List of Past Astronomy Colloquia : 01-Sep-2001 to 31-Dec-2001

Date:   Wednesday 19-Sep-01
Speaker:   Dr. Joan Centrella (NASA Goddard Space Flight Center)
Title:  "Gravitational Waves - Cosmic Messengers"


Gravitational waves are a new type of astronomical messenger that carry direct information about gravitational dynamics in the universe. This talk will provide an overview of current and future efforts to detect these signals, and offer glimpses of their importance in understanding areas such as black holes, neutron stars, and galaxy formation and evolution.

Date:   Wednesday 26-Sep-01
Speaker:   Prof. Josh Grindlay (Harvard)
Title:  "Stellar Encounters of the Close Kind: Chandra View of Globular Clusters"


The remarkable high resolution of the Chandra X-ray Observatory has opened a new chapter in the study of dense stellar systems and the compact binaries they produce. The rich populations of low luminosity (~10^{30-33} erg/s) x-ray sources discovered in the dense cores of the massive globular 47Tuc vs. the lower mass but core collapsed cluster NGC 6397 present clues to the formation and retention of compact objects (neutron stars vs. white dwarfs; with limits for black holes) and the compact binaries in which they are detected as CVs, quiescent LMXBs and millisecond pulsars. I review results obtained, work in progress, and prospects for deriving fundamental quantities (e.g. neutron star masses and radii) from upcoming Chandra-HST observations.

Date:   Wednesday 03-Oct-01
Speaker:   Prof. Marc Pinsonneault (Ohio State University)
Title:  "Accretion Disks and the Origin of Stellar Rotation"


In the past 15 years observers have obtained an extensive database of rotation periods and velocities for low mass stars. We now have detailed information about the rotation of stars across a wide mass range (from solar analogs to brown dwarfs) and in clusters and associations with a wide range in age (from young star forming regions to systems with the age of the Sun.) Stellar rotation can cause mixing which can alter the surface abundances of stars; the observed spin down of low mass stars is related to the presence of a magnetized wind, which provides clues about the generation of stellar magnetic fields; and the distribution of initial stellar rotation rates provides a window onto the star formation process. I will begin by reviewing the overall results of recent theoretical studies of stellar angular momentum evolution. I will then present new work on the rotation of stars below 0.5 solar masses. These stars provide an excellent test of the initial conditions. We find that the observed spindown of these stars can best be explained by angular momentum coupling between protostars and their accretion disks, and that the inferred distribution of disk lifetimes is consistent with those obtained from other methods. Uncertainties and areas for future work will be discussed. Also see astro-ph/0107061.

Date:   Wednesday 10-Oct-01
Speaker:   Dr. Adam Riess (Space Telescope Science Institute)
Title:  "Cosmological Implications of the Most Distant Supernova (known)"

Date:   Wednesday 17-Oct-01
Speaker:   Prof. Julian Krolik (Johns Hopkins University)
Title:  "The Last Gasp: Dynamics of Matter in the Final Stages of Accretion onto a Black Hole"


Our knowledge of events in the immediate vicinity of black holes is rapidly improving. Diagnostics such as fluorescent X-ray line profiles tell us directly about dynamics of matter not far outside the event horizon. Similar advances have recently been made in theoretical models of the accretion disks that feed black holes: MHD turbulence, driven by a magneto-rotational instability, seems now to be well-established as the underlying mechanism of angular momentum transport.

With these two supports, it has become possible to reconsider the Novikov-Thorne model for the inner regions of relativistic accretion disks. The presence of MHD turbulence undercuts several of its principal assumptions. As a result, the angular momentum and energy per unit mass carried into the black hole may be substantially altered by magnetic stresses in the vicinity of the marginally stable orbit. These stresses may also create the fluctuations in light output that are an observational hallmark of accreting black holes. Large-scale numerical simulations are beginning to yield quantitative accounts of these processes.

Date:   Wednesday 24-Oct-01
Speaker:   Dr. Jean Turner (UCLA)
Title:  "Fine Structure in Starbursts: Supernebulae and Super Star Clusters"


Dense and energetic young nebulae have been discovered in starbursts that require the localized excitation of many thousands of OB stars -- likely young super star clusters. The properties of these nebulae can give clues as to how super star clusters form and evolve.

Date:   Wednesday 31-Oct-01
Speaker:   Dr. Cole Miller (UMd)
Title:  "Origin and Implications of Intermediate-Mass Black Holes"


The discovery of many highly luminous non-nuclear X-ray point sources in starburst galaxies has stimulated speculation about their nature and origin. The strong variability seen in several sources points to massive black holes as the central engines. If the flux is isotropic, the inferred luminosities require masses greater than 10^3 M_sun; at the same time, their off-center locations combined with dynamical friction arguments limit their masses to be less than 10^6 M_sun, implying that this is a new class of black holes. We suggest that these black holes are grown in globular clusters, then released into their host galaxy if the cluster merges with the disk. This model suggests that black holes with masses comparable to 10^3 M_sun could be common in the centers of dense globular clusters, where they would interact primarily with neutron stars and lighter black holes. These objects may therefore be a new source of gravitational waves with unique properties. We will discuss the possibility of detecting this gravitational radiation with future instruments such as LISA and LIGO II, and speculate on current observations that will test our predictions.

Date:   Wednesday 07-Nov-01
Speaker:   Prof. Sylvain Veilleux (UMd)
Title:  "Ultraluminous Infrared Galaxies and the Origin of Quasars"


Ultraluminous infrared galaxies may represent an important stage in the evolution of a wide variety of extragalactic objects including powerful nuclear starbursts, radio galaxies, and quasars. They may also represent a primary stage in the formation of elliptical galaxy cores via the merger of two gas-rich spirals. Ultraluminous infrared galaxies contribute significantly to the extragalactic far-infrared background and perhaps also to the metal enrichment of the intergalactic medium. A review of the most recent results on ultraluminous infrared galaxies will be presented. These new data will be used to quantify star formation and AGN activity in these objects, and to test the model where ultraluminous infrared galaxies represent a critical phase in the evolution of powerful starbursts into optically selected quasars.

Date:   Wednesday 14-Nov-01
Speaker:   Dr. Chuck Dermer (NRL)
Title:  "High Energy Neutrinos from Blazars, and the Evolution of Quasars"

Date:   Wednesday 28-Nov-01
Speaker:   Dr. Ken Sembach (STScI)
Title:  "Hot Gas in the Low Redshift Universe"

Date:   Wednesday 5-Dec-01
Speaker:   Prof. Mitchell Begelman (University of Colorado)
Title:  "Defeating the Eddington Limit: Photon Bubbles in Luminous Accretion Disks"


Magnetized, radiation pressure-supported atmospheres have a tendency to become highly inhomogeneous on small scales, an effect known as "photon bubble instability". When this happens, radiation can leak readily through the low density regions, avoiding the dense regions which are weighed down by gravity. Magnetic tension ties the whole system together, preventing it from blowing apart even if the radiation flux exceeds the Eddington limit. I will explain the physics of this effect and explore whether it might be relevant in luminous accretion disks.

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