An interesting implication of this study is that it may be more difficult than once thought to build planets (and it was already pretty hard)!

Date:   Wednesday 16-Feb-00
Speaker:   Dr. Jens Niemeyer (University of Chicago)
Title:  "Cosmic Fireworks - The Combustion Physics of Type Ia Supernova Explosions"

Type Ia Supernovae (SNe Ia) are believed to be thermonuclear explosions of white dwarf stars composed of carbon and oxygen. The most successful explosion scenarios involve an initial phase of subsonic (deflagration) burning which quickly becomes turbulent owing to the Rayleigh-Taylor instability of the flame front. It was also suggested that a transition to supersonic (detonation) burning may occur later during the explosion. This talk will highlight some aspects of turbulent thermonuclear combustion that are crucial for the development of self-consistent, three-dimensional SN Ia models. Important flame-turbulence interactions take place on scales ranging from a few microns to several thousand kilometers. I will discuss various computational approaches that can help us to understand - and eventually model - these processes and their implications for SN Ia observations.

Date:   Wednesday 23-Feb-00
Speaker:   Dr. Peter Hoeflich (University of Texas)
Title:  "Models for Type Ia Supernovae and Evolutionary Effects with Redshift"

In the last few years, progressive improvements in observations of Type Ia supernovae have placed tight constraints on models. The improvements include the availability of accurate light curves starting a few hours after the explosion, detailed spectral coverage including the optical, IR and X-ray observations, and the measurement of SNe up to redshifts of 1.2. Based on empirical relations, a comparison of low with high redshift SNe Ia allows us to determine cosmological parameters. There are, however, potential problems with systematic changes of these relations with redshift. I will present current theoretical models for SNe Ia, and will discuss how these models may be used to understand the empirical relations and to estimate the magnitude of systematic effects.

Date:   Wednesday 1-Mar-00
Speaker:   Dr. Andrey Kravtsov (Ohio State University)
Title:  "The Abundance of Galactic Satellites in Hierarchical Models: Problems and Possible Solutions"

I will present predictions of popular hierarchical structure formation models regarding the abundance of satellite dark matter halos around galaxies. These models estimate 5-10 times more satellites than are actually observed around the Milky Way and M31. This discrepancy indicates that there are either physical mechanisms at work which greatly reduce the starformation efficiency in most satellite halos (thereby rendering them invisible), or that essential physics is missing in the CDM models. I will discuss some of the proposed solutions, as well as their advantages, problems, and testable implications.

Date:   Wednesday 8-Mar-00
Speaker:   Dr. Jack Hills (Los Alamos National Laboratory)
Title:  "Damage from the Impacts of Comets and Asteroids with Earth"

Comets and asteroids can cause severe damage on impact with Earth. Stony asteroids greater than 40 meters in diameter can cause airblast damage which rapidly increases with asteroid size. The airblast from a stony asteroid 200 meters in diameter can destroy an area the size of Connecticut. Asteroids larger than 200 meters in diameter can cause direct ground impact damage: craters, earthquakes, and tsunami. Tsunami is probably the greatest single danger from these objects. An asteroid the size of the KT impactor that destroyed the dinosaurs 65 million years ago would swamp all of Florida if it impacted in the midAtlantic. An asteroid of this size hitting anywhere in the Pacific would destroy Tokyo and much of the surrounding flat area. Large asteroids would also cause worldwide darkness that could persist for months, which is likely to cause mass starvation in developing countries. Such impacts can be prevented if preparation is made early enough.

Date:   Wednesday 15-Mar-00
Speaker:   Dr. David Bennett (Notre Dame University)
Title:  "The Top and Bottom of the Galactic Mass Function from Gravitational Microlensing Observations"

Gravitational Microlensing surveys first began about a decade ago in order to determine if the dark matter that dominates the mass of the Galaxy is composed of objects in the planetary to stellar mass range. The MACHO and EROS have now excluded most of this mass range from comprising the majority of the dark matter, but gravitational microlensing observations towards the Galactic center by the MACHO and MPS collaborations have revealed tantalizing evidence of objects ranging from low mass planets to isoloated black holes. I present the current microlensing evidence for planets and black holes, and describe how future observations can determine precise black hole masses and detect planets of very low mass.

Date:   Wednesday 29-Mar-00
Speaker:   Dr. Richard Mushotzky (Goddard Space Flight Center)
Title:  TBA

Date:   Wednesday 5-Apr-00
Speaker:   Dr. Vicky Kalogera (Harvard-Smithsonian Center for Astrophysics)
Title:  Compact Objects in Close Binaries: From Birth to Coalescence

Compact objects such as neutron stars and black holes can manifest their presence in a variety of binary stellar configurations, as sources of X-ray emission accreting matter from a close companion or of pulsed radio emission and possibly of gravitational waves in tight relativistic binaries. These systems provide us with a unique opportunity to study the origin and physical properties of neutron stars and black holes. Some of the issues I will discuss in this talk include the measured and theoretically predicted masses of neutron stars and black holes, how the properties of binary pulsars constrain the physical origin of kick velocities imparted to neutron stars at birth, and the exciting possibility that such kicks are associated with black holes as well. I will also briefly review our most up-to-date expectations for the detectability of merging compact objects by interferometers currently under construction, such as LIGO.

Date:   Wednesday 12-Apr-00
Speaker:   Dr. John Mather (Goddard Space Flight Center)
Title:  "The Next Generation Space Telescope"

Date:   Wednesday 19-Apr-00
Speaker:   Dr. Lucy McFadden (University of Maryland)
Title:  "Near-Shoemaker at asteroid 433 Eros: First Results"

The NEAR spacecraft was launched February 17, 1996 to study the Earth-approaching asteroid 433 Eros. It flew past the main-belt asteroid 253 Mathilde in June, 1997 and conducted an Earth fly-by testing all systems in January, 1998. The spacecraft was scheduled to go into orbit in January, 1999 for a year of physical and chemical observations, but a main engine abort caused the mission to be postponed for a year while the spacecraft caught up with Eros. We successfully went into orbit around this 30x13x13 km minor planet on February 14, 2000.

Six scientific instruments will study the asteroid for one year. The early results from these experiments will be reported with an emphasis on what we expect to learn from the remainder of the mission scheduled to continue through March, 2001.

Date:   Wednesday 26-Apr-00
Speaker:   Dr. David Koerner (University of Pennsylvania)
Title:  "Cosmic Habitability: The Origin and Character of Planetary Systems"

Recent imaging of protoplanetary disks and detections of planetary and sub-stellar companions have set in motion a revolution in our picture of extra-solar planetary systems. The steps which lead to planet formation are suggested by a time sequence of circumstellar disks at different stages of development. High-dynamic-range surveys of sub-stellar companions are beginning to provide insight into the end-products of disk evolution. Results to date suggest a highly diverse population of circumstellar bodies, whose inventory is only beginning to come to light. Although the actual extent of life in the universe may remain hidden well into the future, we may understand relatively soon how prevalent are environments that are habitable in principle by life as we know it.

Date:   Wednesday 3-May-00
Speaker:   Dr. Rocky Kolb (University of Chicago/FNAL)
Title:  Seeds of Cosmic Structure: Quantum Fluctuations in the Primordial Soup

If the early universe had a phase of rapid expansion known as inflation, then the pattern of small quantum fluctuations may be imprinted on the universe in the form of large scale structure and fluctuations in the background radiation temperature. In the talk I will explain how something as small as quantum fluctuations can lead to something as large as a galaxy, and review observational tests of the idea.

Date:   Wednesday 10-May-00
Speaker:   Dr. Doug Hamilton (University of Maryland)
Title:  Physics of the Jovian Ring System

Jupiter's tenuous rings have recently been revealed by spectacular new Galileo images. And, as often is the case, with new data has come a new theoretical understanding of the relevant physical processes at work. Consequently, the rings around Jupiter are now the best understood of all the ring systems: we have answers to almost all of the major questions that can be asked about them including age, origin, lifetime, dynamics, structure, particle sizes, etc.

The Jovian ring consists of four components: a relatively narrow bright main ring, a vertically-extended inner halo, and two broad "Gossamer" rings located external to the main ring. The main and Gossamer rings are each associated with small jovian satellites which act as sources to replenish the rings, which are composed primarily of small micron-sized dust grains. Most ring particles are swept inward by Poynting-Robertson drag and lost by collisions with the planet. Resonant dynamics as well as electromagnetic and radiation forces all imprint their signatures on the structure of the ring. This talk will discuss the recent observations and our new theoretical understanding. I will make new predictions and apply results to other ring systems.

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