List of Past Astronomy Colloquia : 01-Jan-2012 to 01-Jun-2012

Date:   Wednesday 25-Jan-2012
Speaker:   Dr. Sean Solomon (DTM-CIW)
Title:  Exploring the Planet Mercury with the MESSENGER Spacecraft: Seeking clues to the formation and evolution of the inner planets

One of Earth’s closest planetary neighbors, Mercury remained comparatively unexplored for the more than three decades that followed the three flybys of the innermost planet by the Mariner 10 spacecraft in 1974–75. Mariner 10 imaged 45% of Mercury’s surface at about 1 km/pixel average resolution, confirmed Mercury’s anomalously high bulk density and implied large fractional core size, discovered Mercury’s internal magnetic field, documented that H and He are present in the planet’s tenuous exosphere, and made the first exploration of Mercury’s magnetosphere and solar wind environment. Ground-based astronomers later reported Na, K, and Ca in Mercury’s exosphere; the presence of deposits in the floors of polar craters having radar characteristics best matched by water ice; and strong evidence from the planet’s forced libration amplitude that Mercury has a fluid outer core. Spacecraft exploration of Mercury resumed with the selection for flight, under NASA’s Discovery Program, of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Launched in 2004, MESSENGER flew by the innermost planet three times in 2008-2009 en route to becoming the first spacecraft to orbit Mercury in March 2011. MESSENGER’s first chemical remote sensing measurements of Mercury’s surface indicate that the planet’s bulk silicate fraction differs from those of the other inner planets, with a low-Fe surface composition intermediate between basalts and ultramafic rocks. Moreover, surface materials are richer in the volatile constituents S and K than predicted by most planetary formation models. Global image mosaics and targeted high-resolution images (to resolutions of 10 m/pixel) reveal that Mercury experienced globally extensive volcanism, including large expanses of plains emplaced as flood lavas and widespread examples of pyroclastic deposits likely emplaced during explosive eruptions of volatile-bearing magmas. Bright deposits within impact craters host fresh-appearing, rimless depressions or hollows, often displaying high-reflectance interiors and halos and likely formed through processes involving the geologically recent loss of volatiles. The tectonic history of Mercury, although dominated by near-global contractional deformation as first seen by Mariner 10, is more complex than first appreciated, with numerous examples of extensional deformation that accompanied impact crater and basin modification. Mercury’s magnetic field is dominantly dipolar, but the field is axially symmetric and equatorially asymmetric, a geometry that poses challenges to dynamo models for field generation. The interaction between the solar wind and Mercury’s magnetosphere, among the most dynamic in the solar system, serves both to replenish the exosphere and space weather the planet’s surface. Plasma ions of planetary origin are seen throughout the sampled volume of Mercury’s magnetosphere, with maxima in heavy-ion fluxes in the planet’s magnetic-cusp regions. Bursts of energetic electrons, seen at most local times, point to an efficient acceleration mechanism operating within Mercury’s magnetosphere on a regular basis that produces electrons with energies up to hundreds of keV on timescales of seconds.

Date:   Monday 30-Jan-2012
Speaker:   Dr. Kevin Schawinski (Yale University)
Title:  The Co-Evolution of Galaxies and Black Holes

What is behind the symbiotic relationship between galaxies and their central black holes? Does the energy liberated by accreting supermassive black holes alter the evolutionary trajectories of their host galaxies?I will discuss recent observational advances in illuminating this relationship and the viability of various ideas for "feedback" in co-evolution scenarios. In particular, I will highlight the emergence of multiple pathways in galaxy evolution accompanied by black hole growth and the increasing evidence for "state switches" in local AGN analogous to the behavior in X-ray binaries. I will also discuss the contributions that citizen scientists taking part in the Galaxy Zoo and other projects have made to these results.

Date:   Wednesday 01-Feb-2012
Speaker:   Dr. Philip Hopkins (UC Berkeley)
Title:  Star Formation, Black Holes, and Feedback in Galaxy Formation

Many of the most fundamental unsolved questions in star and galaxy formation revolve around star formation and "feedback" from both massive stars and accretion onto super-massive black holes. The combination of models with realistic gas and feedback physics have led to huge shifts in our understanding of when and how galaxies grow, where stars form within them, and what their ultimate fate will be. I'll review the current status of our understanding of these feedback processes, and present new models which attempt to realistically model the ISM, star formation, and feedback from stellar radiation pressure, supernovae, and photo-ionization, and their interplay with feedback from luminous quasars. These mechanisms lead to 'self-regulated' galaxy and star formation, in which global correlations such as the Schmidt-Kennicutt law, the BH-host galaxy correlations, and the global inefficiency of star formation emerge naturally. I'll discuss how, within galaxy disks, feedback regulates the structure of the ISM, the collapse of dense gas into star-forming cores, and black hole accretion rates. But feedback also produces galactic super-winds that can dramatically alter the cosmological evolution of galaxies, their behavior in galaxy mergers, and structure of the inter-galactic medium. I'll highlight how a combination of improved theoretical models and observations can elucidate the physical processes driving these winds and their role in phenomena on an enormous range of spatial scales.

Date:   Monday 06-Feb-2012
Speaker:   Dr. Stefan Kraus (University of Michigan)
Title:  Imaging the planet-forming zone in protoplanetary disks

Many of the open questions in the field of star- and planet-formation are related to the structure and physics of the innermost regions of protoplanetary disks, where disk material is transported onto the forming star, ejected in powerful jets & outflows, or interacts with newly-formed planets. Interferometry at infrared and millimeter wavelengths now provides the exciting opportunity to investigate these processes directly.

In this talk I will review some recent studies, in which we employed the Very Large Telescope Interferometer (VLTI) and the CHARA array in order to image the AU-scale inner regions of protoplanetary disks. With an angular resolution of 0.001", these observations allow us to discover and trace the orbital motion of close-in companions, to derive the vertical disk structure in the dust sublimation zone, and to detect viscously-heated gas close to the star. By applying near-infrared interferometric imaging for the first time to a massive (20 M_sun) YSO, we discovered an AU-scale hot dusty disk, yielding new evidence for the accretion-disk hypothesis in high-mass star formation. Interferometric observations with high spectral dispersion (R=12,000) in spectral lines now also enable us to constrain the gas velocity field in the accretion- and outflow-launching region, for which we employ models of Keplerian-rotating disks and magnetocentrifugally-driven disk winds.

Finally, I will discuss the prospects of combining infrared and CARMA millimeter interferometry, providing a unique tool to constrain the 3-D density & temperature distribution, covering sub-AU scales to hundreds of AUs. Such combined constraints are particularly important to unravel planet-induced disk surface structures, such as the tidally cleared gaps in transitional or pre-transitional disks, and are essential to advance our understanding of planet formation and migration.

Date:   Wednesday 08-Feb-2012
Speaker:   Dr. Aurora Simionescu (Stanford)
Title:  Zooming out on the X-ray brightest galaxy clusters

Studies of the diffuse X-ray emitting gas in galaxy clusters have provided powerful constraints on cosmological parameters and plasma astrophysics. I will discuss some of the most recent results obtained from deep observations of bright, nearby clusters of galaxies. I will first present spatially resolved spectroscopic maps of the central regions of these systems, where the observed distributions of multiphase plasma and metallicity can place important constraints on transport processes. I will then move on to larger and larger scales, and finally focus on the faint cluster outskirts. Measurements of the thermodynamic properties of these regions have become possible only recently due to the unique capabilities of the Suzaku satellite, opening a new window for the exploration of the ongoing virialization and equilibration processes that occur as part of large-scale structure formation. I will discuss current and future observations of the cluster "edges", and show how combining the low-surface brightness X-ray observations together with low-frequency radio data and numerical simulations will lead to a complete picture of the physics of cluster growth, and to optimizing the use of clusters as standard candles for cosmology.

Date:   Monday 13-Feb-2012
Speaker:   Dr. Colin Snodgrass
Title:  "What is that x-shaped space thing? The comet that wasn't."

I will present results on the unusual "comet" P/2010 A2 (LINEAR). I observed this object in the context of my wider studies of the icy minor bodies of our solar system, as it was first thought to be an example of the recently identified population of Main Belt comets. These bodies are an important clue into the formation of our solar system, as they hide ice in a region previously thought to be within the snow line. The unusual morphology of P/2010 A2 led to speculation that it was in fact an even rarer phenomenon, a collision between asteroids caught in the act. Using data from both ground-based telescopes and the ESA Rosetta spacecraft, we were able to demonstrate that this was the case, and place tight constraints on the nature of the collision. I will show these results, and discuss the implications these have for the evolution of asteroids and the generation of zodiacal dust, as well as the wider topic of ice within the asteroids and how we can tell comets and collisions apart.

Date:   Wednesday 15-Feb-2012
Speaker:   Dr. Suvi Gezari
Title:  Flares from Starving Beasts: The Tidal Disruption of Stars by Supermassive Black Holes

A dormant supermassive black hole lurking in the nucleus of a galaxy will be revealed when an ill-fated star passes too close to the black hole and is torn apart by its strong tidal forces and consumed, resulting in a luminous flare of radiation. The mass accretion rate of the stellar debris is determined by the mass of the black hole and the type of star disrupted, and thus tidal disruption events provide a clean cosmic laboratory to study accretion physics, and a means to probe the masses of dormant black holes in distant galaxies. I will present a review of the current census of tidal disruption event candidates detected in the X-rays, UV, and optical, including our most recent discovery from the coordinated efforts of the GALEX Time Domain Survey in the NUV and the Pan-STARRS Medium Deep Survey in the optical. The next generation of optical synoptic surveys, such as LSST, will discover thousands of tidal disruption events, which can then be used to study black hole demographics, and to compare measured rates to the predictions of dynamical galaxy models.

Date:   Wednesday 22-Feb-2012

Date:   Monday 27-Feb-2012
Speaker:   Dr. Sarah Dodson-Robinson (University of Texas-Austin)
Title:  "Planetary Archaeology in Space and Time."

Just as an archaeologist explores the origins of human civilization by analyzing the remains of ancient settlements, a planetary scientist may explore the origins of solar systems by investigating the remains of bygone protoplanetary disks. Here I show how my interdisciplinary research involving both numerical simulations of protostellar disks and spectroscopy of planet hosts has yielded an emerging map of how, when and where planets form in the Galaxy. I begin by discussing the newly discovered mineralogical bottleneck in forming the planet species that dominates the set of known planets--the species that grows "bottom-up" from colliding planetesimals on close-in orbits. I then move to a second species of planet--that which forms via "top-down" collapse of a protostellar disk--and present evidence that newly formed such planets await discovery in the wide inner cavities that populate a subset of accreting protostellar disks. I close with a discussion of how the dominant species of forming planet may have changed during the Milky Way's history due to Galactic chemical evolution and reveal future plans for investigating changes in planet formation across cosmic time.

Date:   Wednesday 29-Feb-2012
Speaker:   Dr. Michael C. Cooper (UC Irvine)
Title:  Survey Science at z < 2: Understanding the Dominant Modes of Galaxy Evolution

Evolution in the global galaxy population over the past 10 Gyr has been dominated by two principal trends: a dramatic decline in the average level of star-formation activity combined with a substantial growth in the stellar mass density within the red galaxy population. While both of these evolutionary trends are well measured at z < 2, the physical mechanisms responsible remain somewhat poorly understood. Using data from the DEEP2 and DEEP3 Galaxy Redshift Surveys in concert with complementary observations spanning UV to radio wavelengths, I will present recent results that directly constrain the physical processes driving the global transformation in galaxy properties at z < 2. In particular, I will discuss ongoing work to probe the cold gas component of star-forming galaxies at high redshift, which is providing direct constraints on the fuel supply for star formation when the Universe was less than half its current age. Finally, I will conclude by outlining the limitations of the current data sets and how they might be overcome with future ground- and space-based facilities.

Date:   Monday 05-Mar-2012
Speaker:   Dr. S. Bradley Cenko (UC Berkeley)
Title:  The Time Domain Renaissance: From Gamma-Ray Bursts to Advanced LIGO

With convergence of both science (i.e., community interests) and technology (the advent of new facilities enabled by Moore’s Law), time-domain exploration of the sky promises to be a frontier pursuit in the coming decade. In this talk I will review recent results from our efforts to characterize variability across the electromagnetic spectrum with increasing sensitivity and on ever-shorter time scales. At high energies (Swift satellite), we have discovered a new class of relativistic outbursts that appear to result from the tidal disruption of a star by a supermassive black hole. These relativistic tidal disruption flares are likely to be the dominant source of extragalactic transients for future wide-field radio surveys (ASKAP, MeerKAT, SKA), providing a pristine view of both the jet formation process and the nuclei of distant, quiescent galaxies. At optical wavelengths, the Palomar Transient Factory has been remarkably successful in the rapid identification of young supernovae, and I will highlight our efforts to use these discoveries to constrain their progenitor systems. Finally, with Advanced LIGO expected to begin operations in 2015 (or so), I outline a plan to identify the first electromagnetic counterpart of a gravitational wave source. Despite the inherent challenges in such an endeavor, such a discovery would likely serve as one of the signature scientific achievements of our era.

Date:   Wednesday 07-Mar-2012
Speaker:   Dr. Peter Capak (Caltech)
Title:  Large Scale Structure and Extreme Star Formation At z~5

I will show how we are beginning to quantify the importance and influence of large-scale structure on galaxy evolution at z>4 using deep pan-chromatic imaging and spectroscopy in the COSMOS survey. Theory predicts and statistical observations confirm that large-scale structure, and the largest over-densities in particular, have a significant roll in the formation and evolution of the first galaxies. However, the way large-scale structure at high redshift (z>4) influences galaxy formation remains largely un-quantified due to the spatial extent of structures at these redshifts and the faint fluxes of the galaxies that populate them. I will show that the typical structure at z>4 extends over 10's of arc-minutes and that galaxy mergers signified by Quasars and Extreme-Starbursts (Sub-mm Galaxies) appear to preferentially populate these structures. Furthermore, I will show these mergers may account for up the half the star formation activity at these redshifts. Finally, I will link the members of these z>4 proto-clusters to their likely decedents at z~2.

Date:   Wednesday 14-Mar-2012

Date:   Wednesday 21-Mar-2012

Date:   Wednesday 28-Mar-2012
Speaker:   Dr. Nadia Zakamska (Johns Hopkins University)
Title:  Observations of quasar feedback

Black hole feedback -- the interaction between the radiation and outflows from supermassive black holes and their surrounding environments -- is routinely invoked to explain the absence of overly luminous galaxies, the observed high temperatures of intracluster medium and the black hole / bulge correlations. I will review the theoretical basis of this phenomenon, existing observational evidence for feedback in various types of active galaxies and present recent observations of our group illustrating black hole feedback in action.

Date:   Wednesday 04-Apr-2012
Speaker:   Dr. Desika, Narayanan (Arizona)
Title:  The Star Formation Law: Physics at the Intersection of Star Formation and Galaxy Evolution

It is now well-established that the star formation rate (SFR) surface density in molecular clouds and galaxies scales well with the gas surface density. Interpreting the observed relation is a complex topic that leads to interesting physics associated with both star formation and galaxy evolution. For example, simply understanding the physical nature of the high-redshift galaxies that lie on the star formation realtion has spawned a cottage industry of both observational and theoretical research. Similarly, relating the observables on either axis of the relation (e.g. converting bolometric luminosity to star formation rate, as well as CO emission to H2 gas mass) requires understanding the impact of the galactic environment on the physical properties of the ISM across a large range of scales. In this talk, I will present numerical models of galaxies in evolution that aim to understand aspects of the aforementioned topics.

Date:   Wednesday 11-Apr-2012
Speaker:   Dr. Enrico Ramirez Ruiz (UC Santa Cruz)
Title:  Tidal Disruption of Stars by Massive Black Holes

A star interacting with a massive black hole cannot be treated as a point mass if its gets o close to the black hole that it becomes vulnerable to tidal distortions and even disruption. When a rapidly changing tidal force starts to compete with a star’s self-gravity, the material of the star responds on a complicated way, being stretched along the orbital direction, squeezed at right angles to the orbit and strongly shocked. This phenomenon poses an as yet unmet challenge to computer simulations. The art of modeling tidal disruption of stars forms the main theme of my talk. Detailed simulations should tell us what happen when stars of di?erent types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.

Date:   Wednesday 18-Apr-2012
Speaker:   Dr. Jon Jenkins (NASA)
Title:  The Kepler Mission: Good Planets are Hard to Find, and Vice Versa

Kepler vaulted into the heavens on March 6, 2009, initiating NASA’s search for Earth-size planets orbiting Sun-like stars in the habitable zone, that range of distances for which liquid water would pool on the surface of a rocky planet. In the 1000+ days since Kepler began science operations, a flood of photometric data on upwards of 190,000 stars of unprecedented precision and continuity has provoked a watershed of 2300+ planetary candidates (most sub-Neptune in size and many comparable to or smaller than Earth) and a resounding revolution in stellar asteroseismology. Kepler detected the first multiple transiting system, just one of over 360, including Kepler-11, a system of six transiting planets. Transit timing variations allow us to constrain or measure the masses of many of these planets without the need for expensive radial velocity observations, and promises to map out the dependence of planetary size and composition in the critical regime from 1.0 – 2.0 Re (Earth radii), that region where the transition from gassy, watery super-Earths to rocky terrestrial planets occurs. Important milestones include the discovery of the first transiting planet in the habitable zone, Kepler-22b. As the mission data accumulate and the sensitivity of the search pipeline improves, we are pushing down to smaller and smaller planets and to longer and longer orbital periods on the road to determining eta-Earth, that fraction of tars in our galaxy that host potentially habitable wor

Date:   Wednesday 25-Apr-2012
Speaker:   Dr. Sean Andrews (Harvard)
Title:  Planet-Forming Disks Around Young Stars

Young circumstellar disks are thought to experience a rapid "transition" phase in their evolution that can have a considerable impact on the formation and early development of planetary systems. I will present high angular resolution (0.3" = 20-60 AU) millimeter-wave observations from a Submillimeter Array (SMA) survey of such transition disks in nearby star-forming regions. With those data, we directly resolve dust-depleted disk cavities on scales comparable to the extent of our Solar System. Surprisingly, these large cavities are common, comprising at least 1 in 5 (20%) of the disks at the high end of the disk mass distribution. Using these results, I assess the physical mechanisms proposed to account for transition disk structures. I will argue that neither photoevaporation or particle growth alone can reproduce the observations. Instead, the data are more commensurate with the substantial disk structure perturbations expected from dynamical interactions with low-mass (planetary) companions. Finally, I will comment on observations of the cold gas content in these disks and highlight some updates to our approach for modeling such data.

Date:   Wednesday 02-May-2012
Speaker:   Dr. Alycia Weinberger (DTM)
Title:  Circumstellar Disk Composition and Evolution

Circumstellar disks are the birthplaces of planets, and the compositions and architectures of planetary systems depend on planet-disk interaction. I will describe imaging and spectroscopy of disks that span their lifetimes from gas-rich and capable of forming planets to gas-poor but dusty debris systems. I will also discuss nearby young stars as the hosts of disks and in particular, show recent work on the kinematics of the TW Hydrae Association.

Date:   Wednesday 09-May-2012
Speaker:   Dr. Todd Thompson (The Ohio State)
Title:  Feedback Processes in Rapidly Star-Forming Galaxies

I will describe a set of projects aimed at developing a physical picture of the feedback and regulation processes in the ISM of rapidly star-forming and ultra-luminous infrared galaxies. The UV radiation produced by massive stars in the ISM of starbursts is absorbed, scattered, and reprocessed into the IR by dust grains. I will argue that the associated radiation pressure may provide the dominant vertical support against gravity. I will discuss the implications of this important feedback process at the galaxy scale, and at the scale of individual star clusters. Because the system radiates at its Eddington limit (for dust), starbursts supported by radiation pressure achieve a characteristic radiative flux and their ``Schmidt Law'' for star formation changes qualitatively with respect to normal star-forming galaxies. I will also describe observational and theoretical constraints on the magnetic field strength and cosmic ray energy density in starbursts, with direct implications for the origin of the FIR-Radio correlation, the shape of the radio spectra of starbursts, and their observability at GeV energies by Fermi.

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