Dept. of Astronomy Colloquia Schedule for Spring 2010
University of Texas at Austin
 Jan. 19th |
Gamma-ray bursts (GRBs) are the most spectacular transient events in the universe, |
Jan. 26th |
Jay Boisseau University of Texas at Austin "TACC's Capabilities for Enabling World-Class Science" |
Feb. 2nd |
Erin Bonning Yale University "Multiwavelength Observations of Fermi Blazars" Multiwavelength observations of blazars have entered a new era with the launch and successful operation of the Fermi Gamma-ray Space Telescope. With concurrent monitoring programs spanning decades of energies from radio through gamma-rays, the characteristic double-peaked spectral energy distribution (SED) of blazars can be observed evolving on timescales from days to months (and ultimately to years). The low energy peak of the SED is well understood as synchrotron emission from electrons accelerated in a relativistic jet; however, the source of the high-energy emission is as yet poorly understood with multiple models often able to fit a given single-epoch SED. With both spectral and time-variability information available for a large number of sources, physical models for gamma-ray emission can be more effectively constrained. I will discuss recent and ongoing observations of blazars with Fermi, associated multiwavelength campaigns, and theoretical implications of these observations. |
Feb. 9th |
Ravit Helled University of California, Los Angeles "Metallicity of the Massive Protoplanets around HR 8799 if formed by Gravitational Instability" One possible mechanism for giant planet formation is disk instability in which the planet is formed as a result of gravitational instability in the protoplanetary disk surrounding the young star. The final composition of giant planets formed as a result of gravitational instability depends on their ability to capture solid material (planetesimals) during their 'pre-collapse' stage, when they are extended and cold, and contracting quasi-statically. The total mass that can be captured by the protoplanets depends on the planetary mass, planetesimal size, the radial distance of the protoplanet from the parent star, and the local solid surface density. We consider three radial distances, 24, 38, and 68 AU, similar to the radial distances of the planets in the system HR 8799, and planetary masses of 3, 5, 7, and 10 Jupiter masses, and estimate the mass of heavy elements that can be accreted. We find that for the planetary masses usually adopted for the HR 8799 system, the amount of heavy elements accreted by the planets is small, leaving them with nearly stellar compositions. |
Feb. 16th |
Angela Speck University of Missouri, Columbia (Tinsley Visiting Scholar) "Through the Looking Glass: A Re-analysis of Silicate Dust Spectral Features" Silicate dust plays an essential role in many astrophysical environments. The "amorphous" silicate spectral features have been observed in our solar system, young stellar objects, star formation regions, novae, and the diffuse and dense interstellar medium as well as in extragalactic environments such as quasars and AGN. This dust contributes to the physical processes inherent in star formation processes, as well as to several aspects of interstellar processes such as gas heating and the formation of molecules. The discovery of this almost ubiquitous ~10um silicate feature, led to many laboratory studies of potential cosmic dust analogs attempting to determine the exact nature of this dust. New laboratory data combined with re-analysis of observational data for asymptotic giant branch (AGB) stars will be presented. Combined, these data present a challenge to the "common wisdom" on what we think we know about silicates in space. |
Feb. 23rd |
John Johnson California Institute of Technology "The Golden Age of Exoplanet Spin-Orbit Measurements" The angular momentum of the Solar System planets is remarkably well aligned with the spin axis of the Sun. This arrangement supports the hypothesis that the planets and the Sun formed from the same spinning disk of gas and dust. Until very recently, measurement of the spin-orbit alignment of exoplanets had revealed that planets around other stars, most in very close-in orbits (periods less than 10 days), also have well-aligned spin and orbit axes. However, our recent measurements from this past year have revealed that there may be a second class of exoplanets that inhabit very tilted orbits, and that some may even have retrograde configurations. I'll show how spin-orbit measurements are made, and discuss how the distribution of observed spin-orbit angles informs, and challenges, theories of planet migration. |
Feb. 24th (Wed) |
Andrew Blain California Institute of Technology "Discovering and Understanding the Most Luminous Galaxies" The most extreme end of the galaxy luminosity function is revealed most clearly in far-infrared light. By seeking the far-infrared properties of galaxies when star formation and galaxy evolution were at their most intense, their true total power output can be revealed, along with information about the astrophysics in the gas from which stars form and supermassive blackholes grow. I will describe multi-wavelength observations of some of the most extreme galaxies at redshifts 2-3, and highlight some opportunities with ALMA, and NASA's recently launched WISE MIDEX mission, that of 24th February should have imaged more than 20% of the sky from 3.3-23 microns. |
Mar. 2nd |
Evgenya Shkolnik Carnegie Institution Department of Terrestrial Magnetism "Star-Planet Interactions: The Tidal and Magnetic Influence of Hot Jupiters" The interacting processes taking place between a giant planet orbiting its star within 10 stellar radii (also known as a "hot Jupiter") have been getting increasing attention both observationally and theoretically. Our work has shown that such a short-period planet can induce activity on the upper atmosphere of its host star through both tidal and magnetic star-planet interactions (SPI). Evidence for magnetic SPI includes a diverse array of photometric, spectroscopic and spectropolarimetric studies. Because of the small separation (< 0.1 AU), many of the hot Jupiters lie within the Alfven radius of their host stars, allowing direct magnetic interaction with the stellar surface. Models show both the stellar and planetary magnetic fields are strongly affected, possibly influencing the magnetic activity of both bodies, as well as modifying irradiation and non-thermal and dynamical processes. In addition, a hot Jupiter's tidal influence on its star may increase the stellar rotation rate and thus also increase the global stellar activity level. Our recent work has shown that stars with hot Jupiters have twice the UV emission than stars with planets in wider orbits, which is also anti-correlated with the stellar synchronization time scales. Even though the stars with hot Jupiters are not fully synchronized (full synchronization in most cases will take longer than the age of the Universe), they have already undergone some increase in rotation rate, provided that the planets migrated early on in the system's history. Studying both tidal and magnetic star-planet interactions aids our understanding of the formation, migration and evolution of hot Jupiters, and provides the best-available probe of exoplanetary magnetic fields. |
Mar. 23rd |
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Mar. 30th |
Genevieve Graves University of California, Berkeley "The 2D Galaxy Population: Mass-Dependent Star Formation, Stochastic Assembly-Driven Quenching" To zeroth order, galaxies form a one-dimensional family in which their observed properties - morphology, color, metallicity, environment, central black hole masses - all scale with their mass. This makes it difficult to disentangle the critical properties that determine their evolutionary history because everything correlates with everything else. Examining where this 1D sequence breaks down can provide essential clues to galaxy evolution. I present observational evidence that galaxy star formation histories form a 2D family, such that galaxies of the same mass today start out with similar star formation histories but shut down star formation ("quench") at different times. A parallel study using semi-analytic models of galaxy evolution suggests that these differences are due to their underlying halo mass assembly histories. Indeed, the observed 2D family of galaxy star formation histories seems to be a generic result of a mass-threshold for quenching (whether the quenching process is driven by stellar mass, halo mass, or central black hole mass), given a paradigm of mass-dependent evolution coupled with stochastic hierarchical assembly. |
Apr. 6th |
Anders Johansen Leiden Observatory (Tinsley Visiting Scholar) "From Pebble to Planetesimal" In the early stages of planet formation dust grains collect to form km-sized planetesimals. Growth beyond a few cms is frustrated by collisional shattering and rapid radial drift of pebbles and rocks. I will present my work on the dynamics of pebbles, rocks, and boulders in gaseous protoplanetary disks. The loose drag force coupling with the gas causes particles to pile up in large scale geostrophic gas pressure bumps which arise in turbulence driven by the magnetorotational instability. The local density of rocks and boulders becomes high enough to allow a gravitational contraction into gravitationally bound clusters with masses comparable to solid objects of several hundred kilometers in diameter. As 100-km scale protoplanets may form in an environment dense in dust and pebbles, the accretion of small solids is important for their further growth. I show that accretion of pebbles in a gaseous environment is very efficient and that the process leads to prograde spin up of the growing protoplanets, in agreement with the trend for prograde rotation displayed by the largest asteroids. |
Apr. 13th |
Adam Burrows Princeton University (Tinsley Visiting Professor) "A View of the Frontier: Exoplanet Research at the Crossroads" Approximately 500 exoplanets, mostly giant planets in the Jovian mass range, but also more than 70 "Neptune-mass" planets, have been detected orbiting stars in the solar neighborhood. More than 15% of these are transiting their primaries and these have collectively yielded a wealth of structural and physical information. A number of these exoplanets have recently been detected directly by the Spitzer infrared space telescope, NICMOS on HST, Kepler, and CoRoT in secondary eclipse and primary transit. These constitute the first remote-sensing data of extrasolar worlds, and JWST stands in the wings to push this science to the next level. I will present the embryonic theory of such irradiated giant planets and discuss what we have learned about their atmospheres, radii, and compositions. I will also address what we may soon learn from the variety of ground-based and space-based telescopes being planned or already under development as the exploding field of exoplanets accelerates into its future. |
Apr. 20th |
Dara Norman National Optical Astronomy Observatory (NOAO) "A Perspective on Quasar Fueling from LRG-QSO Correlation Functions" Public data from the 2dF quasar survey and 2dF/Sloan Digital Sky Survey LRG and QSO, with their vast reservoirs of spectroscopically located and identified sources, afford us the chance to more accurately study their real-space correlations in the hopes of identifying the physical processes that trigger quasar activity. We have used these two public databases to measure the projected cross-correlation, omega p , between quasars and luminous red galaxies. We find the projected two-point correlation to have a fitted clustering radius of r_0 = 5.3 +/- 0.6 and a slope gamma = 1.83 +/- 0.42 on scales from 0.7 to 27 h -1 Mpc. We attempt to understand this strong correlation by separating the LRG sample into two populations of blue and red galaxies. We measure at the cross-correlation with each population. We find that these quasars have a stronger correlation amplitude with the bluer, more recently star-forming population in our sample than the redder passively evolving population. We compare this result to published work on hierarchical models. The stronger correlation of bright quasars with LRGs that have undergone a recent burst of star formation suggests that the physical mechanisms that produce both activities are related and that minor mergers or tidal effects may be important triggers of bright quasar activity and/or that bright quasars are less highly biased than faint quasars. |
Apr. 27th |
Raffaella Schneider INAF/Osservotorio Astrofisico di Arcetri "Dust in the Early Universe" In the last few years mm and submm observations of high redshift quasars and galaxies have provided a powerful way of probing the very existence and properties of dust within 1 Gyr of the Big Bang. It is generally believed that at these cosmic times dust could have only condensed in the explosive ejecta of supernovae since intermediate-to-low mass stars did not have time to evolve off the main sequence into the dust-condensation stage. To account for the dust masses observed at z > 6, about 1 Msun of dust per supernova needs to be produced, in agreement with the condensation efficiencies predicted by supernova dust nucleation models. However, the largest dust masses measured so far in supernova ejecta are about two orders of magnitude smaller. Conversely, indications for supernova dust beyond z ~ 6 have recently come from observations of dust reddening of the host galaxies of a z = 6.2 quasars and z = 6.3 gamma-ray burst. In this talk, we will review the recent advancements made in this controversial subject. We will present the results of recent models for dust production in supernova ejecta and intermediate mass stars and its subsequent evolution in the interstellar medium of high redshift galaxies. We will also discuss the implications of an early epoch of dust formation for the cosmic transition between Population III and Population II stars. |
May. 4th |
Emily Freeland Texas A&M University "Bend-Double Radio Sources as Probes of Intergalactic Gas in Galaxies: Implications for Missing Baryons and Galaxy Evolution" As the most common environment in the universe, groups of galaxies are likely to contain a significant fraction of the missing baryons in the form of intergalactic gas. The density of this gas is an important factor in the effectiveness of ram pressure stripping on the evolution of galaxies in these systems. I will discuss the intergalactic gas densities that we measure using radio galaxies with bent jets. I will also present evidence that the group environment is altering the atomic hydrogen content of dwarf galaxies through ram pressure stripping. |