University of Toronto Astronomy Colloquia



	ASTRONOMY COLLOQUIA

First half of 2004

Jan 16	Prof. John Monnier (Michigan) *Imaging with Optical Interferometry* In many areas of astrophysics, especially those addressing the most mysterious phases of stellar evolution, progress is stymied by lack of angular resolution in the visible and infrared, which is limited by both diffraction of the collecting telescope and atmospheric turbulence. Interferometry overcomes these limitations by combining light from widely-separated apertures, but is technically challenging and requires sophisticated image reconstruction algorithms. In order to explore the potential for imaging complex objects using optical interferometry, the 10-meter Keck-I telescope has been converted into an interferometric array using aperture masking techniques, allowing near-infrared imaging with the diffraction-limited resolution of ~50 milli-arcseconds. I will discuss a few of our most surprising findings, results which are changing our views of accretion disks around young stellar objects (YSOs), the physics of colliding winds in massive binaries, and the mechanisms of stellar mass-loss from evolved stars. Also I will reveal first results of two new interferometric surveys of YSOs taking advantage of the incredible sensitivity of the long-baseline Keck-Keck Interferometer and the imaging capabilities of the 3-telescope IOTA array. Host: Chris Matzner
Jan 23	Dr Inese Ivans (Caltech) *Tracing the Formation of the Galactic Halo* When we observe old stellar populations of the Milky Way, we are viewing a fossil record of the early dynamical and chemical evolution of our Galaxy. The chemical compositions of long-lived stars provide information that can be employed to trace the nucleosynthetic origins of various elements. In addition to nucleogenesis, chemical abundance analyses of these relics of early star formation and chemical enrichment set constraints for models of stellar and galactic evolution, and galactic structure. In this talk, I will present recent results, highlighting those obtained by combining information about stellar chemical abundances with kinematics in studies of the formation of the Galactic Halo and the contributions from systems which likely evolved beyond our Halo. Host: Stefan Mochnacki
Jan 26 Mon., 2:00PM! MP 1318A!	Dr Mallory Roberts *Vingt Ans Apres: Concerning Certain Recent Observations of Pulsar Wind Nebulae and What Modifications to Models Are Required Thereof* In 1984, a series of papers by Kennel & Coroniti and by Reynolds & Chevalier established the now standard theoretical models of a pulsar wind nebula growing at the center of an expanding supernova blast-wave. These models were sphericaly symmetric and only compared to data from the Crab nebula, since at the time it was the only source whose spectrum, age, size, and pulsar energetics were well known. However, it was (and still is) lacking direct evidence of the blast-wave in the form of a supernova remnant shell, and in many ways is hard to regard as typical. In the last five years deep pulse searches, high resolution X-ray images, and high-quality radio observations have given us an abundance of information about a number of pulsar wind nebulae and shown that there is remarkable variety in their characteristics. I will give a highly biased review of some of these observations and discuss some of the issues future models of these systems will need to address. Host: Maxim Lyutikov
Jan 30	Dr Bart Pindor (UofT) *Searching for Strongly-Lensed Quasars in the SDSS* The Sloan Digital Sky Survey (SDSS) offers an opportunity to substantially increase the known sample of gravitationally-lensed quasars. I will give a brief introduction to the SDSS, describe an algorithm for selecting gravitational lens candidates from amongst SDSS quasars, discuss selection effects associated with the lens search, and present a few of the interesting systems discovered to date.
Feb 6	Dr Henk Hoekstra (UofT/CITA) *Mapping the Dark Matter Using Weak Lensing* In recent year the accuracy with which the lensing signal induced by large scale structure can be measured has improved tremendously. The prospects for the future are excellent and several large surveys are underway enabling us to measure the matter power spectrum with great precision. By itself and in combination with other methods such as CMB weak lensing provides tight constraints on a range of cosmological parameters. In this talk I will discuss the theory and how one goes from images to unbiased shape measurements. I will review recent progress in this rapidly evolving field and discuss what to expect in the next few years focusing on the weak lensing component of the CFHT Legacy Survey which will image 160 square degrees.
Feb 9 Mon., 5PM! MP 203!	Prof. Ray Jayawardhana (Michigan) *New Worlds in the Making: Probing the Origins of Planetary Systems* The discovery of extra-solar planets has invigorated interest in exploring planetary origins. Thanks to recent technological developments, we are now able to probe many of the key stages in the evolution of cirumstellar disks into planets. In particular, young stellar groups in the solar neighborhood provide valuable laboratories for testing our ideas about the origin and initial configurations of planetary systems. Disks around many 10 million-year-old stars appear to have central cavities, perhaps carved out by newborn planets. If planets have indeed formed in these disks, it may be possible to image them directly using adaptive optics on large ground-based telescopes.
Feb 12 Th., 2PM! MP 203!	Dr Scot Chapman (Caltech) *The evolution of luminous, dusty galaxies* Massive elliptical galaxies mark the highest density regions of the Universe (galaxy clusters), and their environment must be an important factor in their evolution. How were these luminous galaxies formed? Are they assembled on a timescale comparable to their star-formation (implying luminous star-bursts emitting in the rest-frame submm wavelengths), or from a more prolonged build-up from passive, sub-components? The formation mechanism for massive elliptical galaxies can be tested by searching for populations of strongly clustered and gas-rich, luminous submm galaxies (SMGs) at z<2-3. We have recently increased the number of spectroscopically identified SMGs by two orders of magnitude, yielding for the first time, strong constraints on the evolution of luminous, dusty galaxies. The volume density of SMGs at z~2 is comparable to that of massive elliptical galaxies. The redshifts span a range of z=1-4, with a median z=2.3 and a quartile range of 2.0-2.7. In this talk, I will discuss the astrophysics, clustering and masses of the SMGs.
Feb 23 Mon., 5PM! MP 203!	Dr Joop Schaye (IAS) *The Chemical Enrichment of the Intergalactic Medium* The enrichment of the intergalactic medium (IGM) with heavy elements provides us with a fossil record of past star formation and a unique laboratory to study galactic feedback processes. I will present some results from numerical simulations of enrichment by galaxies, but the focus of my talk will be on the observed distribution of intergalactic metals. We have developed new statistical techniques for analyzing quasar absorption spectra and applied them to an unprecedented sample of high-quality observations. I will present results from this new study, including the first measurements of the distribution of metals in the diffuse IGM as a function of density and redshift. Finally, I will present evidence for the existence of a very large population of compact, highly enriched gas clouds.
Feb 26 Th., 2PM! MP 203!	Dr Annette Ferguson (MPA) *Probing Galaxy Formation & Evolution with the Local Universe Fossil Record* Understanding the way in which galaxies form and evolve is a major goal of modern astrophysics. Disk formation has proved particularly challenging within the popular hierarchical scenario for structure formation. Sophisticated numerical simulations of cosmological galaxy formation can now make predictions about the internal structure and content of disk galaxies, however the results are somewhat dependent on the details of the adopted star formation and feedback prescriptions. I will describe observations of the resolved fossil record -- i.e. the spatial distribution, ages, metallicities and kinematics of old and intermediate-age stars -- in our nearest massive companions, M31 and M33, and discuss the insight they provide about the galaxy assembly process. This approach to studying galaxy formation and evolution complements studies of the high redshift Universe and I will conclude by discussing how we might extend this type of work to large disk galaxies residing beyond the Local Group.
Feb 27	Prof. Tereasa Brainerd (Boston) *Mass Estimators for Field Galaxies: Dynamics vs. Weak Lensing* Having traveled the world for the past 8 years convincing the community that weak lensing is the only reasonable tool to use to constrain the mass-to-light ratios of galaxies, I'm now ready to admit that dynamical analyses really do have their place. Here I'll discuss the pros and cons of weak ("galaxy-galaxy") lensing mass estimates in comparison to dynamical mass estimates and will argue that there are times when dynamical estimates really are the better choice. I'll present results of dynamical mass estimates for "isolated" host galaxies in the Two Degree Field Galaxy Redshift Survey (2dFGRS) and I'll compare the results to current weak lensing constraints. This is the first dynamical study which attempts to study host galaxies on the basis of morphology, and clear differences in the velocity dispersions of satellites about host galaxies are seen when the sample is divided into E/S0 hosts and spiral hosts. In particular, while the velocity dispersion of satellites about E/S0 hosts increases monotonically with the host luminosity, the velocity dispersion of satellites about spiral hosts is independent of the host luminosity. The latter leads to the conclusion that there is a universal mass for the halos of spiral galaxies, and the implications of this for the "merger hypothesis" of the formation of ellipticals will be discussed briefly. Host: Marten van Kerkwijk
Mar 2 Tu., 4PM! MP 203!	Dr Ludo van Waerbeke (IAP) *Gravitational Lensing and Cosmology* Recent advances in cosmology have revealed the beautiful picture of the very high redshift universe at the photon/matter decoupling, galaxy surveys have shown the three dimensional distribution of light in the nearby universe, the exploration of the deep universe at high redshift with submm, infrared and X-rays brought new insights about large scale structures formation. Yet, cosmology is a young science and new observations are desperately needed in order to consolidate our picture of the universe. In particular a large fraction of the universe's content is not seen directly and its physical nature remains totally unknown. In this talk, I will describe a new way to probe the universe, based on the gravitational optics. This technic enables the possibility to 'see' the dark matter directly and to study its distribution out to high redshift. I will discuss the recent results in this field and put them in perspective within the expected progress in cosmology over the next few years.
Mar 8	Dr Bill Jones (Caltech) *Bolometric Polarimetery: First light with Boomerang* Rich and interesting data sets from bolometric polarimeters are finally becoming avaliable. The first generation of instruments have only recently seen first light. I report on what have we learned from Boomerang, what scientific returns we are expecting shortly, and what lessons may be applied to the next generation of polarimeters. Boomerang performed admirably, achieving a noise per 10' pixel of ~ 4 uK over ~100 deg^2 field, and 30 uK per pixel over ~ 3 % of the sky. In addition, a 20x10 degree swath of the Galactic plane was mapped with polarized detectors at 150, 240 and 345 GHz, at 10', 6.5', and 7' resolution, respectively. Host: Barth Netterfield
Mar 12	Dr Yuri Levin (CITA) *Massive stars in discs around supermassive black holes* Most galaxies harbour supermassive black holes (BH)s in their centers. Some of the holes capture gas from the ambient medium; the gas settles into a disc and eventually accretes onto the BH. The outer edges of the disc may fragment and form massive stars. I shall argue that there is evidence of the circum-BH star formation in our Galactic center. I will speculate on its consequences for other galaxies and show that the future space-based gravitational-wave antennas may detect a signal when a compact remnant of the circum-BH massive star inspirals into the supermassive black hole.
Mar 19	Dr Paul Martini (CfA) *Quasar Lifetimes* The quasar lifetime is one of the most fundamental quantities for understanding black hole and quasar evolution, yet it remains uncertain by several orders of magnitude. If it is long, then only a small fraction of galaxies went through a luminous quasar phase. In contrast, a short lifetime would require most galaxies today to have undergone a quasar phase in their youth. The current best estimates or constraints on the quasar lifetime from black hole demographics and the radiative properties of quasars vary from at least 1 to 100 million years. I will describe several methods for measuring the lifetime that should substantially narrow this range in the next few years, as well as determine if quasar activity is episodic. Host: Bob Abraham
Mar 26	No colloquium: speaker ill
Apr 2	Dr Todd Strohmayer (GSFC) *Burst Oscillations: A New Spin on Neutron Stars* High amplitude X-ray brightness oscillations during thermonuclear X-ray bursts were discovered with the Rossi X-ray Timing Explorer (RXTE) in early 1996. These oscillations are caused by rotational modulation of the burst emission and reveal the spin frequencies of neutron stars in low mass X-ray binaries, a long sought goal of X-ray astronomy. The fast variability during bursts has provided new insights into neutron star physics. I will review the status of our knowledge of these oscillations and describe what they are teaching us about neutron stars, with a focus on the most recent developments. Host: David Ballantyne
Apr 9	No colloquium: Good Friday
Apr 16	Prof. Joe Cassinelli (Wisconsin) *Chandra Observations of X-ray Sources in Hot Star Winds* Chandra spectra have provided spectrally resolved line profiles. This has greatly improved our ability to diagnose the X-ray formation regions in the outer envelopes and winds of Early Type Stars. With the resolved f-i-r lines of Helium like ions we are able to determine the radius at which X-ray lines are originating. We find that although the radii differ drastically from one ion to another, the source location occurs at about optical depth unity in the wind. This is as expected from models in which shock fragments are distributed throughout the winds. However, rather unexpectedly we find that high ion stages come from regions near the base of the wind where the wind speeds and associated shocks should be too weak to produce such high ion stages. For most hot stars the X-ray lines show negligible centroid shifts and unexpectedly minor skewness, suggesting that the winds are rather porous to the transmission of X-rays. Various new ideas are being supported, such as X-rays form in bow shock structures in the winds, and/or from shocks at the interfaces of the winds and magnetically torqued equatorial disks. Host: Marten van Kerkwijk
Apr 23	Prof. You-Hua Chu (Urbana-Champaign) *X-ray Views of Stellar Mechanical Energy Feedback* Massive stars inject mechanical energy into the interstellar medium (ISM) via fast stellar winds and supernova ejecta at velocities of thousands of km/s. The dynamical interactions sweep ambient medium into shells and generate gas at X-ray-emitting temperatures, 10^6 to 10^8 K. Thus, X-ray observations of the hot gas are essential in the study of stellar mechanical energy feedback. In the first part of my talk, I will present X-ray views of shocked fast stellar winds in bubbles with a wide range of sizes. In the second part, I will use the energy budget in a superbubble to illustrate that only a small fraction of stellar mechanical energy is retained as the thermal and kinetic energies of the ambient ISM. Host: Chris Matzner
Apr 30	Prof. David Guenther (Saint Mary's) *Asteroseismology Through the Ages: January - April* We believe nearly all main-sequence stars with surface convection zones support low amplitude (velocity amplitudes are comparable to the speed with which your eyes are skimming through the words of this abstract) nonradial oscillations similar to those seen on the Sun. For over twenty years astronomers have been trying, mostly unsuccessfully, to observe oscillations on other stars. The first observations to convince some of the skeptics have only appeared within the past year. And the first telescope in space dedicated to observing nonradial oscillations, MOST, has just begun observing (December 2003) Sun-like stars. Following a short introduction to asteroseismology and the MOST satellite, I will describe a new strategy to interpret oscillation spectra and show what we have so far learned from some of the most convincing ground-based observations. I also hope to be able to discuss some of the early results from MOST. Host: Slavek Rucinski
May 7	Prof. Ermanno Borra (Laval) *Liquid Mirrors* Liquid surfaces are very smooth and follow equipotential surfaces so that one readily gets optical quality mirrors. I review the progress made since the concept was reborn 20 years ago. The technology of rotating mercury mirrors is well-proven both in the laboratory and in observatory settings, where several Liquid Mirror Telescopes (LMTs) have successfully been operated for years. We are now concentrating our efforts on a new nanotechnology that can coat any liquid with a layer of self-assembling particles. We have successfully coated oils and ferrofluids with reflecting layers of silver and gold. Coating viscous oils should allow us to tilt parabolic rotating LMTs by a few tens of degrees. Coating ferrofluids should allow us to make a new class of versatile high-performance optical elements shaped by magnetic fields. Prototype mirrors have been made and tested. We also have studied a new class of liquid mirrors shaped by thermal fields. I will present the laboratory work done so far. Host: Barth Netterfield
May 14	Prof. Geoff Blake (Caltech) *High Resolution Spectroscopy and Imaging of Protoplanetary Disks* Infrared and millimeter-wave continuum surveys have placed strong constraints on the evolutionary timescales of circumstellar disks and any attendant planetary system. Rather less is known about the nature of the gas in protoplanetary accretion disks and how it may evolve with time. This talk will summarize recent millimeter-wave aperture synthesis studies of a suite of molecules in a small sample of the disks encircling T Tauri and Herbig Ae stars, and highlight future possibilities in both the near- and far-term. Probing the inner, planet-forming region of such disks is not presently possible at (sub)millimeter wavelengths, but high resolution spectroscopy in the mid-infrared with 8-10m class telescopes is beginning to address this pivotal environment. The available data have been analyzed with detailed two-dimensional Monte Carlo radiative transfer codes, and the results of these analyses will be compared with the predictions of theoretical chemical models of protoplanetary disks. Host: Yanqin Wu
May 21	Prof. Thebe Medupe (NASSP, South Africa) *Pulsational Properties of the roAp stars* I review the pulsational properties of the rapidly oscillating Ap (roAp) stars. Mention will also be made of how oscillations in stars can be used to infer their internal properties. One of the interesting observational results of studying these stars is the fact that their photometric amplitudes decline more rapidly with wavlength of observation than is theoretically expected. We also find that their photometric pulsation phases show large variations with wavelength in some of these stars. I discuss attempts to explain these and other spectroscopic results. Host: John Percy
May 28	Prof. Slavek Rucinski (UofT) *MOST: The first Canadian space telescope* MOST, the first micro-satellite for astronomy, has been launched in June 2003 and started providing data for its primary targets in January 2004. The satellite is to explore the p-mode osciallations of bright stars, variability of hot stars and reflection properties of known extra-solar planets. The relatively long testing and commissioning phase has led to an improvement in pointing from the expected 15-20 arcsec to now routinely achieved 3-5 arcsec and to a drastic improvement in data quality. Exciting and unexpected results will be presented for several targets observed during the testing period, but the excellent performance of the satellite tells us that many great results are still to come.

Last modified: 28 Jun 2004, 17:27:55
Marten van Kerkwijk / MP 1404D / 416-946-7288 / 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