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Fall 2009

Sep 18 Prof. Lee Ann Willson (Iowa State University)
Mass Loss and Red Giants, Prof. Lee Ann Willson (Iowa State University)
There are dozens of mass loss formulae in use or proposed for use in stellar evolution calculations, particularly for the heavy mass loss epoch that occurs near the tip of the AGB and perhaps along the RGB. How can we choose the one(s) that are most appropriate? Observations and theory both give some guidance, but recent observations also challenge our understanding of mass loss from evolved stars. Why do M, S, and C type AGB stars have similar winds, when dust is suspected of playing a key role and they should have very different chemistry? Why do some, but not all, red giants in clusters have moderately high mass loss rates? Why do the cluster stars with mass loss appear to satisfy (1/M) dM/dt ≈ (1/L) dL/dt over a wide range in luminosity? These puzzles challenge our understanding of how and when stars lose mass.
Sep 25 Prof. David Charbonneau (Harvard-Smithsonian Center for Astrophysics)
Detecting Habitable Exoplanets: The Small Star Opportunity
When exoplanets are observed to transit their parent stars, we are granted direct estimates of their masses and radii, and we can undertake studies of their atmospheres. Such systems have profoundly impacted our understanding of giant exoplanets akin to Jupiter or Neptune, yet no definitive examples of transiting rocky exoplanets have yet been found. By targeting nearby low-mass stars, a transit search using modest equipment is capable of discovering planets as small as 2 Earth radii in their stellar habitable zones. The discovery of such planets would provide fundamental constraints on the physical structure of planets that are primarily rock and ice in composition. Moreover, by differencing spectra gathered when the planet is in view from those when it is occulted by the star, we can study the atmospheric chemistry of potentially habitable worlds.
Oct 09 Dr. Pauline Barmby (University of Western Ontario)
Young star clusters in Andromeda
The Andromeda galaxy contains a number of luminous blue clusters which were previously confused with old, metal-poor globulars. We have used Hubble Space Telescope imaging of these objects to determine their ages, masses, and structural properties. The Andromeda clusters have sizes and masses comparable to those of similar-age clusters in the Magellanic Clouds. Massive young clusters in M31, the Clouds, and the Galaxy all trace approximately the same age-size relation. The young Andromeda clusters are expected to dissolve within a few Gyr and will not survive to become old, globular clusters. However, they do appear to follow the same fundamental plane relations as old clusters; if confirmed with velocity dispersion measurements, this would be a strong indication that the star cluster fundamental plane reflects universal cluster formation conditions.
Oct 16 Prof. Mike Reid (U of T)
Clump Mass Functions in the Age of Herschel and SCUBA2
In principle, the distribution of the masses of pre-stellar clumps in a star-forming region should be well correlated with the mass function of the stars formed from those clumps/cores. The relationship between the mass functions of clumps/cores and stars can help us distinguish among different theories of star formation. Measurements of the clump mass function to date have shown that, in fact, it does agree well with the stellar initial mass function. However, I believe that the clump mass function is much more difficult to interpret than it seems and that the results to date are likely misleading. In this talk, I will summarize the existing measurements of the clump mass function and demonstrate, using analysis of the existing data as well as new simulations, why I believe them to be unreliable. I will then suggest how the clump mass functions produced from Herschel and SCUBA2 data can be more reliably interpreted.
Oct 23 Dr. Nick Law (U of T)
Cool Things Around Cool Stars
Cool dwarfs are the most common stars in our galaxy. Yet, despite their prevalence, the planetary population around cool dwarfs is essentially unknown, and there is still uncertainty about their substellar and stellar companion populations. I will discuss new ground-based techniques developed to comprehensively search cool stars for planets and substellar companions. These techniques range from tens-of-microarcseconds astrometry and high-contrast adaptive optics imaging, to infrared radial velocity surveys and an M-dwarf transit survey that covers tens of thousands of cool stars simultaneously.
Oct 30 Prof. Tommaso Treu (University of California, Santa Barbara)
Dark matter in galaxies and clusters of galaxies
The standard cold dark matter cosmological model successfully reproduces the properties of the universe on supergalactic scales. However, it is unclear whether it can reproduce the detailed properties of galaxies and clusters themselves. Understanding the relative distribution of dark matter and baryons at these scales, and its evolution with cosmic time, is a key test of the paradigm. I will present new observational results on this topic and discuss the overall implications for galaxy and cluster formation.
Nov 06 Dr. Paul Ho (Academia Sinica Institute of Astronomy and Astrophysics)
Karl Kamper Memorial Lecture
Preparing for ALMA First Science The Submillimeter Array has been in operations on Mauna Kea since 2004. Many interesting and important results have been obtained in that time including planetary studies, dusty circumstellar disks, extremely collimated molecular outflows, circumnuclear disks in nearby galaxies, magnetic fields via dust polarization studies, and dark submillimeter galaxies at high red shifts. These studies are paving the say for the first science projects to be attempted on ALMA, currently under construction in the Atacama Desert. I will show some of our latest results.
Nov 20 Dr. Alicia M. Soderberg (Harvard-Smithsonian CfA)
A Holistic View of Catastrophic Cosmic Explosions
Throughout history, observational supernova (SN) studies have focused almost exclusively on their strong optical emission that dominates the bolometric luminosity. Yet many of the leading breakthroughs in our understanding of SNe and their progenitor systems have been enabled by observations at other wavelengths. For example, through the combination of radio and gamma-ray observations, we now know that about 0.2 percent of all core-collapse have the ability to power relativistic gamma-ray burst jets (GRBs). With the recent serendipitous discovery of shock breakout X-ray emission from an ordinary SN, comes the observational realization of a superior discovery technique for local core-collapse SNe from which data encode the properties of the progenitor star in the moments leading up to explosion. Finally, as thermonuclear Type Ia SN studies are pushed toward higher redshift and trusted to constrain the expansion history of the Universe, the favored single-degenerate progenitor model is increasingly called into question by sensitive radio and X-ray observations that have yet to reveal any evidence for a non-degenerate companion star. Theoretical considerations suggest that progenitor mass, metallicity, angular momentum and binary interaction all play a role in the observed properties of GRBs and SNe across the electromagnetic spectrum. I will discuss a fresh approach to bridging observations and theory of cosmic explosions with an ultimate goal to shed light on the nature of their progenitors, the physics of their explosions, and a better understanding of their utility as high redshift probes.
Nov 27 Prof. Marcin Sawicki (Saint Mary's)
The Lives of the Working Poor: Low-mass, Low-SFR Galaxies at z>2
Observational studies of galaxies in the early universe teach us about how present-day galaxies formed and allow us to test theories of cosmic structure formation. To date, such observational efforts have largely focused on the easier-to-study - but relatively rare - luminous objects that form stars at the very high rates of 10s of 100s of solar masses per year. In contrast, their fainter, less active cousins have thus far been largely ignored. But we ignore them at our peril! Although individually faint, these fainter objects are very numerous and so, collectively, are extremely important contributors to the production of stars and chemical elements in the cosmos. Drawing on recent work, I will describe our current understanding of what these numerous, faint objects are, how they differ from their better-studied, luminous cousins, and what they teach us about how galaxies form.
Dec 04 Dr. David Gilbank (University of Waterloo)
Cosmic star-formation as a function of stellar mass since z~1
Motivated by recent suggestions of `cosmic downsizing' -- a shift towards lower stellar mass galaxies with increasing cosmic time for the location where the bulk of cosmic star-formation is taking place -- we have recently completed a survey aimed at directly exploring for the first time the star-formation activity in low mass galaxies (8.5< log Ms <9.5) at z~1. I will present results from ROLES (the Redshift One LDSS-3 Emission line Survey) which, combined with other higher mass surveys, has measured the cosmic star-formation rate density (SFRD) as a function of stellar mass at z~1. By comparison with a local sample drawn from the SDSS (which is also used to assess the calibration of our SFR indicators), we find that the SFRD has decreased equally for galaxies of all stellar masses over half the age of the Universe.
Dec 11 Prof. Joshua Winn (MIT)
Exoplanets and their Odd Orbital Orientations
In the Solar system, the planets follow orbits that are aligned with the Sun's equatorial plane to within about 7 degrees. What about planets around other stars? Recently we have measured the orbital orientations (relative to their parent stars' equators) of more than a dozen different exoplanets, using a technique first theorized in the 19th century. Many systems have good alignment, as in the Solar system -- but there are a few surprises. I will discuss these results and their implications for theories of planet formation and migration.
Last modified: 07 Sep 2011, 10:42:27

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