Spring 2003 Astronomy Colloquia - University of Texas at Austin

February 4, 2003
Frank Shu
National Tsing Hua University
"Jets from Young Stellar Objects"

This talk reviews the modern view that the sun and planets both formed from an accretion disk that is built up by continuing infall from a surrounding molecular cloud core. A difficulty with this idea is that pre-main-sequence sunlike stars rotate much more slowly than naive expectations predict. A solution to the difficulty is possible if young stellar objects (YSOs) are strongly magnetized and truncate the inwardly drifting material of their accretion disks before the disks can reach the stellar surfaces. The resulting interactions drive powerful bipolar outflows that collimate slowly into protostellar jets. A prediction of the model is that the highly linear appearances of the outflows at their base, as imaged by the Hubble Space Telescope, are optical illusions. Using quantitative calculations of the collimation and heating and ionization processes, we compare the theoretical models with images and spectroscopic observations of YSO jets.

February 11, 2003
Rocky Kolb
University of Chicago and Fermi Lab
"Search for Nearby Isolated Black Holes"

Using the Sloan Digital Sky Survey and the Rosat All-Sky Survey, we searched for nearby, isolated, accreting, "stellar-mass" (3 to 100 solar mass) black holes. The colloquium will report on the search strategy and resulting candidates.

February 18, 2003
Volker Bromm
Harvard University
"The End of the Dark Ages: The Formation of the First Stars and Quasars"

How and when did the cosmic dark ages end? I present simulations of the formation of the first stars and quasars, discuss their feedback on the IGM, and describe ways to probe their signature with NGST and MAP. The first supernovae are responsible for the initial metal enrichment of the IGM, and I address the impact of this initial enrichment event on the subsequent history of structure formation. Finally, I describe the properties and statistics of high redshift GRBs and SNe that result from the first generation of stars.

February 20, 2003
Oleg Gnedin
"Formation of Globular Clusters: In and Out of Dwarf Galaxies"

Formation of Globular Clusters: In and Out of Dwarf Galaxies
Despite rapid observational progress in studying the structure and distribution of young massive star clusters, the formation of globular clusters remains poorly understood. I discuss a generic scenario for their formation within the progenitor galaxies of the Milky Way, supported by high-resolution hydrodynamical simulations. The oldest clusters formed around redshift z = 9, but the process continued at least until z = 4. The metallicity of 0.01-0.1 solar is provided by the preceding generation of small star clusters. Depending on the infall rate of fresh gas, the feedback from massive clusters may halt the first episode of star formation in the dwarf halos. Because of their high density, globular clusters survive when their progenitor galaxies are disrupted by the Galactic tidal field.

February 25, 2003
Massimo Ricotti
University of Cambridge
"Radiative Feedback in the Early Universe: Clues to the Origin of Dwarf Spheroidals"

The theory of galaxy formation predicts that the creation process is hierarchical: small objects form first, and large galaxies form later from mergers of smaller subunits. The first galaxies are believed to have formed 100 million years after the Big Bang, at redshift z ~ 30. In these primordial galaxies, the first stars emitted light into a previously dark universe. The radiation emitted at this time influenced the subsequent evolution of the universe in a still-unknown way. This process of self-regulation, in which the radiation emitted by galaxies influenced the surrounding intergalactic medium and the future formation of other galaxies, is termed "radiative feedback" from galaxy formation. Using 3D cosmological simulations with radiative transfer we find that the first galaxies are characterized by a bursting star formation that is self-regulated by a feedback process acting on cosmological scale. In contrast to massive galaxies, which reionize voids first, small primordial galaxies partially ionize only the dense filaments, while leaving the voids neutral.

Preliminary results indicate similarities between the properties of simulated primordial galaxies and the bulk of dwarf spheroidal galaxies (dSphs) in the Local Group and Andromeda. I briefly discuss observational tests that could help in understanding the impact of a population of small primordial galaxies on the cosmic evolution.

February 27, 2003
Eiichiro Komatsu
Princeton University
"The WMAP First Results: What Have We Learned About Cosmology?"

The First Year results of the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, which is currently mapping the entire microwave sky to unprecedented accuracy and precision, are presented. The primary goal of the mission is to produce high fidelity maps in order to image the cosmic microwave background radiation (CMB). We begin by describing the first-year maps and the power spectra of the CMB temperature and polarization fluctuations, and then focus on the implications of the results for our understanding of cosmology, including precision determinations of the cosmological parameters, and the evidence for early reionization of the universe.

March 4, 2003
Andrew Benson
California Institute of Technology
"Beyond the CMB: Galaxy Formation as Cosmology"

The rapid progress in observational measurements of anisotropies in the cosmic microwave background means that we now understand the global structure and matter content of the Universe to good precision. One of the main goals of cosmology now is to understand how those anisotropies grow into the objects we see in the Universe today, namely galaxies. I will describe how our understanding of galaxy formation as a cosmological process is progressing, highlighting recent results in the areas of galaxy clustering and the origins of dwarf galaxies and will examine the possible existence of dark galaxies.

March 18, 2003
E. Sterl Phinney
California Institute of Technology
"Physics and Astronomy with LISA (Laser Interferometer Space Antenna)"

The NASA/ESA Laser Interferometer Space Antenna (LISA) mission, planned for launch in 2010 (with a test mission scheduled for 2006) will detect and measure gravitational radiation from astronomical sources at frequencies 0.0001 Hz to 0.1 Hz (compared to 30-1000 Hz for ground-based detectors like LIGO). LISA will detect continuous signals from: thousands of exotic binary stars in the Milky Way, merging supermassive black holes in the nuclei of merging galaxies at redshifts from 0 to 100, and compact stars scattered into supermassive black holes. The latter will provide precision tests of strong-field predictions of relativity, such as the No-Hair Theorem and energy extraction from rotating black holes. We will describe the principles of operation of LISA, and the known and unknown physics, astrophysics, and signal-processing which govern the sources it will detect and the information we will gain.

March 25, 2003
Shri Kulkarni
California Institute of Technology
"Gamma-ray Bursts: The Brightest Explosions in the Universe"

A few times a day the sky is lit up by brilliant flashes of gamma-rays. We now know that these bursts are located at cosmological distances and perhaps even at the edge of the Universe. These bursts are then the most brilliant astronomical objects. There are good reasons to suspect that the bursts may well be responsible for the highest energy cosmic rays in the Universe and are attractive targets for novel telescopes of the future (gravitational wave interferometers, Terra Electron Volt telescopes and neutrino telescopes). There is evidence that GRBs result from the death of massive stars and perhaps may signal the formation of rapidly spinning black holes.

Viatcheslav Mukhanov
Theoretische Physik, Ludwig-Maximilians Universitaet Muenchen
"Inflation: Conjectures vs. Facts"

The inflationary paradigm will be discussed. The robust model independent predictions of inflation will be stressed and elementary explanation of the physics behind them will be presented. The current and future observations testing these predictions will be discussed.

April 1, 2003
Andy Ellington
University of Texas at Austin, Dept. of Chemistry & Biochemistry
"The Difference Between Astronomy and Astrobiology: You Have A Better Time Machine, But At Least We Can Do Experiments"

In attempting to understand the origin of life, the Universe, and everything, it would be marvelous if we could both have a complete record of what has occurred in the past, and be able to carry out experiments in which individual variables in the timeline could be altered to our specifications. Of course, neither of these whims is even remotely a possibility. However, astronomers have a wealth of data available to them in the form of the cosmos, and much of this data is in effect a historical record, since it has taken so very long to get here. In contrast, astrobiologists have very little in the way of old data. There are blobs in rocks that may be ancient microbial fossils, and some DNA evidence ensconced in amber. By and large, though, astrobiologists must rely on inferences derived from comparative biology to discern origins. Fortunately, these inferences can in many instances be tested and validated (or overturned) by laboratory experiments. Dr. Ellington will present the intellectual underpinnings of the so-called 'RNA world,' the period on Earth before the invention of protein catalysts, and will then fall back on a discussion of relevant experiments in his lab.

April 8, 2003
John C. Mather
GSFC, NGST Project Scientist
"JWST Science and Technology Plans"

The James Webb Space Telescope (JWST) will extend the discoveries of the Hubble Space Telescope by deploying a large cooled infrared telescope at the Sun-Earth Lagrange point L2. With a 6 m aperture and three instruments covering the wavelength range from 0.6 to 28 Ám, it will provide sensitivities orders of magnitude better than any other facilities. It is intended to observe the light from the first galaxies and the first supernovae, the assembly of galaxies, and the formation and evolution of stars and planetary systems. In this talk I will review the scientific objectives, the hardware concepts and technology, and the predicted system performance.

April 15, 2003
Ramesh Narayan
Harvard University
"Type I X-ray Bursts in Neutron Stars and Black Holes"

Type I X-ray bursts are very common in accreting neutron stars. They occur when the accreting material undergoes thermonuclear explosions on the surface of the neutron star. Type I bursts exhibit a variety of interesting phenomena, including quasi-periodic oscillations associated with the spin of the star.

Not all accreting neutron stars have bursts. The talk will describe a theoretical formalism that enables us to understand why only some neutron stars burst while others do not. The formalism will then be applied to accreting black hole candidates, which are known from observations never to burst. It will be argued that the only plausible reason for the lack of bursts is that these objects lack surfaces. Thus, the absence of bursts provides strong evidence that black hole candidates truly are black holes, with event horizons.

April 22, 2003
Chris Stubbs
University of Washington
"SuperMacho and Supernovae, Astrophysics in the Time Domain"

A number of forefront problems can be addressed with time-domain observations. I will describe two projects in this vein; 1) a next-generation LMC microlensing survey and 2) a project that is attempting to determine the equation of state of the Dark Energy using type Ia supernovae. Both of these are being used as precursor projects to face up to the software challenges of the Large Synoptic Survey Telescope (LSST), which is presently under development.

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