Talks & Events
Astronomy and Astrophysics Colloquia - Usually Wednesdays, 3:30 PM, ERC 161, unless otherwise specified. Reception starts at 4:30 PM in Hubble Lounge; persons with a disability who believe they may need assistance, please call the departmental secretary in advance at 773-702-8203 or email deptsecoddjob.uchicago.edu. See also the list of KICP Wednesday Colloquia which alternate with the Astronomy and Astrophysics Colloquia.
Current & Future Astronomy Colloquia
Past Astronomy Colloquia
Feedback from a central supermassive black hole is an essential component of galaxy evolution models. Without it, those models cannot produce realistic massive galaxies and galaxy clusters. However, the black-hole feedback mechanism remains mysterious. Somehow, accretion of matter onto the central black hole of a massive galaxy becomes precisely tuned so that it regulates radiative cooling and condensation of gas in a volume many orders of magnitude larger than the black-hole's gravitational zone of influence. I will discuss how the required coupling can arise through condensation and precipitation of cold clouds out of a galaxy's circumgalactic medium, and will show how a feedback mechanism that suspends the circumgalactic medium in a marginally unstable state can regulate star formation within galaxies.
The Yin and Yang of Slowly-Pulsating B Stars: Asteroseismology and Angular Momentum Redistribution
During their main-sequence evolution, almost all B-type stars will pass through a phase where they are unstable toward oscillation in one or more global internal gravity waves ('g modes'). The g modes, driven by iron and nickel opacity in the stars' outer envelopes, generate surface temperature and velocity changes with periodicities on the order of days.
In the 'Yin' part of my talk, I'll discuss how time-series spectroscopy and photometry of these `slowly-pulsating B' (SPB) stars can be leveraged into asteroseismology --- probing the stars' interiors by careful analysis of their oscillation spectra. I'll highlight in particular how the Kepler mission, together with the MESA stellar evolution code and my GYRE stellar oscillation code, has allowed novel constraints to be established on the internal rotation and mixing physics of SPB stars.
I'll then pivot to the 'Yang' part of my talk. Although we typically regard stellar oscillations as passive tracers of stellar structure, they can also modify this structure. I'll present recent work by my group exploring angular momentum redistribution by g modes. Modeling this process in SPB stars, we find that significant modification of internal rotation profiles can occur on timescales as short as centuries. This suggests that the g modes can impact the stars' life trajectories, a possibility that's been hitherto ignored in stellar evolution calculations.
Why interstellar grain align and why you should care
More than 70 years after the discovery of interstellar polarization, we now have a quantitative, empirically tested, theory of grain alignment giving ride to the observed effect. This Radiative Alignment Torque (RAT) theory predicts that dust grains are spun up by an anisotropic radiation field, if the wavelength of the light is less than the grain diameter. If the grain is made of a paramagnetic material, it will then align with the magnetic field.
A number of specific, observationally testable, predictions follow from the theory, many of which have already been addressed. With a full testing of the theory and quantification of its parameters, polarimetry has the promise to not only allow efficient and reliable tracing of interstellar and interplanetary magnetic fields, but also to provide new and unique probes of the dust and the interstellar environment.
I will review RAT alignment and its observational testing, and discuss some of the probes of ISM environmental parameters and dust that the verified theory allows.
Space astrometry: the Hipparcos and Gaia missions
Alone amongst the space agencies, ESA made its entry into space astrometry with the adoption of the Hipparcos mission in 1981. Outside of the astrometric community, it was viewed at the time as fundamental if not particularly exciting, although Freeman Dyson described it as "... the first time since Sputnik in 1957 that a major new development in space science has come from outside the US". In his ASP Millennium Essay in 2001, Cavendish Professor Malcolm Longair stated that "It is invidious to single out surveys which I find particularly impressive, but I make an exception for the Hipparcos astrometric satellite".
Hipparcos delivered its high-accuracy catalogue of 120,000 star distances and space motions in 1997. As a follow-up, ESA accepted the Gaia mission in 2000. Launched in 2013 and expected to operate into the next decade, Gaia will represent a revolution in its dynamical stereoscopic mapping of our Galaxy, promising a catalogue of more than a billion stars to 20 magnitude at microarcsec-level accuracy. The talk will provide a short historical context and describe the scientific motivation for these missions, outline the essential experimental principles which underpin their measurements, and give an overview of the science objectives, including Gaia's expected yield of many thousands of astrometrically-detected exoplanets.
Michael Perryman obtained his PhD in 1980 (Cambridge, UK) and spent most of his subsequent career with the European Space Agency. He was project scientist for Hipparcos from adoption in 1981 to catalogue finalisation in 1997, holding the dual role of overall project manager (1989-1993) after the satellite failed to achieve its nominal geostationary orbit. With Lennart Lindegren (Lund, Sweden) he was the co-originator of Gaia, and responsible for driving many of its principal attributes. He was study scientist from the Gaia's origins in 1995 to mission adoption in 2000, and thereafter ESA project scientist until the Critical Design Review in 2008.