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Northwestern University

Weekly Astrophysics Seminars 2015-2016

Seminars are held at 4:00 PM on Tuesdays in Room F160
on the first floor of the Technological Institute (2145 Sheridan Road) unless otherwise noted


Spring Quarter 2016

  Date      Speaker / Seminar Host
  Mar. 29

Laura Chomiuk
    Michigan State University
    The E-Nova Project: A Multi-Wavelength Initiative to Probe the
    Ejecta and Environments of Novae

When imagining a nuclear explosion, we often picture strong, spherical shock waves, like a bomb or supernova; however, nature's most common thermonuclear explosions look nothing like this, showing delayed and multiple phases of mass ejection that can last for months after the nuclear fuel is ignited. These most common explosions are novae---thermonuclear runaways on the surfaces of accreting white dwarfs---and their complexities are best revealed with an intensive multi-wavelength observational program highlighting radio and X-ray data---our E-Nova Project. I will discuss our recent results, featuring observations from the Karl G. Jansky Very Large Array, and spotlighting sources like recent novae with giant companions and novae that have recently been detected in GeV gamma rays (an emission process that was not predicted and remains an intriguing mystery).

Fred Rasio,
Sourav Chatterjee

  Apr. 12

Alycia Weinberger
   Carnegie Institution of Washington
   Circumstellar Disks and Young Stars: Tracing the Formation of Planetary Systems

The initially ubiquitous disks of gas and dust around young stars eventually form the diversity of observed mature planetary systems. I will present my recent work using high angular resolution imaging and spectrophotometry from the Hubble Space Telescope, Magellan Adaptive Optics system, and ALMA to show the structure and compositions of disks. I will also present recent results from the Large Binocular Telescope Interferometer; our HOSTS survey is searching for the incidence of tenuous disks at the level of the solar system's zodiacal light.

Fred Rasio

  Apr. 26

Ilse Cleeves
   Harvard-Smithsonian Center for Astrophysics
   Fire and Ice: The Role of Energetic Processes in the Cold Chemistry of Protoplanetary Disks

Planets form from the coldest and densest parts of circumstellar disks around young stars. During this phase, the active nature of the star subjects the disk to relatively high fluxes of UV and X-ray photons and energetic particles. Simultaneously, the local star-forming environment may provide additional external UV and/or radioactive pollutants from recent massive stellar populations. These energetic agents play a vital role in 1) setting protoplanetary disks' turbulent and thermal physics, and 2) regulating the important gas and grain surface chemical reactions, impacting the overall disk molecular composition. Consequently, chemistry can used to "map out" the important ionizing processes in disks using molecular emission. Using data from the Submillimeter Array and Atacama Large Millimeter/Submillimeter Array, we have put strong constraints on the ionization levels in the deep disk of TW Hya, constraining the cosmic ray rate to a value two orders of magnitude below the dense interstellar value. This result has major consequences for the active disk chemistry and, in particular, the chemistry of water. Finally, I will discuss future directions aiming to resolve ionization structure and time variability.

Sourav Chatterjee

  May 3

Matteo Cantiello
   Kavli Institute for Theoretical Physics
   The Stellar Ultrasound: How Asteroseismology Revolutionized Our
   Understanding of Stellar Interiors

Internal rotation and magnetism are key ingredients that largely affect explosive stellar deaths (Supernovae and Gamma Ray Bursts) and the properties of stellar remnants (White Dwarfs, Neutron Stars and Black Holes). However, the study of these subtle internal stellar properties was limited to very indirect proxies. In the last couple of years, new exciting asteroseismic results have been obtained by the Kepler satellite. Among these results are 1) the direct measure of the degree of radial differential rotation in many evolved low-mass stars, and 2) the detection of strong (>10^5 G) internal magnetic fields in the cores of thousands of red giant stars. I will explain how asteroseismology is able to probe rotation and magnetism in the internal regions of red giant stars. I will then show how these asteroseismic observations can now be used to test different mechanisms for angular momentum transport and to study the evolution of internal magnetism in stars. I will conclude with a brief discussion of the important consequences of these recent discoveries.

Sourav Chatterjee

  May 10

Xavier Siemens
   University of Wisconsin-Milwaukee
   The Discovery Potential of Pulsar Timing Arrays

For over a decade, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been using the Green Bank and Arecibo radio telescopes to monitor millisecond pulsars. NANOGrav aims to directly detect low-frequency gravitational waves which cause small changes to the times of arrival of radio pulses. In this talk I will discuss the work of the NANOGrav collaboration and our sensitivity to gravitational waves from astrophysical sources. I will show that a detection is possible in the next few years.

Vicky Kalogera

  May 17

Daniel Jontof-Hutter
   Pennsylvania State University
   The Extraordinary Diversity of Super-Earth Mass Planets Revealed with Transit Timing

Transit timing variations (TTV) in multi-transiting systems enables precise characterizations of low-mass planets and their orbits. The increase in sensitivity to planetary mass with orbital distance with TTVs provides a sample that complements low mass radial velocity detections, by characterizing transiting planets with lower incident fluxes. This pushes exoplanet characterization to the regime of sub-Earth size planets and out to Venus-like distances. Here we demonstrate the astonishing diversity in the density of super-Earth mass planets. We summarize these and other contributions to exoplanet science from TTVs.

Sourav Chatterjee

  May 24

Wen-fai Fong
   University of Arizona
   Setting the Stage for the Era of Gravitational Wave Discovery

The first advanced gravitational wave detectors are newly operational and have brought of the most anticipated discoveries of the century: the direct detection of gravitational waves. The premier gravitational wave sources are the mergers of two compact objects, involving either two neutron stars or a neutron star and a black hole. While the gravitational wave signal will give insight to the basic properties of compact objects, a coincident detection at electromagnetic wavelengths will significantly leverage the event by providing precise sky localization and an association to a galaxy. The main challenge will be how to identify the correct electromagnetic counterpart amidst an otherwise dynamic sky. In this talk, I discuss ongoing efforts to characterize the electromagnetic signatures from compact object mergers. In particular, I present observational evidence linking mergers to two distinct counterparts: short-duration gamma-ray bursts (GRBs) and long-lived transients powered by the radioactive decay of heavy elements (kilonovae). I then address how these results can help us develop observing strategies for this revolutionary era of gravitational wave discovery.

Vicky Kalogera,
Niharika Sravan, and Fani Dosopoulou

  May 31

Jonathan Trump
   Pennsylvania State University
   The Birth and Growth of Supermassive Black Holes:
   Coming of Age with Space Telescope Imaging Spectrograph Surveys

The past 20 years have revealed that supermassive black holes play an essential role in the formation and growth of galaxies. Every massive galaxy hosts a supermassive black hole in its center, and the black hole's mass is tightly coupled to the mass of the galaxy. Remarkably, the black hole - galaxy connection has been "self-maintained" from the adolescent universe (z~2) to the current epoch, from Milky-Way progenitors to massive cluster galaxies, governed by coupled black hole accretion and galaxy star formation. Until recently the "chicken-or-egg" birth of galaxies and supermassive black holes has remained mysterious. I will show how imaging spectrograph surveys with the Hubble Space Telescope are revolutionizing our understanding of black hole formation, revealing a fossil record of massive black hole seeds in tiny galaxies. Similar imaging spectrographs are flagship survey instruments on the upcoming JWST, WFIRST, and Euclid space telescopes, enabling an exciting future for understanding the birth of primordial galaxies and their black hole seeds.

Daniel Angles-Alcazar

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Past Astrophysics Seminars