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Fall 2008

Sep 12 Prof. Sarah Gallagher (W. Ontario)
Compact Galaxy Groups: Local Windows into High Redshift Galaxy Evolution
The high densities and low velocity dispersions of compact galaxy groups -- small neighborhoods with member separations comparable to a galaxy radius -- make them ideal for studying star formation and morphological transformation under the influence of multiple, ongoing gravitational encounters. A Spitzer survey of infrared properties of these systems reveals evidence for rapid evolution of the galaxy populations in this environment. Within a local sample of 12 Hickson Compact Groups (HCGs), we identify two primary tracks of group evolution. The first, characterized by strong interactions and mergers of gas-rich galaxies, leads to an elliptical-dominated group with a bright X-ray halo. In the second, enhanced secular evolution consumes cold gas resulting in dry mergers at later epochs and only weak X-ray emission. Individual groups within the local sample typify the main stages in this proposed two-pronged evolutionary sequence. We speculate on the factors that influence the chosen track, and the implications for galaxy evolution in dense environments.
Host: Howard Yee
Sep 19 Prof. Josh Eisner (U. Arizona)
Building Blocks of Planets around Young Stars
Planetary systems form in disks of dust and gas around young stars, and observations of these disks can shed light on the physical processes by which planets form. I will describe millimeter wavelength interferometric observations that constrain the mass content of hundreds of protoplanetary disks. I will show that most disks are probably less massive than the nebula from which our solar system formed, and argue that Jupiter-mass planets are rare if they form by the core accretion or disk fragmentation mechanisms currently proposed. I will also briefly discuss recent near-IR observations of the innermost regions of protoplanetary disks. While millimeter wavelength observations probe the properties of cool outer disks where giant planets likely form, near-IR interferometry can probe disk terrestrial regions where giant planet migration and rocky planet formation occur.
Host: Dae-Sik Moon
Sep 26 Prof. Peter Martin (U. Toronto, CITA)
Mapping the complex structure of the cool ISM using the Balloon-borne Large Aperture Submillimeter Telescope (BLAST)
With BLAST we have carried out a number of Galactic plane surveys in star-forming regions, including large-area maps covering the Vela Molecular Ridge and Cyg X, and ranging from relative quiescence in IC 5146 to very energetic conditions near eta Carina. These maps reveal the stunning complexity of the interstellar medium as imaged in this new spectral window (250, 350, 500 microns), highlighting dust in a variety of neutral and ionized environments in the cycle of interstellar material, spanning diffuse emission and molecular complexes to elusive dense cold self-gravitating prestellar cores and the somewhat warmer envelopes of protostars and protoclusters. With unique spectral coverage near the peak of the spectral energy distribution, BLAST enables us to quantify the temperatures, envelope masses, and luminosities of hundreds of compact sources, exposing the range of evolutionary states in the L – M plane. But BLAST also sees dust so cold that its presence is missed even in far-infrared emission, including structures such as infrared dark clouds, signposts of the earliest stages of star formation. Unlike surveys from the ground at slightly longer wavelengths, mapping with BLAST is not heavily spatially filtered, so that the hierarchy of larger-scale structures in the embedding molecular clouds in captured as well. Correlations of submillimetre emission with ancillary data that also measure column density (e.g., far-infrared IRAS, extinction, CO) constrain physical parameters such as temperature and dust opacity. The power spectrum of this interstellar froth is of interest in evaluating the cirrus confusion which ultimately limits the detection of faint compact sources. As intended, these BLAST surveys provide striking realizations of the rich potential anticipated for the submillimeter imaging to be carried out using the SPIRE camera on the Herschel Space Observatory.
Host: Dae-Sik Moon
Oct 3 Prof. Peter Goldreich (IAS, Princeton)
Reading the Record of Ancient Impacts
Debris from asteroids and comets continually bombards the Earth. Big impacts launch ejecta on trajectories that carry them thousands of kilometers from the impact site. These are recorded in discrete layers of submillimeter size spherules in sea floor sediments. Larger, aerodynamically shaped,objects are found on land. I will describe the physical processes that occur in energetic impacts with emphasis on the evolution of the fireball and determination of the range and size of the ejecta.
Host: Yanqin Wu
Oct 10 Prof. Lars Bildsten (KITP, UCSB) (Refreshment starts at 1:45 pm!)
Progenitors of Type Ia Supernovae: Challenges and Opportunities
Type Ia supernovae are one of the key tools for cosmological studies of the distant universe. However, our ignorance remains vast as to how they are made, how they explode, and how they may evolve in time. Recent supernovae surveys have revealed a diversity of Type Ia supernovae consistent with our naive expectation that there are many ways to get a white dwarf to explode. I will review the observational evidence for these distinct populations and I will discuss recent work we have done on a new kind of thermonuclear event from double white dwarfs. These ``.Ia'' supernovae (one-tenth as bright for one-tenth the time as a Type Ia supernovae) should be found in the upcoming surveys at the rate of a few per month, and would directly reveal the population of double white dwarfs in distant galaxies.
Host: Dae-Sik Moon & Marten van Kerkwijk
Oct 17 Prof. Tim de Zeeuw (ESO)
ESO: Present and Future
The European Southern Observatory is an intergovernmental organization for astronomy, created in 1962 by Belgium, France, Germany, Sweden and The Netherlands, and today has 14 member states. Headquarters are located in Garching near Munich. ESO’s mission is to enable scientific discoveries by constructing and operating powerful observational facilities that are beyond the capabilities of individual member states, and to organize collaborations in astronomy. ESO operates medium-sized optical telescopes on Cerro La Silla, the Very Large Telescope and the Interferometer on Cerro Paranal, widely considered to be the most advanced optical/infrared observatory in the world (to which soon the VISTA and VST survey telescopes will be added), and the sub-millimeter observatory APEX on Llano Chajnantor, all located in Northern Chile. ESO represents Europe in a partnership with North America and East Asia that is constructing the transformational Atacama Large Millimeter/sub-millimeter Array on Chajnantor, to be completed in 2013. ESO is designing an Extremely Large Telescope with a 42m primary mirror and adaptive optics built-in, to be constructed in the next decade. The talk will summarize the current program, include scientific highlights, and will outline plans for the future.
Host: Ray Carlberg
Oct 24 Prof. Fred Adams (U.Michigan)
Effects of Young Star Clusters on their Constituent Solar Systems
Most stars -- and hence most solar systems -- form within groups and clusters. This talk explores how these star forming environments affect the solar systems forming within them. The discussion starts with the dynamical evolution of young clusters with N = 100 - 1000 members. We use N-body simulations to study how evolution depends on system size and initial conditions. Multiple realizations of equivalent cases are used to build up a robust statistical description of these systems, e.g., distributions of closest approaches and radial locations. These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models then determine the efficacy of radiation in removing gas from the systems and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine the probability of solar system disruption. The main result of this work is thus a quantitative determination of the effects of clusters on forming solar systems. Along the way, we set constraints on the possible birth environments for our solar system. By studying orbits in these extended mass distributions, we have also discovered a robust orbit instability that operates when the potential is sufficiently triaxial. This instability not only affects young star clusters, but also arises in the dynamics dark matter halos.
Host: Ray Jayawardhana
Oct 31 Dr. Maryam Modjaz (UC Berkeley)
Elucidating the SN-GRB Connection from the SN Perspective
Supernovae of Type Ib/c are core-collapse supernovae (SN) whose massive progenitors have been stripped of progressively larger amounts of their hydrogen and helium envelopes. The link between long-duration Gamma-Ray Bursts (GRBs) and Type Ic supernovae is now well established, but we still do not fully understand the exact conditions that produce each kind of stellar explosion. I will discuss a number of observational venues that attempt to probe the progenitor environments and explosion conditions of SN, GRB and SN-GRBs. I will show data that suggest that asphericity is common during the explosion of even normal SNe Ib/c and not an exclusive property of SN-GRB. Furthermore, I will present host-galaxy environments and measured metallicities at the sites of SN with and without observed GRBs. Though the sample size is small, the observations are consistent with the hypothesis that low metal abundance (~<0.3 Z_solar) is the main cause of some very massive stars becoming SN-GRB. Lastly, I will discuss SN Ib 2008D, which was discovered serendipitously in January 2008 with the NASA Swift satellite via its X-ray emission and has generated great interest by astronomers (10 papers and counting). I will present extensive optical and near-infrared light-curves and spectra (ranging from 17 hours to 109 days after the X-ray outburst), as well as our analysis of the Swift UV/optical data and of the initial Swift X-ray outburst. I will discuss the significance of this SN, whether it harbored a jet, and its implications for the SN-GRB connection. I will conclude with the most promising venues of upcoming research that can clarify how massive stars die.
Host: Dae-Sik Moon & Marten van Kerkwijk
Nov 7 Prof. Craig Heinke (U.Alberta)
The Accreting Millisecond Pulsar SAX J1808.4-3658
The accreting millisecond pulsar SAX J1808.4-3658 (1808) may be a transition object between low-mass X-ray binaries and millisecond radio pulsars. Its relative proximity (3.5 kpc), frequent outbursts (six since 1996), and relatively low extinction make it an excellent candidate for detailed studies. Studying 1808's X-ray flux in quiescence, we find only nonthermal X-ray emission, with the thermal emission from the neutron star (NS) surface constrained to L_NS < 6e30 ergs/s. This constraint, coupled with the well-constrained mass transfer rate onto the NS, requires very rapid neutrino cooling from the NS core, constraining the interior composition of the NS. Our simultaneous Gemini observations of the optical light curve in quiescence verify that the observed X-ray flux cannot explain the large optical modulation, requiring another source of irradiation of the companion star--perhaps a pulsar wind. Our modeling of the optical light curve requires a larger (and more massive) companion than expected, and indicates a high mass (>1.8 Msun) for the NS. I will conclude with some of the remaining mysteries surrounding 1808.
Host: Dae-Sik Moon & Marten van Kerkwijk
Nov 14 Dr. Oliver Guyon (NOAJ)
New technologies for direct imaging of exoplanets from ground and space
Our group is developing new coronagraph and wavefront control technologies for high contrast imaging. These technologies are prototyped in the laboratory and constitute the core of both the Subaru Coronagraphic Extreme-AO (SCExAO) system and the space-based Pupil remapping Exoplanet Observer (PECO) space mission concept. The SCExAO system, currently under assembly, is an upgrade of the HiCIAO coronagraphic differential imaging camera, mounted behind the 188-actuator curvature AO system on Subaru Telescope. This platform includes a 1000-actuator MEMS deformable mirror for high accuracy wavefront correction and a PIAA coronagraph which delivers high contrast at 0.05" from the star (5 AU at 100 pc). The project is designed to be highly flexible to continuously integrate new technologies with high scientific payoff. The PECO NASA-funded probe-class ($600M to $800M) Advanced Mission Concept Study is a 1.4-m telescope aimed at imaging and characterizing extra-solar planetary systems at optical wavelengths. Our studies show that a probe-scale PECO mission with 1.4 m aperture is extremely powerful, with the capability of imaging at spectral resolution R~15 the habitable zones of already known F, G, K stars with sensitivity sufficient to detect planets down to Earth size, and to map dust clouds down to a fraction of our zodiacal cloud dust brightness. PECO will acquire narrow field images simultaneously in 10 to 20 spectral bands covering wavelengths from 0.4 to ~1.0 micron and will utilize all available photons for maximum wavefront sensing and imaging/spectroscopy sensitivity. This approach is well suited for low-resolution spectral characterization of both planets and dust clouds with a moderately sized telescope.
Host: Ray Jayawardhana
Nov 21 Prof. Adam Burrows (Princeton)
Theory for the Atmospheres and Radii of Extrasolar Giant Planets
Approximately 300 exoplanets, mostly giant planets in the Jovian mass range, but also more than 40 "Neptune-mass" planets, have been detected orbiting stars in the solar neighborhood. More than 50 of them are transiting their primaries and a large subset of these have collectively yielded a wealth of structural and physical information which theorists are attempting to interpret. A number of these giant planets have been detected directly by the Spitzer infrared space telescope. These constitute the first remote-sensing data of extrasolar worlds. I will summarize the embryonic theory of such irradiated giant planets and discuss what we have learned about their atmospheres and origin. We are witnessing the birth of a new astronomical discipline and I will describe some of its first notable findings.
Karl W. Kamper Memorial Lecturer
Host: Dae-Sik Moon, Ray Jayawardhana & Marten van Kerkwijk
Nov 28 Dr. Chris Willot (HIA/NRC)
Probing the tail-end of reionization with high-redshift quasars
The epoch of reionization marks the end of the Cosmic Dark Ages. Reionization was most likely achieved as a result of UV photon emission by the first stars and accreting black holes. Observations of the reionization epoch allow us to infer the details of early galaxy formation. I will talk about the Canada-France High-z Quasar Survey (CFHQS) - a survey designed to find quasars at redshifts z>6, which can be used as probes of reionization. I will describe the method used to discover quasars, the current status and followup observations focussed on reionization and black hole growth.
Host: Howard Yee
Dec 5 Prof. Gerry Gilmore (Cambridge)
How Cold is Cold Dark Matter
The nature, number and observable properties today of the first bound structures to form at high redshift, arguably local dSph satellite galaxies, is determined by a mix of astrophysical history and the physical properties of the dark matter particles. Recent advances in discovering, and in detailed photometric, dynamical and chemical abundance studies of the local dwarfs has revolutionised our knowledge of their properties, and their use as probes of the very early Universe. I present an update and overview of this new information, and its implications.
Host: Ray Carlberg
Last modified: 22 Jan 2009, 22:43:05

Yanqin Wu (wu@astro) /Dae-Sik Moon (moon@astro) / Ray Jayawardhana (rayjay@astro) / Marten van Kerkwijk (mhvk@astro)

 Colloquia are Fridays, at 2 PM, in Cody Hall. They are followed by refreshments in the lounge, AB 201.

Instructions for speakers

Schedule with abstracts

Schedule without abstracts

Student seminars

CITA seminars

Physics colloquia

Past colloquia: 2003A, 2003B, 2004A, 2004B, 2005A, 2005B, 2006A, 2006B, 2007A, 2007B, 2008A, 2008B, 2009A, 2009B, 2010A, 2010B, 2011A

Current colloquia: 2011B