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First half of 2006

Jan 11
We.3PM
MP 137!		 Prof. Tim Slater (Arizona)
Are You Really Teaching Astronomy IF No One is Actually Learning?
The repeated call for a more student-centered approach to teaching due to the ineffectiveness of lecture has been gaining prominence in the astronomy teaching community. To study the large-enrollment introductory astronomy survey course, we found that pretest scores of 30% were only increased to 52% due to lecture alone. Alternatively, when using curriculum materials created from a basis of astronomy education research, we find that the posttest average score grows beyond 70%. Each 15-minute Lecture-Tutorial poses a carefully crafted sequence of conceptually challenging, Socratic-dialogue driven questions, along with graphs and data tables, all designed to encourage students to reason critically about difficult concepts in astronomy previously identified through systematic research. Quantitative and qualitative results strongly suggest that the Lecture-Tutorials help students make significant conceptual gains.
Host: John Percy
Jan 13 Dr Suzanne Aigrain (IoA)
The Monitor project: Transits and rotation in young open clusters
We have started an intensive photometric monitoring program of a dozen nearby star forming regions and young open clusters aged 1-200 Myr. Our primary goal is to search for eclipses by very low mass stars, brown dwarfs and planets around low mass members of the clusters. Combined with radial velocity follow-up, the resulting dynamical mass and radius measurements will provide unprecedented constraints on the formation and early evolution of these objects. Our data is also ideally suited to searching for rotation periods in cluster members down to the brown dwarf regime over much of their pre-main sequence lifetime. I will discuss the motivation behind and design of the survey, and present early results, including a membership and rotation study of M34 (200 Myr) and a discussion of the first eclipse candidates.
Host: Ray Jayawardhana
Jan 20 Prof. Mike Brown (Caltech)
Sedna and the birth of the solar system
The recent discovery of Sedna, in a distant eccentric orbit well beyond the outer edge of the Kuiper belt, is difficult to explain in the currently understood solar system. Sedna could not have formed in its current orbit, nor could it have been perturbed to its current orbit by any of the planets in the solar system. I'll discuss possible formation scenarios and suggest that Sedna is a fossil record that we will eventually be able to read to tell us about the earliest history of the solar system.
Host: ASX & Yanqin Wu
Jan 27 Prof. Matt Bershady (Univ. of Wisconsin/UofT)
Progenitors of Dwarf Elliptical Galaxies
Luminous compact blue galaxies (LCBG) selected to be photometrically homogenous between 0.3<z<1 have diverse morphology. Yet they have consistently narrow kinematic line-widths and ``enthusiastic'' levels of star-formation. They are perhaps a bridge population in mass and star-forming intensity between Lyman-break galaxies at z>3 and today's more quiescent systems. The most extreme LCBG are dynamically hot, unrelaxed, and truly compact. We have recently found these sources appear in abundance in both the field and clusters, but are absent from cluster cores. We marshal this information to evaluate contested claims for what these sources evolve into today.
Feb 3 Prof. Norm Murray (CITA)
Radiation Pressure, Starburst Disks, and AGN Fueling
Radiation pressure is dynamically important in the inner regions of starburst galaxies and ultraluminous infrared galaxies (ULIRGs). This insight leads to a simple Eddington-like argument which shows that a galaxy with a velocity dispersion σ has a maximum luminosity L~4fgσ4c/G, where fg is the fraction of the galaxy's mass in the form of gas. The luminosity might come either from stars or a central black hole. This strongly suggests that radiation pressure is involved in both the Faber-Jackson relation (linking the luminosity of an elliptical galaxy to its velocity dispersion) and the M-σ relation. I will argue that radiation pressure is also the key missing ingredient in models of black hole feeding on scales of a few tenths to a few tens of parsecs. The radiation is supplied by star formation in marginally Toomre-stable disks. Simple disk models can explain many of the features of starbursts, ULIRGs, Seyferts, and quasars. I will describe some observational tests of these disk models on scales of 0.1-1 parsec in nearby Seyfert II galaxies, using water masers. Adaptive optics in the near infrared offer the possibility of testing the models on somewhat larger scales.
Feb 17 Dr Gregg Wade (Royal Military College)
What is the origin of magnetism in intermediate-mass and high-mass stars?
Some main sequence A, B and O stars host strong, globally-ordered surface magnetic fields. The physical impact of these fields is clear: atmospheric structure, photospheric chemical abundances, mass loss, rotation, and ultimately stellar evolution are all modified, to various extents, due to the interaction of the magnetic field with the stellar plasma. Furthermore, because the ultimate descendants of stars in this mass range are both white dwarfs and neutron stars, their magnetic fields may provide significant clues toward understanding the magnetic characteristics of stellar remnants. Remarkably, the origin of magnetism in intermediate-mass and high-mass stars remains a total mystery. In order to trace the presence of these fields back to the earliest observable evolutionary stages, we have undertaken an extensive search for magnetic fields in the Herbig Ae/Be (HAeBe) stars, the pre-main sequence progenitors of the main sequence intermediate-mass stars. In this talk, I will review the details of our investigation and discuss our results.
Host: Tom Bolton
Feb 24 Dr Dawn Erb (CfA)
Mass, Metallicity, Kinematics and Star Formation in Galaxies at z~2
The redshift interval 1.5<z<3 hosted a large fraction of the assembly of stellar mass in the universe and the peak of AGN activity, but the quantitative study of galaxies in this redshift range from large spectroscopic samples has only recently become feasible. I will discuss the results of a large H-alpha survey of star-forming galaxies at z~2, including kinematic properties inferred from line widths and spatially resolved emission, star formation rates, evidence for galactic-scale outflows, and the comparison of stellar, dynamical and inferred gas masses. The galaxies show a strong correlation between stellar mass and gas phase metallicity (the first detection of the mass-metallicity relation at high redshift), which is best explained by a model in which galaxies of all masses lose gas to outflows at a rate higher than the star formation rate.
Host: Norm Murray
Mar 3 Prof. Scott Kenyon (CfA)
Formation and Evolution of Planets and Debris Disks
In the solar neighborhood, there are now several hundred stars with a debris disk of dusty material with a size comparable to the Kuiper Belt in the Solar System. In this talk, I will show how theoretical calculations of planet formation naturally produce dusty disks of debris with properties similar to those observed in known debris disks (including the Kuiper Belt). I will also describe several theoretical predictions of the calculations and show how current/future facilities can test these predictions.
Host: Ray Jayawardhana
Mar 10 Dr Beate Stelzer (Osservatorio Astronomico di Palermo)
X-rays probing the limits of stellar dynamos: from hot stars to brown dwarfs
Magnetic activity, manifest in chromospheric and coronal emission, is an ubiquituous phenomenon on late-type stars. The observational phenomena, most prominently Halpha and X-ray emission, are believed to trace back to a dynamo that operates in the interior of the star. In solar-like stars strong fields are produced in the overshoot layer between radiative core and convective envelope. Radiative stars of high and intermediate mass lack such a zone, and the solar-stellar connection breaks down. Winds or cool companion stars have been held responsible for the observed activity. Similarly, fully convective very low-mass stars and brown dwarfs, if magnetically active must be driven by a type of dynamo which differs from the solar case.
X-ray emission is an observational tool to examine the nature of activity in these critical regimes. I discuss recent observations with Chandra and XMM-Newton from stars of different mass and age.
Host: Ray Jayawardhana
Mar 17 Prof. Janna Levin (Barnard College)
Black hole pairs, chaos, and gravitational waves
We are on the verge of a truly remarkable observational possibility: the direct detection of black holes. Or at least as direct as is theoretically possible. We won't use our eyes, telescopes, or any other detector of electromagnetic radiation. Instead, we hope to be able to observe black holes through gravitational waves. Yet the dynamics of realistic, astrophysical black hole pairs is surprisingly not well understood, difficult to solve, and possibly even chaotic. Understanding the orbits of black hole pairs is crucial to the current and future gravitational wave observatories. With the success of these observatories we will have a new portrait of black holes and of the sky.
Host: Marten van Kerkwijk
Mar 24 Prof. Eugene Chiang (Berkeley)
Rings in Resonance: The Kuiper Belt and Beyond
The music of the spheres can be heard in resonant systems. The fixed gaze of the Moon, the commensurate orbital periods of the Galilean satellites, and the arms of grand-design spiral galaxies testify to how dynamical resonances organize the universe. We discuss how resonances have ordered our view of one of the newest frontiers of planetary science: the Edgeworth-Kuiper Belt, that great expanse extending beyond the orbit of the last known planet in our solar system. This space is strewn with icy, rocky bodies of which Pluto is merely one (and not the largest) member and which occupy orbits of a formerly unimagined variety. We identify newly discovered populations of resonant objects in the Kuiper belt, including the new class of Neptune Trojans; describe how these objects inform our understanding of planetary migration and planetary accretion; and highlight unsolved problems. Connections are drawn between the Kuiper belt and extra-solar debris disks which afford glimpses of planetesimals at ages of 10-100 Myr. As a coda, we recount the mystery of the famously puzzling 1-Myr-old T Tauri star, KH 15D, and how its resolution provides yet another example of the power of resonance.
Host: Yoram Lithwick
Mar 31 Prof. Philipp Podsiadlowski (Oxford)
Pulsar Kicks and the Formation of Compact Objects
It has long been realized that most neutron stars receive a substantial kick when they are born due to an asymmetry either in the supernova mass loss or the explosion mechanism itself, but the details of the kick mechanism are still poorly understood. Supernova kicks strongly affect the properties of the resulting X-ray binaries (their orbital properties as well as their space distributions) and the birthrates of compact binaries which are potential progenitors of gamma-ray bursts and gravitational-wave sources. In this talk I will emphasize the implications of observed kicks for models of core collapse and present recent empirical evidence and theoretical support for a dichotomous kick distribution, where neutron stars born in close binaries receive relatively small kicks. Implications for binary pulsars, including the double pulsar, will also be discussed.
Host: Natasha Ivanova
Apr 7 Prof. Frits Paerels (Columbia Univ.)
X-ray Spectroscopy of the Intergalactic Medium
With the advent of high resolution X-ray spectroscopy, provided by the diffraction grating spectrometers on Chandra and XMM-Newton, we can now search for the putative highly ionized phase of the IGM that may contain up to half of all the baryons at the current epoch. Searches for X-ray resonance absorption lines have been performed, on lines of sight to multiple bright extragalactic continuum sources. I will describe the results of this work, the prime results of which are:
  1. The positive detection of a large amount of highly ionized gas around the Milky Way galaxy (the intragroup medium of the Local Group?); and
  2. A lack of absorption by more distant gas.
I'll put these results into context, and briefly discuss the prospects for detection of X-ray line emission from the IGM.
Host: Marten van Kerkwijk
Apr 14 No colloquium: Good Friday
Apr 21 Prof. Hsiao-Wen Chen (Chicago)
GRB Afterglows as a New Probe of the ISM and IGM at High Redshift
Spectroscopic observations of Gamma-ray Burst (GRB) afterglows can serve as a probe of the chemical and dust content in high redshift galaxies. Because of the transient nature of GRB afterglows, this investigation is complementary to traditional quasar studies and is expected to uncover a large sample of intervening damped Lyman-alpha absorbers for follow-up surveys of the absorbing galaxies. We have been conducting a rapid spectroscopic follow-up of well-localized GRB afterglows on a number of ground-based facilities. I will discuss the results we have learned so far.
Host: Ray Jayawardhana
Apr 28 Prof. Susana Lizano (Univ. Nac. Aut. de Mexico)
Dissipation of the magnetic field during the gravitational collapse to form stars
Magnetic fields are observed in molecular clouds, the cradles of low mass stars. Their strengths are large enough to support the clouds against self-gravity and they are difficult to get rid of. It is known that the magnetic flux must be annihilated at some point during the gravitational collapse to form stars or the compressed stellar magnetic fields would be too large. Here I will discuss another obstacle presented by magnetic fields to the current view low mass star formation: magnetic braking by the strong compressed fields will also prevent the formation of centrifugally supported disks. I will also discuss an illustrative model of Ohmic dissipation at very high densities during the gravitational collapse which show that this process is able to destroy enough magnetic flux to solve both problems.
Host: Chris Matzner
May 5 Prof. Rachid Sunyaev (MPA & Space Res. Inst., Moscow)
The sky in the hard X-Rays.
This talk is based on the recent results from INTEGRAL spacecraft and includes following topics:
  1. The extremely bright and narrow electron-positron annihilation line from the central parts of our Galaxy gives information about the properties of interstellar matter.
  2. The central supermassive black hole of our Galaxy SGR A* was a million times brighter 400 years ago: Using the SGR B2 molecular cloud as a natural mirror.
  3. Heavily obscured High Mass X-Ray binaries in the Galactic plane.
  4. The population of the heavily obscured local Seyfert galaxies discovered during the INTEGRAL shallow sky survey
  5. Eclipse of the bright X-Ray sky by the dark Earth. The spectrum of Cosmic X-Ray Background Radiation measured during observations of the Earth by INTEGRAL. The growth of supermassive black holes in AGNs and the intensity of X-ray background.

Host: CITA/Sackler
May 12 Dr Debra Shepherd (NRAO)
Links between star and planet formation
Beginning with the formation of solar type stars, I will review our current understanding of the link between star and planet formation. This relatively new field of study poses intriguing questions about the formation of planets and ultimately the formation of life. Visible and near-infrared images of star forming regions show dark molecular cloud boundaries disrupted by spectacular explosions that occur during the formation process. Images made at centimeter and millimeter wavelengths penetrate into the cloud core where the star formation process begins. These observations, combined with theoretical models, have shown that more massive protostars (>10 solar masses) may have accretion disks with significantly increased turbulence and higher temperature. Will these extreme conditions limit the ability of massive stars to form planetary systems? Further, most stars, including our sun, are created in massive star forming clusters. The proximity of a solar type star to the turbulence and radiation field created by a more luminous(proto)star may influence the evolution of a proto-planetary disk into a stable, Keplerian planetary system.
Host: Marten van Kerkwijk
May 19 No colloquium: Too many people away

Close galaxy pairs provide a useful probe of the frequency and nature of galaxy mergers, and the role of mergers in the evolution of galaxies. 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 use the Millennium Run LCDM N-body simulation (Springel et al. 2005) to carry out a census of close galaxy pairs at redshift zero, identifying pairs in both real space and redshift space. We then perform a direct comparison with a well-defined sample of spectroscopic galaxy pairs drawn from the Sloan Digital Sky Survey.
May 26 Dr Subu Mohanty (CfA)
A Disk around the Planetary Mass Companion to a Brown Dwarf
The recent discovery of 2MASS J1207B, an apparently planetary mass object orbiting a nearby young brown dwarf, has been immensely exciting. As the first directly imaged planetary mass companion, and the first such body discovered around a brown dwarf, 2MASS J1207B offers crucial insights into the formation and properties of planetary mass objects, binarity in brown dwarfs, and star and brown dwarf formation in general. We present here new infrared photometry and spectroscopy of this remarkable object. Our data reveal that 2MASS J1207B is in fact hotter and somewhat more massive than estimated previously (though still in the plantetary mass regime). Most importantly, it shows strong evidence of a circum-planetary edge-on disk. I will discuss the analysis leading to these conclusions, and the attendant implications for both the origin of brown dwarfs and the formation and properties of planetary mass objects around them.
Host: Ray Jayawardhana
Last modified: 15 Aug 2006, 16:30:12
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