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Northwestern University


CIERA Special Seminars 2016-2017

*Tech = Technological Institute (2145 Sheridan Road)
**Db = Dearborn Observatory (2131 Tech Drive)


Fall Quarter 2016



  Date/Time      Visitor Host
  Oct. 31
   Tech F160
   10:30am

Elad Steinberg
    Hebrew University of Jerusalem
    New Insights on the G2 Gas Cloud and the Connection Between Asteroids and
    Terrestrial Planet Formation

Recent observations have shown that a gas cloud, called G2, is in the midst of being tidally disrupted in our galactic center. I will present novel simulations of the G2 gas cloud and discuss it's agreements and disagreements with observations. The Kepler mission has shown that terrestrial planets are extremely common in our galaxy. However, there is still ongoing debate regarding their last stage of formation. I will try and shed some light on this issue utilizing observations from asteroids in our solar system and relating their spin magnitude to terrestrial planet formation theories.

Yoram Lithwick
  Nov. 11
  F160
  12:00pm

Benjamin Montet
    University of Chicago
    Making the Most of Exoplanet Searches

The Kepler mission has revolutionized our understanding of the number and diversity of planetary systems in our galaxy, and TESS will soon follow in its footsteps. By observing hundreds of thousands of stars to search for subtle brightness variations induced by transiting planets, Kepler has also enabled major advances in stellar and galactic astronomy well beyond the core mission requirements. In this seminar, I will discuss some of the lagniappe science results enabled by both Kepler and future missions which significantly add to their legacies. Specifically, I will describe how the large number of stars observed by Kepler has allowed us to understand spacecraft systematics, enabling the success of the extended Kepler mission, K2. I will also outline ongoing work to search for long-term brightness variations of stars in the Kepler field caused by stellar magnetic cycles. I will conclude with a look to the future, describing how the WFIRST microlensing mission will continue the transiting planet revolution.

Fred Rasio
  Nov. 14
  F160
  12:00pm

Ondrej Pejcha
    Princeton University
    Cool and Luminous Outbursts from Merging Binary Stars

Many binary stars pass through a phase of dramatic energy, mass, and angular momentum loss. The existing theoretical uncertainties in this short-lived interaction phase significantly affect our understanding of the binary evolution pathways and their rates, including formation of close binaries composed of black holes, neutron stars and white dwarfs. The discovery of V1309 Sco, a contact binary with rapidly decreasing orbital period followed by an outburst, recently established a connection between these astrophysically critical, catastrophic interactions and a group of astronomical transients characterized by their red color and the luminosity in the gap between novae and supernovae. I will present an exploration of the dynamics of outflows from mass-losing binary stars and the associated menagerie of transients. I will interpret the unprecedentedly detailed pre-merger data on V1309 Sco and argue that these transients can provide a fresh observational input into the open problem of violent binary interactions.

Fred Rasio
  Nov. 17
  Dearborn 23
  11:00am

Siyao Xu
    Peking University, Beijing
    The Power Law in the Radio Sky

Turn on the radio light, and you will see the elephant in the room.

As shown by overwhelming observational evidence, the interstellar medium (ISM) is turbulent and magnetized. The ubiquitous magnetized turbulence in the ISM connects the physical processes happening on local small scales (~10^7 cm) to global large scales (~ 100 pc), through the power-law spectrum of turbulence. The turbulent spectrum has been measured in both the warm ionized medium, known as “the big power law in the sky”, and cold neutral medium. The power-law nature of the interstellar turbulence is widely imprinted in fluctuating observables in the ISM. An in-depth understanding of the power-law statistics of magnetized turbulence is the key to unlocking many mysteries in radio observations that have been puzzling for decades. These observables in turn provide valuable information on interstellar turbulence and density distribution in a straightforward way. In the light of radio observations of Galactic pulsars, I will show you what I have seen in our room, i.e., the Galactic ISM, and in the fast radio burst’s room, extragalactic ISM.

Giles Novak,
Farhad Yusef-Zadeh
  Nov. 18
  F160
  12:00pm

Wen-fai Fong
    University of Arizona
    Unfolding the Dynamic and Colorful Lives of Compact Object Mergers

The mergers of two compact objects (neutron stars: NS and/or black holes: BH) in a binary system serve as signposts of gravitational wave emission, potential sites of heavy element nucleosynthesis, and laboratories for high-energy astrophysical processes not seen elsewhere in the universe. With the recent onset of the advanced era of gravitational wave detectors, there will soon be an unprecedented rise in the detected rate of these systems. At the end of the decade, the arrival of the Large Synoptic Survey Telescope will also lead the community to unexplored territory regarding the timescales, luminosities, and rates at which we discover explosive transients. In this talk, I will present observational challenges for this upcoming revolution in transient astrophysics, with a particular focus on the ground-breaking detections of electromagnetic counterparts to gravitational wave sources. I will also describe ongoing and future efforts across the electromagnetic spectrum to confront these challenges.

Fred Rasio
  Nov. 21
  F160
  12:00pm

François Foucart
    Lawrence Berkeley National Laboratory
    Electromagnetic and Gravitational Wave Signals from Merging Black Holes
    and Neutron Stars

Black hole-neutron star and neutron star-neutron star mergers are powerful emitters of gravitational waves, and can produce a wide range of bright electromagnetic transients. They also provide us with a remarkable environment to study nuclear interactions in the cold, dense core of neutron stars, and may significantly contribute to the production of many heavy elements whose origin remains unexplained. Given the increasing sensitivity of existing gravitational wave detectors, we expect gravitational waves from neutron star mergers to be observed in the coming years. In order to extract as much information as possible from these observations, reliable models of the gravitational wave and electromagnetic signals powered by mergers are necessary. In this talk, I will discuss our efforts to study binary mergers using general relativistic simulations. I will in particular focus on recently developed gravitational wave models which account for tidal effects in neutron stars, on the electromagnetic transients powered by radioactive decays in the matter ejected by neutron star mergers, and on the outcome of nucleosynthesis in that ejecta. Tidal effects in neutron star mergers can provide us with information about the size of neutron stars, an important observable in nuclear physics, while radioactively powered transients can offer both improved localization of the merger events and additional information about the properties of the merging objects.

Fred Rasio
  Nov. 28
  F160
  12:00pm

Jennifer van Saders
    Carnegie Observatories
    Broken Clocks and Fields in Flux: Making Sense of Stellar Rotation Observed with Kepler

Gyrochronology utilizes the spin-down of stars as a function of time as an indicator of stellar age. This technique has the potential to yield precise ages for large samples of stars, providing unprecedented chronological information for studies of the Milky Way and extrasolar planets. However, gyrochronology is in its adolescence: it has been tested under limited scenarios, but its weaknesses and limitations have hitherto been largely unexplored. With data from the Kepler mission we can address these gaps: we now have access to datasets of rotation periods for tens of thousands of stars, as well as independent asteroseismic ages and rotation periods for a few hundred old (main sequence) stars. I will discuss my comparisons of theoretical rotation models to these Kepler data, which have yielded unexpected insights into the rotational and magnetic lives of stars (and the Sun!), as well as a better understanding of the power and peril of gyrochronology as a tool.

Fred Rasio
  Nov. 30
  F160
  4:00pm

Sasha Tchekhovskoy
    University of California - Berkeley
    New Frontiers in Simulating Black Hole Accretion and Jets

Black holes are responsible for a wide variety of astrophysical phenomena. They devour stars, eject relativistic jets, affect star formation and galaxy evolution, and enrich the Universe with heavy elements. In the next several years, the Event Horizon Telescope will produce resolved images of infalling gas and jets on the event horizon scale that promise to revolutionize our understanding of black hole physics. However, until recently, no first-principles models to quantitatively interpret these observations existed. I will present the first such models, the simulated spectra and images, and the constraints on the near event horizon physics coming from the comparison to the observations of the supermassive black hole at the center of our galaxy. I will then use simulations to constrain black hole physics in several other astrophysical contexts. I will finish by making connections to my future research plans..

Fred Rasio


For more information, contact: ciera@northwestern.edu


Past CIERA Special Seminars