University of Toronto Astronomy Colloquia



	ASTRONOMY COLLOQUIA

Spring 2007

Jan 12	Dr. Alexie Leauthaud (Marseille) *First Lensing Results from the COSMOS Survey* With a primary goal of conducting precision weak lensing measurements from space, the COSMOS survey has imaged the largest contiguous area observed by the Hubble Space Telescope to date using the Advanced Camera for Surveys. I will begin by presenting some of the first lensing results from COSMOS including a 3D lensing analysis as well maps of the Dark Matter that have recently been published in Nature. I will next present early results from a galaxy-galaxy lensing analysis and attempt to shed further light on the question: to what extent does the surrounding dark matter environment shape the observational properties of luminous galaxies? Weak gravitational lensing techniques have the unique ability to probe the full dark matter distribution and to reveal how galaxies are related to their dark matter haloes. Using the galaxy-galaxy lensing technique, I have mapped dark matter around an ensemble of galaxies and groups of galaxies within the COSMOS field. Using the exqusite ACS imaging to classify galaxies according to their morphology, I will show that the dark matter environment does indeed play a role in shaping the galaxies that we observe. Measuring this connection is not only a powerful test of the CDM paradigm, but also provides an essential ingredient towards understanding the physics of galaxy formation. Host: Kevin Bundy
Jan 26	Dr. Jeff Crelinsten (Impact Group) *Einstein's Jury: The Race to Test Relativity* While Einstein's theory of relativity ultimately laid the foundation for modern studies of the universe, it took a long time to be accepted. Between 1905 and 1930, relativity was poorly understood and Einstein worked hard to try to make it more accessible to scientists and scientifically literate laypeople. Its acceptance was largely due to the astronomy community, which undertook precise measurements to test Einstein's astronomical predictions. The well-known 1919 British eclipse expeditions that made Einstein famous did not convince most scientists to accept relativity. The 1920s saw numerous attempts to measure light-bending, as well as solar line displacements and even ether-drift. How astronomers approached the "Einstein problem" in these early years before and after the First World War, and how the public reacted to what they reported, helped to shape attitudes we hold today about Einstein and his ideas. Host: John Percy
Feb 2	Prof. James Graham (Berkeley) *The Signature of Primordial Grain Growth in the Polarized Light of the AU Microscopii Debris Disk* We have used the Hubble Space Telescope Advanced Camera for Surveys coronagraph to make the first polarization maps of the AU Microscopii debris disk. The polarization rises from 5% at 20 AU to 40% at 80 AU. The polarization is perpendicular to the disk, indicating that the scattered light originates from micron-sized grains in an optically thin disk. Disk models show that interior to the ``birth ring'' (40-50 AU) there is a hole in the dust distribution where micron-sized dust is depleted by a factor of more than 300. The disk is collision dominated, and grains that fall inward due to drag forces undergo a destructive collision. The presence of this hole implies that the localized enhancements in surface brightness that occur at projected radii interior to the birth ring are caused by nonaxisymmetric structures in the outer disk. The grains exhibit strong forward scattering and high polarization. Spherical grains composed of conventional materials cannot reproduce these optical properties. A Mie/Maxwell-Garnett analysis demands highly porous (91%-94%) particles. In the inner solar system, porous particles form in cometary dust, where the sublimation of ices leaves a ``bird's nest'' of refractory material. In AU Mic, the grain porosity may be primordial, because the dust birth ring lies beyond the ice sublimation point. The observed porosities span the range of values implied by laboratory studies of particle coagulation by ballistic cluster-cluster aggregation. To avoid compactification, the upper size limit for the parent bodies is in the decimeter range, in agreement with theoretical predictions based on collisional lifetime arguments. Consequently, AU Mic may exhibit the signature of the primordial agglomeration process whereby interstellar grains first assembled to form macroscopic objects. Host: Ray Jayawardhana
Feb 9	Prof. Harvey Richer (UBC) *White Dwarfs in Globular Star Clusters* New HST and Gemini observations are providing interesting new data on white dwarfs in globular star clusters. Some of the most intriguing new results will be presented in this seminar ranging from white dwarf "kicks", through ages of globular clusters determined via white dwarf cooling and their role in understanding the epoch of cluster formation in the Universe, to some delightful serendipity. Host: Chris Matzner
Feb 16	Dr. Buell Januzzi (NOAO) *A Different View of the Evolution of Galaxies and Large Scale Structure* How do the luminous and gaseous components of the Universe trace the formation and evolution of structure? How are they related to each other? How are they connected to the underlying distribution of dark matter? I will review selected results from three surveys that are designed to address these questions: HST Observations of z < 1.6 Gaseous Universe Direct Comparison between Quasar Absorption Line Systems and LSS traced by Galaxies NOAO Deep Wide-Field Survey I will use selected examples of results from these surveys to demonstrate how large, well constructed surveys can be used in conjunction with simulations to gain improved understanding of the astrophysics that is driving the evolution we observe in the properties of galaxies and LSS in the Universe. Host: Colin Borys
Mar 2	Dr. Mark Sullivan (Toronto) *Type Ia Supernovae as Probes of Dark Energy* Type Ia supernovae (SNe Ia) represent the best probe of the expansion history of the Universe and hence of the "dark energy" driving the cosmic acceleration. I will present the cosmological results from the first three years of the Supernova Legacy Survey (SNLS), a five year program designed to find and follow around 500 distant SNe Ia, including the latest constraints on the equation of state of the dark energy, w. I will give an overview of various tests on the systematic uncertainties in using SNe Ia for cosmology that SNLS allows, including evidence that the rates and properties of SNe Ia vary as a function of their progenitor stellar population. I will then discuss the prospects for future dark energy surveys. Host: Ray Carlberg
Mar 9	Dr. Nathan Smith (Berkeley) *Feedback from Massive stars and Triggered Star Formation in Orion and Carina* Feedback from massive stars can have a profound impact on the star formation process, and I will review some recent multiwavelength observational results of studying externally-irradiated protoplanetary disks, jets, and embedded sources behind advancing ionization fronts in HII regions. Two regions in particular -- the Orion Nebula and the Carina Nebula -- are excellent laboratories for studying these phenomena in detail. I will emphasize new observational results from the Carina Nebula (including results from Spitzer and HST), which is just beginning to reveal its rich star formation content. A new Spitzer/IRAC survey of Carina shows a high IR-excess disk fraction across a large area of the nebula, implying that star formation triggered by stellar winds makes a substantial contribution to the total stellar mass of the region, and affects the IMF. I will also discuss the relevance of Carina as a possible analog of the cradle of our Solar System. Host: Norm Murray
Mar 16	Prof. Els Peeters (UWO) *Polycyclic Aromatic Hydrocarbons as Astrophysical Probes* The infrared spectra of a wide variety of objects associated with dust and gas are dominated by the well-known emission features at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7 micron, the so-called Unidentified Infra-Red (UIR) features. These UIR bands are generally attributed to fluorescence from FUV pumped large Polycyclic Aromatic Hydrocarbon (PAH) molecules. As a class, PAHs are the most complex molecules known in space and may well have played an important role in the prebiotic evolution of life. This talk will focus on the spectral characteristics of these UIR bands and their dependence on the local physical conditions of their environment. This will be tightly coupled with the ongoing laboratory studies and the theoretical calculations on the IR characteristics of PAHs underway at the NASA Ames Research Center. Subsequently, we focus on the use of the UIR bands as tracers of star formation. We will discuss different PAH-based IR diagnostic tools to determine the contribution of star formation to the total power budget of IR emission from galactic nuclei and assess the use of PAHs as quantitative tracers of star formation activity. Host: Ernie Seaquist
Mar 23	Prof. Marta Volonteri (Michigan) *The role of environmental hazards in building up massive black holes* I'll discuss models for the hierarchical growth of supermassive black holes, feeding pregalactic black hole seeds. Accretion properties, black hole mergers and dynamical interactions, as well as their dangers, implications and observational signatures, will be critically addressed. Host: Ray Jayawardhana
Mar 30	Prof. Douglas Leonard (SDSU) *The Geometry of Supernova Explosions* Is the mechanism that drives the explosion of massive stars spherical? While simple to pose, this question belies a menacing observational challenge, since all extragalactic supernovae are unresolvable during the critical early phases of their evolution. Fortunately, geometric information is encoded in the polarization properties of supernova light, and it is through the technique of spectropolarimetry that we have begun to address explosion geometry from an observational standpoint. In this talk I will review the small but growing database of spectropolarimetry of core-collapse events, with an emphasis on how polarization and, hence, degree of inferred asymmetry, changes as a function of progenitor envelope mass. By focusing particular attention on SN 2004dj, the closest and most well-observed supernova of the past decade, I will argue that the innermost regions of these stellar explosions are severely distorted, the result of an explosion mechanism that is strongly non-spherical in nature. Host: Andy Howell
Apr 13	Dr. Anna Watts (MPA-Garching) *Neutron star seismology - towards a relativistic Richter scale* The detection of seismic vibrations in the aftermath of giant flares from two magnetars has opened up the prospect of using seismological techniques to study neutron star properties. Initial results have included the first direct estimate of the thickness of the neutron star crust, a strong constraint on the nuclear equation of state. I will discuss current efforts to improve our models of the starquake process, focusing in particular on the effects of the strong field on the oscillations, their excitation, damping and detectability. I will also cover prospects for future observations using both electromagnetic and gravitational wave telescopes. Host: Marten van Kerkwijk
Apr 20	Prof. Isabelle Baraffe (Lyon) *Birth and fate of hot-Jupiter and hot-Neptune planets* The number of exoplanets newly discovered increases rapidly with time, providing new and sometime puzzling information about their formation and their structure. In this talk, I will describe the properties of planets orbiting their parent stars at close orbital distances. The former undergo irradiation effects from their parent stars and eventually evaporation effects. Both processes can severely affect the properties of close-in planets, in terms of atmosphere structure, emitted flux and evolution. I will also discuss the discrepancy found between the observed radii and the model predictions for half of the transit planets detected up to now. Host: Ray Jayawardhana
Apr 26 2pm, Cody Hall	Prof. Gilles Chabrier (Lyon) *Dense astrophysical plasmas : from giant planets to neutron stars* In this talk, I will briefly examine the properties of the dense plasmas characteristic of the interior of giant planets and of the atmosphere of neutron stars. Special attention will be devoted to the equation of state of hydrogen and helium at high density, and the constraints arising from high-pressure shock wave experiments, and to the effect of strong magnetic fields on the properties of dense matter. Joint CITA seminar / Astro colloquium Host: Chris Thompson
May 11 2pm, Cody Hall	Dr. Alessandro Morbidelli (Nice) *Crucial dynamical phases in solar system formation* The formation and evolution of the giant planets of our Solar System presents several problems: the cores of the planets should have been driven into the Sun by Type I migration, faster than they could accrete their massive gaseous atmosphere; once formed, Jupiter and Saturn should have suffered Type-II migration towards the Sun, becoming hot or warm giants, like most of the extra-solar planets known so far; the planets most likely underwent a late reorganization of their orbital architecture, as indicated by the Late Heavy Bombardment (LHB) of the Moon, which suggests that a massive reservoir of small bodies suddenly became unstable. Without pretension of providing any definitive answer, I will present a scenario of the formation and evolution of the giant planets that addresses these problems. More specifically I will present simulations of the dynamics of planetary cores in the vicinity of a `planet trap', which can exist at the transition between the active and the dead zones of the disk. I will illustrate how the dynamics of the fully formed planets in the gas disk leads to one of 6 possible mutual configurations, that are stable and avoid significant migration towards the Sun. Finally I will describe our model for the origin of the LHB and how it connects with some of these mutual stable configurations. Host: Roman Rafikov
June 13 Wed 2pm Cody Hall	Dr. William Wall (INAOE, Mexico) *Comparison of 13CO Line and Far-Infrared Continuum as a Diagnostic of Dust and Molecular Gas Physical Conditions --- Implications for the N(H2)/I(CO) Conversion Factor* Far-infrared continuum data from the COBE/DIRBE instrument were combined with Nagoya 4-m 13CO J=1-0 spectral line data to infer the multiparsec-scale physical conditions in the Orion A and B molecular clouds, using 140mic/240mic dust color temperatures and the 240mic/13CO J=1-0 intensity ratios. Two-component models fit the Orion data best. The models require that the dust-gas temperature difference is 0+/-2 K. If this surprising result applies to much of the Galactic ISM, except in unusual regions such as the Galactic Center, then there are a number implications. These include dust-gas thermal coupling that is commonly factors of 5 to 10 stronger than previously believed and an improved explanation for the N(H2)/I(CO) conversion factor or X_factor. This improved formulation for the X-factor quantifies the statement that the velocity-integrated radiation temperature of the 12CO J=1-0 line, I(CO), "counts" optically thick clumps using the formalism of Martin et al. (1984). One important suggestion of this formulation is that virialization of entire clouds is irrelevant. The densities required to give reasonable values of the X-factor are consistent with those found in cloud clumps (i.e. roughly 10^3 H_2 cm^-3). Thus virialization of clumps, rather than of entire clouds, is consistent with the observed values of the X-factor. And even virialization of clumps is not strictly required; only a relationship between clump velocity width and column density similar to that of virialization can still yield reasonable values of the X-factor. The underlying physics is now at the scale of cloud clumps, implying that the X-factor can probe sub-cloud structure. While this formulation improves upon that of Dickman et al. (1986), it has shortcomings of its own. These include uncertainties in defining the average clump density and neglecting certain complications, such as non-LTE effects, magnetic fields, turbulence, etc. Host: Peter Martin

Last modified: 21 Oct 2008, 14:11:23

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