U of T logo ASTRONOMY COLLOQUIA DAA logo

Second half of 2005

Sep 16 Prof. Deepto Chakrabarty (MIT)
The Spin and Magnetic Evolution of Old Neutron Stars: A Rotational Speed Limit for Millisecond Pulsars
Pulsars are magnetized, rotating neutron stars formed in the supernova collapse of massive stars at the end of their nuclear-burning lifetime. Old pulsars can be ``spun up'' to millisecond periods by accretion torques exerted by a binary companion star. These millisecond pulsars are spinnning near their centrifugal breakup limit, with surface velocities over 10 percent the speed of light. However, recent X-ray measurements show that the pulsar spin frequency distribution cuts off sharply at the fast end, well before the predicted centrigual break-up limit is reached. Although the braking mechanism that prevents further spin-up is not yet known, these data support theoretical sugggestions that gravitational radiation losses may be responsible. These objects may thus be interesting targets for gravitational-wave interferometers.
Host: Marten van Kerkwijk
Sep 23 Prof. Rocky Kolb (Fermi Lab & Univ. of Chicago)
Thoughts on Dark Matter, Dark Energy, Inflation
Remarkable observational results and bold theoretical ideas have resulted in a standard cosmological model. This model, the `Lambda-Cold-Dark-Matter' model, seems capable of precision predictions of many cosmological observables. However the model posits the existence of dark matter, dark energy, and an early inflationary period. We know nothing about the nature of these three ingredients.
Host: Ray Carlberg
Sep 30 Prof. David Helfand (Columbia Univ.)
The Youngest Neutron Stars
Supernova rates and pulsar population statistics suggest that there should be about 10 neutron stars in the Milky Way younger than the one in the Crab Nebula and about 100 less than 10,000 years old. Such young objects are crucial for determining the intitial distributions of rotation rate, magnetic field strength, and velocity which constrain supernova explosion scenarios, while their temperatures provide a window on the behavior of matter at supranuclear densities. Prior to the launch of Chandra and XMM, we knew of fewer than half a dozen such objects. I will describe progress in this area over the past five years using data from these X-ray satellites as well as our new survey of the Galactic plane with the Very Large Array which promises to reveal some of the youngest pulsars in the Galaxy.
Host: Marten van Kerkwijk
Oct 7 Prof. Stan Owocki (Barthol Research Inst., Univ. of Delaware)
Winds that Sail on Starlight
We normally think of light as a conduit of energy or information. But light also carries momentum, and in the brightest objects in the universe, matter that scatters light can actually be propelled completely away by this "light pressure", forming an outflowing, supersonic "wind". This talk will use everyday analogies like sailing to explain the principles and sometimes puzzling properties of such light-driven winds, and discuss the many important consequences for the evolution of stars, the interstellar medium, galaxies, quasars, and even the early evolution of the universe itself.
Host: Tom Bolton
Oct 14 Prof. Debra Fischer (San Fransisco State Univ.)
On the Frontiers of Planet Hunting
In the past 10 years, more than 150 extrasolar planets have been discovered by Doppler techniques. Most of these planets are gas giants, similar to Jupiter or Saturn in our solar system and were detected with 3 m/s precision Doppler observations. However, our current development efforts are aimed at sub-meter-per-second Doppler precision, needed to detect lower amplitude systems and rocky planets. I will discuss the standing of these new development efforts and review a recent and definitive analysis of the planet-metallicity correlation. Exploiting this correlation, the N2K consortium was established one year ago, and some surprises and trends from this search for hot jupiters will be presented.
Host: Norm Murray
Oct 21 Dr Dirk Froebrich (Dublin Inst. for Advanced Studies)
Star Formation in small clouds
Stars form not only in large molecular clouds but also in small isolated globules. These objects give us the unique opportunity to investigate the star formation process in a simple (laboratory like) environment and in great detail. In this talk I will summarise our investigations of the small cloud IC 1396W that is known to actively form new stars. The striking feature of this cloud is that it harbours three parallel outflows, a rare situation in star formation. The cause for this conundrum might be found in a well ordered magnetic or angular momentum field in this globule. The talk will review our recent work on this object, including all techniques applied so far to build up an as complete as possible picture of how the stars form in IC 1396W (near infrared imaging, extinction mapping, low-resolution and echelle spectroscopy).
Host: Aleks Scholz
Oct 28 Prof. Roman Rafikov (CITA)
Origin of Giant Planets
Recent numerous discoveries of extrasolar giant planets have brough into focus the question of their origin. At present there two competing theories of gian planet formation - core (nucleated) instability and gravitational instability. The former has much wider acceptance as it naturally explains the presence of dense cores in some giant planets and their enrichment in refractory materials. For a long time it has been thought though that planet formation via this avenue requires exceedingly long time. To address this issue a gravitational instability in a massive protoplanetary disk has been invoked as a means of forming planets directly. I will critically review these two ideas, emphasizing observational evidence of different kinds. I will also demonstrate that giant planets can hardly form by the gravitational instability at distances of less than tens of AU from their parent stars, leaving core instability as a much better candidate for the giant planet formation mechanism in the Solar System and the extrasolar systems discovered through the radial velocity searches.
Nov 4 Prof. Bill Herbst (Wesleyan Univ.)
Rotation of Low-Mass Pre-main Sequence Stars
Photometric monitoring programs employing wide-field imagers have now measured rotation periods for more than 1500 pre-main sequence stars in young clusters such as Orion, NGC 2264 and IC 348. The data allow us to address the mass dependence of rotation and to build an empirical rotation evolution model for solar-like stars from the PMS to the ZAMS. The important role of an accretion disk in influencing rotation will be demonstrated and there will be discussion of the possible influence of environment on rotational properties of stars. Some implications of these results for the time scale of giant-planet formation and our understanding of X-ray production in pre-main sequence stars will be touched upon. Time permitting I will also describe a few very odd stars we have stumbled upon and discuss why they are important objects to pursue.
Host: John Percy
Nov 11 Prof. Martha Haynes (Cornell)
ALFALFA: The Arecibo Legacy Fast ALFA Survey
The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map some 7000 square degrees of the high galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range from -1600 to +18000 km/s with 5 km/s resolution. Exploiting Arecibo's large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the HI mass function in the local universe and will provide a census of HI in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local and nearby superclusters, will allow measurement of the HI diameter function, and will enable a first wide-area blind search for local H I tidal features, H I absorbers at z<0.06 and OH megamasers in the redshift range 0.16<z<0.25. Expected to take five years to complete, ALFALFA will detect on the order of 20,000 extragalactic HI line sources out to z~0.06. During only 35 hours of precursor observations in 2004, we detected three objects with HI masses of less than 10 million solar masses. The full ALFALFA survey should detect 100 times as many.
Host: Ue-Li Pen
Nov 25 Prof. Julio Navarro (Univ. of Victoria)
Galaxy Formation, and the Formation of the Galaxy
The Cold Dark Matter paradigm of structure formation has proved remarkably successful at accounting for the large-sale properties of the Universe, as measured by the clustering and motions of galaxies, as well as by the detailed structure of the temperature fluctuations in the Cosmic Microwave Background. Many of its "natural" predictions regarding the hierarchical assembly of galaxy systems, however, appear at odds with observational data on the dynamics of individual galaxies. I will examine the constraints that these data place on the small scale structure properties of a CDM-dominated universe. I will argue that current observational constraints are not inconsistent with the predictions of the CDM paradigm; rather, they highlight the importance of astrophysical processes in shaping the appearance of individual galaxies embedded in an evolving population of dark matter halos.
Host: Bob Abraham
Dec 2 Dr Roeland van der Marel (STScI)
Structure and Kinematics of the Large Magellanic Cloud
I will review our understanding of the structure and kinematics of the Large Magellanic Cloud (LMC), with a particular focus on recent results. This is an important topic, given the status of the LMC as a benchmark for studies of microlensing, tidal interactions, stellar populations, and the extragalactic distance scale. I will address issues such as the observed morphology and kinematics of the LMC; the angles under which we view the LMC disk; its in-plane and vertical structure; the LMC self-lensing contribution to the total microlensing optical depth; the LMC orbit around the Milky Way; and the origin and interpretation of the Magellanic Stream.
Host: Howard Yee
Dec 9 Prof. Charles Gammie (UIUC)
Numerical Models of Black Hole Accretion Flows
Black hole accretion flows probably power active galactic nuclei and galactic microquasars, and may power the gamma-ray bursts. It is now possible to study some aspects of these flows numerically, by evolving first-principles equations that fully include the effects of electromagnetic fields and general relativity. I will describe recent models, with a focus on the "Blandford-Znajek" effect and implications for jet production.
Host: Marten van Kerkwijk
Dec 16 Prof. Shantanu Basu (UWO)
From Clouds to Cores to Protostars and Disks: New Insights from Numerical Simulations
Stars form when interstellar molecular clouds, which are turbulent and magnetized, form dense cores that collapse to form protostars and disks. A star is then built up by accretion from the disk and parent core envelope. Our numerical simulations of various stages of this process reveal several new features of turbulence, fragmentation, and disk accretion. We find that turbulent motions, even when driven from local sources, can quickly fill the cloud with motions in which most of the power is present on the largest scales. The densest regions of the clouds contain subsonic or transsonic turbulent motions, and here the core formation process may be driven by gravity. Magnetic fields strengths consistent with a near critical mass-to-flux ratio can enforce a unique fragmentation process that can be identified with the isolated mode of star formation. Nonaxisymmetric cores are formed and ultimately collapse to form a protostar and disk. The mass accumulation of the star starts in a well-known "smooth mode" of accretion, but then proceeds through a newly discovered "burst mode" that can explain the FU Ori phenomenon.
Host: Peter Martin
Last modified: 19 Dec 2005, 12:30:00
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