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Second half of 2006

Sep 15 Prof. Steve Desch (Arizona State)
The Astrophysical Origins of the Short-Lived Radionuclides in the Early Solar System
When meteorites formed at the birth of the Solar System, they incorporated many radionuclides with half-lives on the order of only millions of years. Evidence for aluminum 26 is well known, but recently evidence for two other isotopes has been discovered: beryllium 10 and iron 60. Both decay with a half-life of only 1.5 Myr, but their modes of production are entirely different: neutron-rich Fe 60 is produced only by nucleosynthesis but Be 10 is consumed by stellar nucleosynthesis and must be produced by spallation. Both these and the other short-lived radionuclides must have been produced during or just prior to the formation of our Solar System and potentially probe the Sun's star formation environment. In my talk I will critically examine three general models for their origins: inheritance, irradiation and injection. I conclude that Be 10 Galactic cosmic rays trapped in the Sun's molecular cloud core yield Be 10 levels in meteorites consistent with observed values, but that the other radionuclides were injected into the Solar System by a nearby supernova. I discuss the implications for the Sun's birth environment, and I present our recent calculations into the effects of a nearby supernova on a protoplanetary disk.
Host: Chris Matzner
Sep 22 Prof. Richard Ellis (Caltech)
New Observational Constraints on the Star Formation Rate at Redshift 10
Recent progress in conducting the census of luminous star forming galaxies at redshifts z>5 has suggested a marked decline with look-back time in their number. This conclusion seems at variance with the presence of fairly massive and mature stellar populations seen at z~5-6 and the UV photon density necessary for cosmic reionization. One possible explanation is the rapid build up of these mature galaxies from a large, as yet, undetected population of much lower luminosity systems at earlier times. Strong gravitational lensing offers possibly the only immediate way to locate such an intrinsically faint population. I will describe current Keck/HST/SST surveys ongoing to detect such systems at z~8-10 and the implications of the results obtained so far.
Host: Ray Carlberg
Sep 29 Dr. Alceste Bonanos (Carnegie DTM)
Distances to Local Group Galaxies with Eclipsing Binaries
Eclipsing binaries provide a direct means of obtaining accurate and geometric distances to nearby galaxies. They have recently been used to obtain direct distances to the LMC, SMC and M31. I present the first direct distance determination to a detached eclipsing binary found by the DIRECT Project in M33. The goal of the project is to replace the current anchor galaxy of the extragalactic distance scale, the LMC, with the more suitable spiral galaxies M31 and M33. We derive a distance to M33 accurate to 6%. I will discuss the implications of our result for the value of the Hubble constant.
Host: Ray Jayawardhana
Oct 6 Prof. Christine Wilson (McMaster)
Luminous Infrared Galaxies with the Submillimeter Array: Probing the Extremes of Star Formation
Luminous and Ultraluminous infrared galaxies (ULIRGs) contain the most intense regions of star formation in the local universe. Because molecular gas is the fuel for current and future star formation, the physical properties and distribution of the warm, dense molecular gas are key components for understanding the processes and timescales controlling star formation in these merger and merger remnant galaxies. In this talk, I will present new results from a legacy project on the Submillimeter Array which is producing high resolution images of a representative sample of galaxies with log(L(FIR) > 11.4 and D < 200 Mpc.
Host: Ernie Seaquist
Oct 13 Prof. Steve Eikenberry (Florida)
Black Hole Astrophysics in the Infrared & the FLAMINGOS-2 Galactic Center Survey
I will review the relatively young field of black hole studies using multi-wavelength (particularly infrared) observational techniques. I will place special emphasis on the insights this is providing into the physics of "microquasars" -- stellar-mass black hole binaries exhibiting relativistic jet outflows. I will also discuss our plans for the immediate future of this field using the upcoming FLAMINGOS-2 near-infrared multi-object spectrograph on the 8-meter Gemini South telescope to conduct a large-scale spectroscopic survey of the Galactic Center region.
Host: Dae-Sik Moon
Oct 20 Prof. Kim Venn (UVic)
Observational Tests of Hierarchical Galaxy Formation in the Local Group
Cold dark matter scenarios for galaxy formation predict that our Galaxy formed through the merger of smaller systems over the first half of the age of the Universe. These scenarios also predict that many of these smaller systems have remained intact and some will have become dwarf galaxies. A novel test of this scenario is to compare the chemistries of old stars in the Galaxy with those in the dwarf systems, which is now possible with the large aperture telescopes and high efficiency spectrographs. We initiated an examination of the kinematics and metallicities of hundreds of stars in three Local Group dwarf galaxies (Sculptor, Fornax, and Carina) using the ESO VLT with Giraffe multi-object spectrograph, and we are determining the detailed chemical compositions for approx.ly 100 of these stars in each of these galaxies as well. With these new and extensive data sets, we are able to study the stellar populations in the dwarf galaxies and retrace their evolution. This includes studying the unexpected complexity in their kinematic structures, differences in their metallicity distributions compared to that determined for the Galaxy, and the abundance ratios in the dwarfs that differ significantly from those of metal-poor stars in the Galaxy. These results will be discussed as observational constraints to the merging history of dwarf galaxies in the formation of our Galaxy.
Host: Kevin Bundy
Oct 23 Dr. Michael Shara (AMNH)
Classical Novae, Dwarf Novae, Tramp Novae
Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, via an accretion disk, to its white dwarf companion. Theory predicts that the white dwarfs in all (accretion-powered) dwarf novae must eventually accrete enough mass to undergo (thermonuclear-powered) classical nova eruptions... and then revert back to dwarf nova or hibernating binary status. I'll report the discovery of a shell of the prototypical dwarf nova Z Camelopardalis that is an order of magnitude more extended than those detected around many classical novae. The derived shell mass matches that of classical novae, and is inconsistent with the mass expected from a steady dwarf nova wind. This shell observationally links, for the first time, a prototypical dwarf nova with an ancient nova eruption and with the classical nova process. Metamorphic transformations of these systems also occur on timescales of Gigayears, with observable consequences. Finally I'll point out that intergalactic tramp classical novae are the best tools available to determine the space density of intergalactic stars.
Host: John Percy
Room: Bahen 1190 *Note special date and location*
Nov 2 Prof. Shri Kulkarni (Caltech)
The Space Interferometer Mission: Parallaxes, Planets and More
The Space Interferometry Mission PlanetQuest (SIM) a 9-m Michelson interferometer operating in the visible band. With an anticipated accuracy of 4 microarcseconds (all sky, wide angle) and precision of 1 microarcseconds (narrow angle). These two modes will enable to make fundamental advances in Galactic astronomy, determine the matter makeup of our Galaxy and the Local Cluster, decisively define the cosmic distance scale and undertake comprehensive search for planets down to the mass of Earth. The speaker serves as the Interdisciplinary Scientist for SIM and is also the Co-PI of EIPICs, the largest key project for planet search.
Host: Marten van Kerkwijk
Joint Astro colloquium / CITA seminar
NU Cody Hall, 3pm *Note special date, time, and location *
Nov 3 Prof. Derek Fox (PSU)
The Nature of Short-Hard Gamma-Ray Bursts
I will discuss the revolution in our understanding of short-hard gamma-ray bursts that has been provoked by the discovery of the first afterglows of these elusive events. The existence of the short bursts as a distinct population was first suggested in 1974 and confirmed in 1993. Until 2005, however, the afterglow revolution that revealed the origins of long-duration gamma-ray bursts in the deaths of massive stars had passed the short bursts by. With eleven afterglows and multiple redshifts now in-hand, the short bursts are revealed as a cosmological phenomenon, like their long-duration counterparts. Compared to long bursts, however, short bursts are less energetic, lower-redshift explosions produced by a much longer-lived progenitor, and lack associated supernovae to very deep limits. I will review the viable models for these cosmic explosions, and derive quantitative constraints on the local rate and progenitor lifetimes of short bursts by placing the recent events in the context of the full BATSE burst catalog (1991-2000). If the short bursts are produced by compact object merger events, these constraints translate directly into event rate estimates for LIGO, VIRGO, and future ground-based gravitational-wave detectors.
Host: Dae-Sik Moon
Nov 10 Dr. Victor Debattista (Univ. Washington)
Secular Evolution of Galaxies: From Dark Halos to Nuclei
One path by which disk galaxies evolve is via internal secular evolution. By breaking the symmetry of the system, bars play an important role in driving such evolution. I describe some recent results concerning aspects of this evolution relating to dark halos, disk density profiles, bulges and central nuclei.
Host: John Dubinski
Nov 17 Prof. Dave Patton (Trent)
Close Galaxy Pairs in the SDSS and the Millennium Simulation
Close galaxy pairs provide a useful probe of the frequency and nature of galaxy mergers. With the availability of large high resolution N-body simulations, it is now possible to directly compare observed pair samples with those predicted by models of galaxy formation. We identify a sample of spectroscopic pairs drawn from the Sloan Digital Sky Survey. We then perform a direct comparison with the Millennium Run semi-analytic catalogue of Croton et al. (2005). We find surprisingly good agreement between the close pair statistics of SDSS and Millennium, and infer that major mergers are most common in L* galaxies.
Host: Marten van Kerkwijk
Nov 24 No colloquium: CSA Space Astronomy Workshop in Montreal
Dec 1 Dr. Alexander Scholz (Toronto)
The Infancy of Brown Dwarfs: Accretion and Dusty Disks
Young brown dwarfs appear to share many similarities with T Tauri stars. Specifically, many of them exhibit clear signs of ongoing accretion and evidence for the presence of dusty disks. Using data from Spitzer, IRAM, Magellan, and ESO/VLT, we are carrying out an extensive program to characterize substellar disks. In the first part of the talk, I will focus on the inner accretion zone. By analysing the variability of emission lines, we can obtain a close-up view on the accretion flow of brown dwarfs. I will present several case studies, demonstrating that accretion on brown dwarfs is funneled and can be highly variable. The second part will focus on the investigation of the dusty disks. Analysing their properties and evolution is a main probe for brown dwarf origins, and allows us to investigate efficiency and universality of planet formation. Our disk surveys in Taurus and Upper Scorpius provide first constraints on disk masses and radii for substellar objects. They show that dust evolution processes like grain growth and dust settling, the prerequisites for planet formation, are ubiquitous in brown dwarf disks.
Host: Ray Jayawardhana
Dec 8 Dr. Kevin Bundy (Toronto)
The Mass Assembly History of Galaxies: Downsizing, Quenching, and the End of Star Formation
Growing evidence suggests that the evolution of galaxies since the peak in global star formation activity at z~2 proceeds in a mass-dependent fashion. The most massive galaxies mature first while smaller systems appear to complete their evolution later. This "downsizing" pattern is intriguing in the context of the hierarchical Cold Dark Matter framework which suggests that the most massive structures should be the last to evolve. In such scenarios, downsizing likely indicates the presence of poorly understood physical mechanisms that play an important role in driving galaxy evolution and quenching star formation. I will present results from Palomar infrared imaging and the DEEP2 Galaxy Redshift Survey that help reveal and quantify the nature of downsizing and shed light on potential physical processes, such as AGN feedback, that may help reconcile the observed evolution with hierarchical models.
Host: Bob Abraham
Last modified: 26 Nov 2006, 15:31:53

Chris Matzner (matzner@astro) / Ray Jayawardhana (rayjay@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