2021 Fall Schedule

13 October 2021: Dr. Tyrone Woods (Herzberg Astronomy & Astrophysics Center, Canada)

  • Title: The First Stars and the Birth of the Most Massive, High-redshift Quasars

  • Abstract: The discovery of billion-solar-mass quasars at redshift ~ 7 challenges our understanding of the early Universe — how did such massive objects form in the first billion years, and what can this tell us about their environments at Cosmic Dawn? Observations and theory increasingly favour a "heavy seed" or "direct collapse" scenario, in which the rapid accretion possible in some primordial halos leads to the formation of uniquely supermassive stars, which collapse to form the initial seeds of supermassive black holes. In this talk, I'll present systematic, self-consistent simulations of the evolution of these objects under realistic formation conditions, and propose observational diagnostics to decisively test the origin of high-z quasars using next generation electromagnetic and gravitational wave observations. I'll also discuss the expected multiplicity of such supermassive stars and their subsequent interactions, as well as the unique observational signatures of primordial stellar populations which are intermediate in mass between supermassive objects and "typical" Pop III stars applicable in the era of JWST and beyond.

27 October 2021: Prof. Evan Schneider ( University of Pittsburgh, USA)

  • Title: The Origin of Multiphase Galaxy Outflows

  • Abstract: Star-forming galaxies are often observed to host outflows - gas that is flowing away from the galaxy in phases ranging from cold molecular clouds to hot X-ray emitting plasma. While these multiphase outflows are routinely observed, theoretically constraining their origin and evolution has proven difficult. Explaining the prevalence and velocity range of the cool ionized phase (T~10^4 K) in particular poses a challenge. In this talk, I will discuss a theoretical model that explains this cool gas. Using results from the CGOLS project — a series of extremely high-resolution simulations run with the GPU-based Cholla code — I will show that in high star formation surface density systems, dense disk gas can be pushed out by the collective effect of clustered supernovae, explaining observed low-velocity material. Subsequent shredding and mixing of these clouds creates gas with intermediate densities and temperatures that is prone to radiative cooling, allowing momentum to transfer between phases and producing high velocity cool gas. In addition to explaining the nature of outflows themselves, these multiphase winds could potentially be a source of the cool photo-ionized gas that is found in abundance in galaxy halos.


10 November 2021: Prof. Erin Kara (Massachusetts Institute of Technology, USA)

  • Title: Reverberation light echoes in black hole X-ray binaries

  • Abstract: Stellar mass black holes in binary systems with normal stars can become some of the brightest X-ray sources in the sky, as they accrete material from the stellar companion. Material accretes through the accretion disk, releasing copious amounts of energy in the form radiation, relativistic jets and outflows, yet the causal connection between accretion and ejection is not well understood. In this talk, I will describe recent effort on probing the disk-corona-jet connection during black hole outbursts using the technique of X-ray reverberation lags, where we measure light travel time delays between different emitting regions in order to map out the geometry and dynamics of the accretion flow. I’ll describe the recent observational breakthroughs using data from the NICER Observatory, an X-ray telescope on the International Space Station.

24 November 2021: Prof. Anthony Brown (Leiden University, Netherlands)

  • Title: The Gaia mission and Gaia EDR3 science highlights

  • Abstract: The European Space Agency's Gaia space mission, launched in 2013 and expected to operate through 2022, is designed to measure the brightnesses, colors, positions, distances, and motions (in three dimensions) of almost two billion of the Milky Way's hundred billion stars. These measurements are yielding new insights about the internal structure and formation history of the Milky Way, thanks in part to a series of increasingly comprehensive data releases that any member of the astronomical community can access. In this talk, I will introduce the Gaia mission and briefly summarize the latest data release, Gaia EDR3. This discussion will be complemented by highlights of the science results from Gaia DR2 and Gaia EDR3, showcasing among others the impact of Gaia on solar system studies, the Milky Way's accretion and recent dynamical histories, and understanding matter in extreme states.


8 December 2021: Prof. Tim Linden (Stockholm University, Sweden)

  • Title: TeV Halos: A New Class of Gamma-Ray Sources Powered by Pulsars

  • Abstract: Observations by the HAWC and HESS telescopes have found extended TeV emission consistent with a handful of young and middle-aged pulsars. In this talk, I will show that these detections have significant implications for our understanding of both pulsar emission and TeV astrophysics. First, the spectrum and intensity of these “TeV Halos” indicates that a large fraction of the pulsar spindown energy is efficiently converted into electron-positron pairs. This provides observational evidence necessitating pulsar interpretations of the rising positron fraction observed by PAMELA and AMS-02. Second, the isotropic nature of this emission provides a new avenue for detecting nearby pulsars with radio beams that are not oriented towards Earth. Third, these observations indicate that the total emission from unresolved pulsars produces the majority of the TeVgamma-ray flux observed from the Milky Way.

22 December 2021: Dr. Nikolaos Karnesis (Aristotle University of Thessaloniki, Greece)

  • Title: Detecting Gravitational Waves in Space: Status and future prospects

  • Abstract: In recent years, the improved sensitivity of our detectors has allowed us to measure a plethora of new gravitational wave signals. These new catalogues have contributed to the improvement of our understanding of the nature of such cataclysmic events. Now, the community is implementing new detector ideas, with improved sensitivity, focusing on different parts of the Gravitational Wave spectrum. In this talk I will present an overview of the new detectors being developed, focusing on the implications to astrophysics and cosmology.

19 January 2022: Prof. Katariina Nykyri (Embry-Riddle Aeronautical University, USA)

  • Title: Kelvin-Helmholtz Instability and its Role On the Plasma Transport and Heating in the Heliosphere

  • Abstract: As space plasmas are highly collisionless and involve several temporal and spatial scales, understanding the physical mechanisms responsible for energy and mass transport between these scales is a challenge. In this seminar I give background on the Kelvin-Helmholtz Instability (KHI) and discuss its role on the plasma transport, heating and acceleration in the heliosphere. The Kelvin-Helmholtz (KH) waves have been observed in the solar corona, boundary of the Coronal Mass Ejections, and in the boundary layers of many planets. While KHI was traditionally considered as an ideal instability (and hence unable to directly transfer mass), observations and simulations during last 20 years have shown that secondary processes (such as magnetic reconnection, kinetic plasma wave modes, other instabilities, and turbulence) can develop within KH vortices. These secondary processes can lead to rapid heating of the plasma and transport over magnetic boundaries. Learning from these multi-scale, velocity-shear driven processes in the heliosphere may also be helpful when studying plasma transport in other astrophysical systems, as well as in laboratory plasmas, e.g., when designing transport barriers for different magnetic field geometries.