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Spring 2008

Jan 18
 Dr. Martin Durant (IAC)
High time-resolution astrophysics: the unusual black hole SWIFT J1753.5-0127
Whereas most astronomers worry about spectral coverage and resolution, spatial resolution and object statistics, one remaining avenue for new exploration is temporal variability. Many systems show a wide variety of time-variability due to orbital motions, acoustic oscillations and dynamical processes such as accretion. Timing has for long been important for radio observations, particularly for pulsars, but high time resolution (>10Hz) has also become possible in X- and gamma-rays due to high effective-collecting-area satellites, and in the optical due to fast readout, low noise detectors. I will discuss some of the applications that this can be turned to. SWIFT J1753.5-0127 is a transient X-ray binary system which appeared as a new, bright X-ray source in 2005. Unlike some X-ray transients, this system never entered a thermal-dominated state, challenging the standard cyclic picture of accretion. The source remained relatively bright after the outburst, rather than returning to quiescence, so we had the chance for a thorough observational campaign to try to understand the system, with multi-band optical photometry, optical spectroscopy, X-ray spectroscopy and timing and high-frequency multi-band photometry from ULTRACAM. I will present the results, which perhaps raise more questions than they answer.
Host: Marten van Kerkwijk
Jan 25
 Prof. Brian McNamara (Waterloo)
AGN Feedback in Clusters and Galaxies
The gaseous atmospheres of galaxies and clusters serve as repositories for the energy output from AGN over cosmic time. X-ray observations have shown that cooling of the dense, keV gas in the cores of galaxies and clusters is strongly suppressed, and that feedback from supermassive black holes is a likely heating agent. The energy pouring out of the nuclei of central cluster galaxies often rivals that of quasars. Unlike quasars, their vast power output is channeled into mechanical energy rather than light.
Host: Ue-Li Pen
Feb 8
 Prof. Doug Gies (GSU)
Mass and Angular Momentum Transformations in Massive Binary Stars
Massive stars love company. Most are found among the clusters and associations in which they were born, and most appear to have one or more gravitationally bound companions. Here I will review the results of observational surveys for binaries among massive O-type stars, and I will explore some of the consequences of companions for the evolution of close binaries. A key issue is the transfer of angular momentum and the spin up of the mass gainer, and I will discuss the evidence that many of the rapidly rotating Be stars were formed through binary interactions. Be stars lose angular momentum through outflowing disks, and I will describe recent observations with the CHARA optical long baseline interferometer of Be star disks. After the supernova explosion of the donor star, the systems may appear as runaway stars and/or X-ray binaries, and I will discuss recent observational results on several such binaries, including the black hole binaries Cyg X-1 and SS 433. Finally, I will offer some speculations about how these observations may constrain models for the formation of massive stars.
Host: Tom Bolton
Feb 15
 Prof. Pieter van Dokkum (Yale)
WANTED (Dead or Alive): The Progenitors of Massive Galaxies
Studies of the formation and evolution of massive galaxies provide strong constraints on galaxy formation models. One of the most intriguing aspects of these models is their long-standing prediction that massive galaxies (mostly giant ellipticals) should have assembled recently, through mergers. I will present recent observational evidence that elliptical galaxies may indeed have had a violent recent past, even though their stellar populations are very old. The properties of massive galaxies at redshifts of 2 to 3 will also be discussed, along with recent evidence that the stellar initial mass function may have been different at these early epochs than it is today.
Host: Ray Jayawardhana
Feb 22
 Dr. Rachel Rosen (UNC)
A Non-radial Oscillation Model for Radio Pulsars
We present a non-radial oscillation model that can successfully reproduce many properties of drifting subpulses in pulsars. By demonstrating the presence of oscillation modes in pulsars, we hope to pave the way for astroseismology of neutron stars. This model is an alternative to the drifting spark model of Ruderman and Sutherland (1975). Our non-radial oscillation model reproduces pulsar morphology in terms of the total intensity and linear polarization. We associate the displacement and velocity of the oscillations to two polarization modes. From this, we recreate the Stokes' parameters to model the intensity, linear polarization, and polarization angle. We use our model to qualitatively reproduce the behavior of several pulsars, including PSR 1919+21 and PSR 1237+25. This behavior includes the time-averaged pulse properties, the individual pulse properties, and the subpulse phase jumps. In addition, we are able to reproduce the orthogonal polarization modes observed in most pulsars. We then show that our model can be successfully fit to the data by conducting a quantitative analysis of PSR 0943+10.
Host: Marten van Kerkwijk
Feb 29
 Prof. Lee Hartmann (Michigan)
New approaches to star formation
Star formation is increasingly being understood as a dynamic, time-dependent phenomenon. I argue that understanding the fragmentation of molecular clouds into stars requires an understanding of cloud formation and subsequent evolution. I suggest that molecular clouds are formed by large-scale flows, and that instabilities in post-shock, dense gas provide the "seeds" for protostellar cores. The effects of global gravity provide a natural mechanism to produce massive cloud filaments and star clusters. I will conclude with a brief consideration of the possible implications of these findings for large-scale star formation in galaxies.
Mar 7
 Prof. Ethan Vishniac (McMaster)
Disk Dynamos and the Origin of Magnetic Fields in Galaxies
One of the more challenging problems in cosmology is understanding the origin of galactic magnetic fields. Proposed seed mechanisms give large scale magnetic fields which are typically about 14 orders of magnitude weaker than currently observed. Moreover, observations at high redshift give no indication of systematically weaker galactic fields. This has led to a large number of proposals involving novel physics at very high temperatures and very early epochs in the universe. Here I will discuss progress in understanding large and small scale dynamo effects in highly conducting fluids. This leads to the prediction that galaxies will reach equipartition between disk turbulence and disordered magnetic fields within a few hundred million years. The long wavelength tail of these fields will be about 1.5 orders of magnitude below current values and should reach them within another few hundred million years.
Mar 14
MP 202		 Prof. Kip Thorne (Caltech)
LIGO: The Laser Interferometer Gravitational Wave Observatory
This seminar will be a technical overview of LIGO, from its roots in the 1970s through its conception in the 1980s, construction in the 1990s, initial interferometers and their gravitational-wave searches in 2001-2007, enhanced interferometers in 2007-2010, advanced interferometers in 2011-... , and beyond. The seminar's focus will range from experimental design and noise sources (both predicted and unexpected, and how they are dealt with), through data analysis, gravitational wave sources, and the information the waves carry.
* Special Sackler Technical Lecture *
Host: CITA
Mar 20
3 PM		 Dr. Edo Berger (OCIW)
Gamma-Ray Bursts: From Progenitors to Probes
The launch of the Swift satellite in late 2004 heralded a new, and intensive, era in the study of gamma-ray bursts (GRBs). Thanks to unprecedented rapid and accurate localizations for over 200 bursts, we have been able to make progress on several fronts. In this talk I will focus on three key aspects of current GRB research: (i) using GRBs as probes of metal enrichment and interstellar medium properties at high redshift; (ii) identifying the counterparts of damped Ly-alpha systems; and (iii) unveiling the physics and progenitors of the short-duration GRBs. Along these lines I will show that, unlike the long GRBs, the progenitors of short GRBs are related to an old stellar population, with a wide age distribution of ~0.1-10 Gyr. In the context of ISM properties at high redshift I will show that GRBs now provide the first step towards an investigation of the luminosity-metallicity relation at z>2.
CITA seminar
Host: Dae-Sik Moon
Mar 28
 Prof. Andrea Ghez (UCLA; H.S. Hogg Lecturer)
Massive Stars at the Galactic Center: A Paradox of Youth?
With a strong case for a supermassive black hole at the center of our Galaxy, we need to address the prevelence of the massive stars in this region. The origin of these stars is difficult to explain, given the strong tidal forces and low present-day gas densities. Kinmetics of this population offers helpful clues. Understanding the origin of these stars may provide key insight into the growth of the central black hole and the origin of the hypervelocity stars that have been discovered in the halo of our Galaxy.
Host: Marten van Kerkwijk
Apr 4
 Prof. Doug Welch (McMaster)
Light Echoes from LMC and Galactic Supernovae
The SuperMACHO Project first reported the discovery of light echo systems associated with centuries-old supernova remnants in the LMC in late 2005. Since that time, we have made considerable progress in exploiting this observational "time machine" which provides opportunities to study both the spectrum of outburst light after the fact and to determine other important characteristics of nearby supernovae. I will review the our spectroscopy of LMC supernova light echo systems and describe recent discoveries including the detection of echo systems associated with the historical galactic supernovae Tycho and Cas A. The first results from a program of Gemini GMOS-S spectroscopy of SN1987A echoes will be also described. These suggest differences in outburst spectrum as a function of echo azimuth (perspective).
Host: Bob Abraham
Apr 11
 Prof. Jonathan Tan (UFL)
The Galactic Star Formation Hierarchy: Massive Stars, Star Clusters and Global Rates
I review our current understanding and lack of understandingÂof how massive stars and star clusters form in the Galaxy. Areas of active debate include theÂinitial conditions (how close to virial equilibrium),Âthe basic accretion mechanism (core accretion versus competitive accretion), and cluster formation timescales (few or many free fall times). I describe the Turbulent Core Model for massive star formation and the Equilibrium Star Cluster Formation Model, and how they are consistent with observations. I describe the Orion Nebula Cluster in particular detail. Finally, I discuss the connection of star cluster formation with global star formation rates in disk galaxies, motivating a model of triggering of star formation activity by the collisions of relatively long-lived giant molecular clouds in the shearing disk.
Host: Chris Matzner
Apr 18
 Dr. Jennifer Sokoloski (Columbia)
Observations of Stellar Explosions and Jets
Recent observations by Patat et al. indicate that at least some normal type Ia supernovae arise from white dwarfs that accrete from the wind of a red-giant companion. These wide binaries, called symbiotic stars, can also produce spectacular bipolar jets. I will use the most recent outburst of the symbiotic star RS Ophiuchi as a framework for a discussion of novae, the progenitors of type Ia supernovae, and stellar jets. Some of the phenomena observed in RS Ophiuchi also have bearing on the study of supernova remnants. Others strengthen the link between jets from white dwarfs and those from both young stellar objects and microquasars.
Apr 23
Wed 3pm
Cody Hall		 Drs. Kim Tait and Ian Niklin (ROM)
The meteorite collection of the Royal Ontario Museum: an overview.
The meteorite collection of the Royal Ontario Museum has undergone dramatic growth over the last decade.  Compared with other institutional collections worldwide it is still  relatively small in terms of overall numbers.  However it is one of the most significant collections in the world in terms of the rarities it contains.  Some of the more notable examples being: NWA 3163, a 1.1 kilogram lunar meteorite; a 1.3 kilogram sample of Murchison; NWA 3171, a 416 gram Martian shergottite; a 52 gram specimen of Tagish Lake carbonaceous chondrite, which will be the only sample of the so called "pristine" material on display anywhere as well as a newly acquired 3 kilogram brachinite presently the largest known.  These meteorites and nearly 200 hundred others will be on display in the new Teck-Cominco suite of Earth Sciences galleries slated to open late this year.   Sorry you're going to miss the talk but you and any others from Astronomy are more than welcome to come by the ROM for a visit and see the collection in person.  We'll be issuing an invitation at the talk since we're hoping that this will produce some interesting joint research projects.
Host: John Percy
May 9
 Prof. Mercedes Richards (PSU)
Images of Accretion Flows in Interacting Binaries
The ability to make direct Cartesian images of gas flowing between stars in interacting binaries has evaded us for a long time because nearly all of these systems are unresolved even by the largest telescopes. However, indirect images of the gas flows have been made possible with the aid of tomography. The basic technique provides 2D velocity images of gas flows confined to the orbital plane of the binary. Since the images of multiple emission sources are superimposed in the tomograms, a new spectrum synthesis code was developed to systematically extract the individual emission sources in order to sequentially isolate the images of the disk, gas stream, and other more compact sources. This tool has enabled us to determine the essential properties of the disk and gas stream more accurately than previously possible. More recently, an extension of the tomography procedure has provided the very first 3D views of gas flows for the entire class of interacting binaries.
May 15
Th 11am
Cody Hall		 Drs. Bob Simcoe and Ian Shelton
Digitizing with Commercial Scanners
There are many small and medium size collections of photographic astronomic spectra and small wide field plates around the world whose curators would like to have digitized. In the 1970s and 1980s there were a number of digitizing instruments designed to analyse spectral plates in particular and that were later sometimes adopted to do area scans for projects like the Hubble guide star catalog. A question that keeps coming up in the community is "Are modern inexpensive scanners really good enough to do scientific work?" This project looks at two scanners, a Nikon CS9000 film scanner and an Epson V750 flatbed scanner to see if these scanners can produce results good enough to be used for scientific analysis. In particular we wanted to look at replacing the PDS photometer for scanning spectra plates. This talk will discuss the findings, so far, of this project.
Host: Lee Robbins
June 13
 Dr. Rene Breton (McGill)
Relativistic Spin Precession in the Double Pulsar
The double pulsar, the first relativistic binary in which both neutron stars are observable radio pulsars, is a celebrated laboratory for testing general relativity in the strong-field regime via the precise timing of its radio pulses. A very fortunate alignment of our sight line with the orbital plane of this system allows us to observe eclipses when one of the pulsars passes behind its companion. These eclipses display a complex structure of flux modulations that, according to a model proposed by Lyutikov and Thompson, originate from synchrotron absorption by plasma trapped in the magnetosphere surrounding the companion pulsar. I will present four years of eclipse monitoring conducted at the Green Bank Telescope, with which I tested this eclipse model. This work not only accurately reproduces the eclipse light curves using the Lyutikov and Thompson model, but also provides a quantitative measurement of relativistic spin precession of the companion pulsar. This qualitatively new test of gravity in the strong-field regime agrees remarkably well the value predicted by general relativity.
Host: Marten van Kerkwijk
Last modified: 21 Oct 2008, 14:15:01

Dae-Sik Moon (moon@astro) / Yanqin Wu (wu@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