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Fall 2011

Sep 16 Dr. Sijime-Jan Paardekooper (University of Cambridge)
Migration of rocks, planets and hurricanes in protoplanetary discs
Planets form in discs of gas and dust around young stars. Since the discovery of the first extrasolar planet around a solar type star, a gas giant planet on an orbit of just a few days, it has become apparent that orbital migration should play a major role during planet formation. While planet migration is able to explain this new class of planets, the Hot Jupiters, theoretical migration rates were alarmingly fast, essentially predicting that *all* gas giant planets should be Hot Jupiters. I will discuss the latest insights in planet migration,and show that recent progress allows for slower migration rates. At the same time, new types of migration keep showing up, indicating that all planetary building blocks, from the very small to the very large, are very mobile inside protoplanetary discs.
Host: Charles Dyer
Sep 20
Tu, 3PM		 Dr. Hilke Schlichting (UCLA)
What the Solar System tells us about Planet Formation
I will discuss recent insights that we have gained into planet formation form our solar system. In the first half of my talk, I will focus on the Kuiper belt, located at the outskirts of our planetary system, which provides a snapshot of earlier stages of planet formation and is therefore an ideal laboratory for testing planet formation theories. In the second half of my talk, I will review geochemical constraints from the Earth, Moon and Mars and discuss their implications for the last stage of terrestrial planet formation.
Host: Charles Dyer
Sep 23 Dr. Jason Kalirai (Space Telescope Science Institute)
A Deep, Wide-Field, Panchromatic Survey of 47 Tuc and the SMC
Nearby resolved stellar populations such as Galactic star clusters anchor our understanding of the Universe. Clusters are ideal testbeds to advance our knowledge of stellar evolution and structure, and provide a calibration of astrophysical relations that aim to interpret light across the Universe. Yet, over the past century of work, most of this effort has focused on visible light investigations given the lack of a high-resolution, sensitive, and wide-field infrared facility. This talk will highlight the first results from a new HST imaging survey of the best studied globular star cluster in the sky, 47 Tuc. The survey strategy uses 121 orbits with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) and both the UVIS and IR channels of the newly installed Wide Field Camera 3 (WFC3) instrument. The full analysis of the data set reveals unprecedented panchromatic color-magnitude diagrams exhibiting complete stellar sequences that extend from the hydrogen burning limit, up to the brightest giants, and down to the coolest remnants in the cluster. These observations will enable a new level of calibration for fundamental astrophysical relations. The talk will also describe the unchartered discovery space that JWST will explore through ultra-deep IR imaging and MOS spectroscopy of nearby resolved stellar populations.
Sep 27
Tu, 3PM		 Dr Andrew Youdin (CfA)
The Exoplanet Census: Using Surveys to Better Understand Planet Formation
We are now in the golden age of exoplanet discoveries. After a decade that delivered more than 500 exoplanets, the Kepler transit survey is on pace to discover 2000 planets. This wealth of planets offers unprecedented opportunities to statistically characterize the planet population and refine planet formation theories. I will describe a general method to determine the planet distribution function from exoplanet survey data. This method accounts for selection effects and the existence of multiple planets per system. Applying the technique to Kepler data reveals interesting trends in the size distribution of planets versus orbital period. I will describe the implications of these trends for the accretion and retention of atmospheres on Earth-sized planets.
Host: Charles Dyer
Sep 30 Andrei Beloborodov (Columbia)
Physics of Gamma-Ray Bursts
Gamma-Ray Bursts (GRBs) are emitted by ultra-relativistic jets from compact objects. The jets may be strongly magnetized outflows driven by rotation of the central object. Alternatively, the jets may be driven by thermal pressure due to neutrino heating around the compact object. Both scenarios can supply the observed luminosities, but only under extreme conditions that will be discussed. A long-debated problem is how the jet emits the observed GRB radiation. A simplest model resembles the big bang: the opaque hot plasma expands to transparency and releases its thermal photons. The expected spectrum from a radiation-dominated jet is Planckian with a peak around 1 MeV. The observed GRB spectra do peak around 1 MeV, however they have non-Plankian shapes, with extended high-energy tails. Physical processes generating non-Plankian radiation will be discussed. The popular phenomenological model of synchrotron emission from internal shocks has serious problems — it contradicts both shock physics and observations. I will describe a different emission mechanism: internal motions in the neutron-proton jet generate electron heat via nuclear and Coulomb collisions, and electrons radiate the received energy. This mechanism invokes no phenomenological parameters, and the produced radiation is calculated from first principles. Remarkably, the predicted spectra agree with observations.
Oct 7 Renbin Yan (NYU)
The Puzzle of LINERs and the Warm Ionized Gas in Early-type Galaxies
Ever since their discovery, the nature of low ionization nuclear emission-line regions (LINERs) has been hotly debated. Some authors treat them as AGNs,others argue they are not AGNs but powered by shocks or hot old stars.No universal agreement has been reached. On the other hand, early-type galaxies frequently contain spatially extended warm ionized gas and have spectra similar to LINERs. How is this large-scale emission related to the nuclear LINERs? Because LINER is the most common spectral type found in massive galaxies in both nuclear and integrated spectra, understanding its nature is important to numerous topics in astrophysics. In this talk, you will hear a story of how I converted from a supporter forAGN-powering of LINER line emission to an opponent, and what kind ofconvincing evidence made me convert. I will also discuss what we can learn about the warm ionized gas in early-type galaxies from this line emission, given that it is not an AGN indicator.
Oct 14 Linda French (Wesleyan)
Oct 18
Tue		 Stuart Clark
Oct 25
Tue, 3PM		 Ben Zuckerman (UCLA)
Nov 11 Crystal Brogan (NRAO)
Nov 18 Andreas Eckart (U. Köln)
Nov 25 Matthew Johnson (Perimeter)
Last modified: 23 Sep 2011, 15:31:02

Mike Reid (mreid@astro) / Bob Abraham (abraham@astro) / Mariangela Bonavita (bonavita@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