List of Past Astronomy Colloquia : 01-Jan-2009 to 01-Jun-2009

Date:   Wednesday 04-February-2009
Speaker:   Dr. Andrew Youdin (Princeton University)
Title:  "Getting the Ball Rolling: the Early Stages of Planet Formation"

The fast-paced discovery of extrasolar planets has heightened interest in the mechanisms by which planets form. The early stages of planet formation -- growth from dust grains to "planetesimals" larger than a kilometer -- is a long-standing problem which sets the state for subsequent growth phases. Simple growth by collisional sticking stalls as collisions become more violent with increasing particle size. The gravitational collapse of many small solids into a larger planetesimal is a promising mechanism. However stirring of particles by turbulent gas opposes this collapse, and led to the abandonment of the hypothesis. I will describe recent dynamical work which is resurrecting the collapse mechanism for planetesimal formation. A key point is that turbulence not only stirs particles but creates transient clumps, which can act as seeds for gravitational collapse. The streaming instability provides a robust mechanism for drag forces to promote particle clumping (Youdin & Goodman, 2005; Youdin & Johansen, Johansen & Youdin 2007). Simulations by Johansen et al. (2007, Nature) confirm the viability of the collapse hypothesis when initial particle sizes are above tens of centimeters. I will describe ongoing analytic work on gravitational collapse which can be extrapolated across parameter space, including toward smaller particle sizes. A surprising and robust conclusion is that planetesimals can form with initial sizes significantly larger than the canonical value of a few km. I will describe implications both for the detailed properties of our Solar System, and for the formation of planets and exoplanets in general.

Date:   Wednesday 11-February-2009
Speaker:   Dr. Eric Gawiser (Rutgers University)
Title:  "Probing the Dark Matter-Galaxy Formation Connection with Lyman Alpha Emitting Galaxies"

I will describe how our understanding of cosmological structure formation is used to probe the dark matter properties of high-redshift galaxies and to identify their present-day descendants. We studied the clustering properties and multiwavelength spectral energy distributions of a complete sample of 162 Lyman Alpha Emitting (LAE) galaxies at z=3.1 discovered in deep narrow-band imaging of the MUSYC-ECDFS field. The LAEs exhibit a moderate clustering bias of b=1.7, which implies median dark matter halo masses of 10^11 M_sun. The evolution of galaxy bias with redshift predicts that z=3.1 LAEs evolve into typical present-day galaxies with L~=L*, whereas other z>3 galaxy populations, including Lyman Break Galaxies and Active Galactic Nuclei, typically evolve into more strongly clustered galaxies such as massive ellipticals and cluster members. A two-population fit to the LAE spectral energy distribution finds that the typical LAE has low stellar mass (10^9 M_sun), moderate star formation rate (2 M_sun/yr), a young component age of 20 Myr, and little dust (A_V<0.2). This represents our first direct knowledge of the progenitors of spiral galaxies like the Milky Way seen when the universe was only 2 Gyr old.

References: Gawiser et al. 2007 (ApJ 671, 278), Francke et al. 2008 (ApJL 673, 13)

Date:   Wednesday 18-February-2009
Speaker:   Dr. Phil Armitage (University of Colorado)
Title:  "Dynamical Evolution of Solar and Extrasolar Planetary Systems"

The giant planets of the outer Solar System have near circular orbits that appear to be outliers to the broad eccentricity distribution of extrasolar planets. This observational distinction is commonly attributed not merely to selection effects but rather to different dynamical processes - scattering of a disk of left over debris in the Solar System versus relaxation among an unstable system of massive planets in extrasolar planetary systems. I will discuss preliminary results from an extremely large set of N-body simulations that includes both types of dynamics as limiting cases. The simulations quantify a predicted transition from "exoplanet-like" to "Solar System-like" dynamics that is a function of planet mass, system architecture, and host metallicity. The Solar System may well be close to the most massive planetary system that is likely to have near circular orbits for its giant planets.

Date:   Wednesday 25-February-2009
Speaker:   Dr. Daniel Whalen (Los Alamos)
Title:  "Primordial Supernovae and the Assembly of the First Galaxies"

The first stars in the universe formed in small dark matter halos at redshifts of 20 - 30. Forming in isolation (one per halo), these stars were very massive, forming H II regions 2.5 - 5 kpc in radius. Nonequilibrium cooling in the relic H II regions after the death of the star likely led to the formation of a second less massive star in the halo of its progenitor. Meanwhile, gravitational mergers assembled these halos into the first primitive galaxies at z ~ 10 - 15. Thus, in this paradigm the first galaxies were composed of a few dozen stars that were relatively massive. However, the cycle of star formation, local ionization and creation of a second star in a given halo may have been punctuated by the prompt formation of an entire generation of chemically-enriched low mass stars in the supernova remnant of the progenitor. If so, hundreds or perhaps thousands of low mass stars may have been swept up into the first galaxies instead of a few dozen very luminous ones. I will describe our initial numerical surveys of primordial supernova remnants and preview upcoming models that will ascertain if prompt stellar populations indeed arose from the debris of the first stellar explosions in the universe.

Date:   Wednesday 04-March-2009
Speaker:   Dr. Zhi-Yun Li (University of Virginia)
Title:  "Star Formation in Turbulent Magnetic Clouds"

Stars are formed in molecular clouds that are both turbulent and magnetized. The role of the turbulence and magnetic field in star formation is uncertain. In the first part of the talk, I will describe a scenario in which the star formation is accelerated by turbulent compression but regulated by a strong magnetic field. This scenario may be relevant for regions of distributed, inefficient star formation such as the Taurus molecular cloud complex. For regions of more efficient, clustered formation, such as the parsec-scale dense clump associated with NGC 1333 in the Perseus molecular cloud, the star formation may be regulated primarily by the turbulence driven by protostellar outflows instead. I will illustrate the different roles of the turbulence and magnetic field in different modes of star formation using numerical simulations. In the second part of the talk, I will discuss the effects of magnetic fields on the collapse of individual cloud cores, focusing on the suppression of disk formation through magnetic braking.

Date:   Wednesday 11-March-2009
Speaker:   Dr. Randall Smith (SAO/GSFC)
Title:  "Science with the International X-ray Observatory"

In July 2008, the International X-ray Observatory (IXO) was announced to the astronomical community. IXO is a joint X-ray observatory with participation from ESA, NASA and JAXA, superseding the Constellation-X mission concept. The baseline for IXO features a single large (3 m^2 EA) X-ray mirror and an extensible optical bench with a 20-25m focal length and moveable focal plane. The instruments include an X-ray wide field imaging spectrometer, a high spectral resolution non-dispersive X-ray spectrometer, an X-ray grating spectrometer, a hard X-ray imaginer, a high-speed spectrometer, and an X-ray polarimeter.

I will discuss the IXO science objectives and how it will help answer the following questions: How do super-massive Black Holes grow and evolve? Does matter orbiting close to a Black Hole event horizon follow the predictions of General Relativity? What is the Equation of State of matter in Neutron Stars? How does Cosmic Feedback work and influence galaxy formation? How does galaxy cluster evolution constrain the nature of Dark Matter and Dark Energy? Where are the missing baryons in the nearby Universe? When and how were the elements created and dispersed? How do high energy processes affect planetary formation and habitability? How do magnetic fields shape stellar exteriors and the surrounding environment? How are particles accelerated to extreme energies producing shocks, jets, and cosmic rays?

Date:   Wednesday 18-March-2009
Speaker:   NO COLLOQUIUM (Spring Break)
Title:  "Spring Break"

Date:   Wednesday 25-March-2009
Speaker:   Dr. Mercedes Lopez-Morales (DTM)
Title:  "Characterizing Extrasolar Planet Atmospheres"

The field of extrasolar planets is quickly evolving from just discovering the planets to efforts to characterize their atmospheric physical properties. A subset of extrasolar planets - the transiting planets - provide now the means to measure several properties of these objects, in particular their bulk atmospheric characteristics. I will summarize in this talk the current stage of the new field of extrasolar planet characterization, including the latest results and expected upcoming developments.

Date:   Wednesday 01-April-2009
Speaker:   Dr. Dan Scheeres (University of Colorado)
Title:  "The Life-cycles of Small Asteroids"

The recent verification that small asteroids are rubble piles and are subject to the YORP effect (i.e, that solar radiation pressure makes asteroid spin rates change over relatively short time spans) has wide-ranging consequences for the life cycles of small asteroids. As the spin rate of an asteroid changes, its minimum energy configuration can change and lead to profound shifts in how its mass is distributed. If the spin rate continues to increase it becomes possible for an asteroid to fission into multiple pieces, forming a binary asteroid. These proto-binary asteroids can follow several fundamentally different paths as a function of their initial morphology, including mutual escape, re-impact, and transition into a stable binary system. Recent in-situ and remote observations of asteroids support this picture, and imply that the smallest members of the asteroid population lead an active and interesting life.

Date:   Wednesday 08-April-2009
Speaker:   Dr. Jennifer Wiseman (GSFC)
Title:  "Exploring Other Worlds: NASA's Laboratory for Exoplanets and Stellar Astrophysics at the Goddard Space Flight Center"

Scientists at the Exoplanets & Stellar Astrophysics Laboratory at NASA's Goddard Space Flight Center study the formation and evolution of stars and planetary systems using advanced telescopes and theoretical techniques. Many active collaborations are underway that include University of Maryland faculty, graduate students, and postdocs, and I will discuss related opportunities. Recent successes in exoplanet studies include transit spectroscopy of exoplanets and now direct imaging of an exoplanet using coronagraphic observations. I will discuss new technology and mission concepts developed in our lab that will help us to understand circumstellar disks and jets and to conduct searches for Earth-like planets and habitable environments around other stars. Of particular excitement right now are our intense preparations for the final servicing mission (SM-4) of the Hubble Space Telescope.

Date:   Wednesday 15-April-2009
Speaker:   Dr. Angela Speck (University of Missouri)
Title:  "The Nature of Stardust: Astromineralogy and Circumstellar Dust Around Evolved Stars"

Intermediate-mass stars (0.8-8.0 solar masses) are major contributors of new elements to interstellar space. These stars eventually evolve into asymptotic giant branch (AGB) stars. During the AGB phase, these stars suffer intensive mass loss leading to the formation of circumstellar shells of dust and neutral gas, including the new elements formed during the star's life. Using a combination of observing techniques (e.g. infrared (IR) spectroscopy, visible, IR and sub-mm imaging) and laboratory IR studies, combined with theoretical considerations (e.g. kinetics and thermodynamics of the dust-forming region; nucleosynthesis models and changing stellar chemistries) and meteoritic evidence, I investigate the structure and evolution of the circumstellar dust and its environment.

Date:   Wednesday 22-April-2009
Speaker:   Dr. Romeel Dave (University of Arizona)
Title:  "How the Cosmic Feedback Cycle Governs Galaxy Formation"

Feedback from galactic outflows is a critical driver of galaxy evolution,particularly during the most active cosmic epoch of star formation. I present cosmological SPH simulations incorporating heuristic models of outflows, and study their impact on the properties of galaxies and intergalactic gas from the reionization epoch until today. I will focus particularly on metal enrichment as a unique observational tracer of such outflows. Remarkably, incorporating outflows based on what is seen in local observations of starbursts fairly uniquely yields a good match to a wide range of galaxy and IGM observables. Our simulations contradict many longstanding notions, to wit: The constancy of high-z CIV absorption is not indicative of pre-enrichment; the mass-metallicity relation is not governed by the ability of winds to escape from galactic potentials; and OVI is not a good tracer of missing baryons. Instead, I offer new interpretations for these and other observational trends, in terms of a cosmic feedback cycle of mass, metals, and energy connecting galaxies and intergalactic gas at all epochs.

Date:   Wednesday 29-April-2009
Speaker:   Dr. Matt Bradford (Caltech)
Title:  "The History of the Dust-Enshrouded Universe: Sensitive Spectroscopy of Galaxies in the Far-IR and Submillimeter"

Half of the integrated background radiation from stars and galaxies is the far-infrared, the result of interstellar dust absorbing and reradiating energy generated by stars and black-hole accretion. This cosmological far-IR background indicates that dust-obscured regions at all epochs hide processes which helped shape our modern Universe: the conversion of gas into stars, the buildup of heavy elements, the growth of black holes. The best method for studying dust-obscured astrophysical processes is spectroscopy at long wavelengths. Spectral lines in the far-IR to submillimeter are immune to dust extinction, and directly measure the physical conditions at the locations where stellar or AGN power impinges upon its surrounding ISM. Far-IR / submillimeter spectra of populations of galaxies will in aggregate reveal the histories of the stellar and black hole populations and heavy-element abundances.

These measurements require new telescope and instruments. The far- IR and submillimeter has long lagged far behind the optical and near- IR, but is now undergoing a period of rapid growth, particularly in the area of direct detectors which provide the best sensitivity. I am working to develop and field new instruments which employ these technologies to enable survey spectroscopy of high-redshift galaxies in the far-IR and submillimeter. I will focus on results and prospects with two approaches. Z-Spec is a novel mm-wave broadband grating spectrometer operating at the Caltech Submillimeter Observatory, providing instantaneous coverage across the entire 195-305 GHz band. The instrument team is undertaking a study of molecular gas in local-universe LIRGS and ULIRGS as well as high- redshift galaxies. Among the results, I will highlight a measurement of the molecular gas cooling in the Cloverleaf quasar host at z=2.56, where Z-Spec's band corresponds to 270--440 microns rest-frame and contains the peak of the CO spectrum. The measured cooling is consistent with X-ray heating from the active nucleus, and this energy input likely influences the stellar mass function. Looking forward, I am pursuing participation in the Japanese-led SPICA mission with BLISS, a broadband far-IR grating spectrometer. SPICA is a cryogenic 3.5-meter space telescope and when combined with BLISS, offers far-IR sensitivities rivaling those of JWST and ALMA, but in the relatively unexplored far-IR regime.

Date:   Wednesday 06-May-2009
Speaker:   Dr. Kevork Abazajian (UMD - Physics)
Title:  "Cosmological Structure at Small Scales"


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