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Northwestern University

Theory Group Meetings 2016-2017

Theory Group Meetings are held at 12:00 PM in Tech F160 (2145 Sheridan Rd) unless otherwise noted

Theory Group Meeting is the informal weekly discussion group of theoretical astrophysicists at CIERA. Discussions usually center around blackboard talks given by graduate students and postdocs at Northwestern, but can include prepared talks given external speakers, as well. Whatever slides or material the speaker brings in, Theory Group Meeting often becomes a quite detailed discussion of the fundamental theoretical astrophysics topic being addressed. Also, while topics under discussion often relate to theory, observational astrophysicists are definitely welcome!

Spring Quarter 2017

  Date Speaker / Seminar
  Apr. 14

Pablo Marchant
   Binary Black-hole Mergers from Rotational Mixing in Tight Binaries

  Apr. 21

Wei Zhu
   Ohio State University
   Detection of a 1 Earth-mass Planet at 1 AU around a Brown Dwarf

We report the discovery of a 1 Earth-mass planet in a 1 AU orbit around a brown dwarf host using the microlensing parallax method (arXiv:1703.08548). This is the third published planet from the on-going Spitzer microlensing program. I will first explain the principle behind the microlensing parallax method and, in particular, why it can measure the masses in a model independent way. Then I will present some significant and unique discoveries from the Spitzer microlensing program, and discuss our future plans. The status of the K2 microlensing program (K2 campaign 9) and its scientific potential will also be discussed.

Winter Quarter 2017

  Date Speaker / Seminar
  Jan. 13

Benjamin Nelson
   Evidence for Two Hot Jupiter Formation Paths


Alexander Richings
   Molecule Formation in AGN-driven Galactic Winds

  Jan. 20

Kit Lee
   Spirals in Disks

  Jan. 27

Raffaella Margutti
   Multi-wavelength Observations of SN 2014C


Fani Dosopoulou
   Supermassive Black Hole Binaries: The Final Hundred-Parsec Problem

  Feb. 3

Alex Lazarian
   University of Wisconsin - Madison
   New Ways to Study Magnetic Fields and Turbulence

I shall talk about our new techniques of tracing magnetic fields in diffuse interstellar medium. The techniques are based on the properties of MHD turbulence to create gradients of magnetic fields and velocity that are largest perpendicular to the magnetic field direction. I shall demonstrate with both synthetic data and the observational data that magnetic fields are, indeed, well traced by the gradients. In terms of observational data I show this by comparing PLANCK polarization and gradients of synchrotron intensity and velocity centroids. The latter are obtained with GALFA atomic hydrogen data.

  Feb. 10

Claude-André Faucher-Giguère
   A Failure of Star Formation Regulation in Short Dynamical Time Scale Environments:
   Implications for Galactic Nuclei, High-redshift Galaxies, and, Possibly, the Formation of Globular Clusters.


Niharika Sravan
   Predicting SN 2016gkg progenitor's Binary Companion

  Feb. 17

Sam Hadden
   Resonances and Chaos in Planetary Systems


Jason Hwang
   Using SPH to Study Outcomes of Grazing Collisions in Kepler Multis

  Feb. 24

Michael Perryman
   University College Dublin

  Mar. 3

Michael Katz
   Population Synthesis of SMBH Binaries for LISA Analysis


Zach Hafen
   Epistemology and Computer Simulations

Fall Quarter 2016

  Date Speaker / Seminar
  Sep. 30

Sarah Wellons
   Harvard University
   The Diverse Progenitors and Descendants of (Compact Elliptical) Galaxies in Cosmological Simulations

Observations of the high-redshift universe have revealed a population of galaxies which are already very massive (~10^11 Msun at z=2) and have typical sizes of < 2 kpc, much smaller than their counterparts in the local universe. How such dense, massive galaxies form, and why they appear to be less common at low redshift, have been questions of interest for both theorists and observers. I will discuss these questions in the context of the Illustris simulation, a hydrodynamical cosmological simulation in which tens of thousands of galaxies form, evolve, and interact with each other, situated within a cosmological context. I select a group of massive compact galaxies at z=2 in the simulation and trace them back and forth in time to discover both how they formed at high redshift, and what they evolve into at the present day. I find a variety of both progenitors (our compact galaxies form either via central starbursts generally brought on by mergers, or by racing out to the tip of the SF main sequence and forming very early) and descendants (many formerly-compact galaxies lurk at the core of a more massive galaxy today, others were consumed in mergers, and some evolve passively and undisturbed). Finally, I will discuss the implications of these results for observational methods of connecting galaxy populations across redshifts - in particular, the assumption of a constant cumulative comoving number density - and suggest an improvement to this method which takes the complexity and variety of galaxies' evolutionary paths into account.

  Oct. 7

Matt Payne
   Harvard / ITC
   Constraining Planet Nine

Recent publications from Batygin & Brown have rekindled interest in the possibility that there is a large (~10 Earth-Mass) planet lurking unseen in a distant orbit (a~500 AU) at the edge of the Solar System. Such a massive planet would tidally distort the orbits of the other planets in the Solar System. These distortions can potentially be measured and/or constrained through precise observations of the orbits of the outer planets and distant trans-Neptunian objects. I will discuss our recent (and ongoing) attempts to observationally constrain the possible location of Planet Nine via (a) measurements of the orbit of Pluto, and (b) measurements of the orbit of Saturn derived from the Cassini spacecraft.

  Oct. 14

Eliza Kempton
   Grinnell College
   Doppler Diagnostics of Atmospheric Dynamics in Exoplanet Spectra

High resolution spectroscopy of exoplanet atmospheres (R~10^5) is a relatively new tool to probe atmospheric composition, structure, and dynamics. At high resolution, the exoplanet's spectrum can be separated from both the host star emission and the telluric spectrum by its Doppler-shifted motion, allowing for successful ground-based observations. In this talk, I will focus on the signature that atmospheric dynamics -- both winds and planetary rotation -- imprints on exoplanet spectra at high resolution through the Doppler shift mechanism. During transit, strong dayside-to-nightside winds produce a net blueshift in hot Jupiter transmission spectra. In thermal emission, Doppler shifts of order several km/s are predicted as the planet's hot spot rotates into and out of view of the observer. We have modeled these effects on the spectra of hot Jupiters by coupling the results of 3-D general circulation models (GCMs) to a radiative transfer solver that self-consistently generates spectra at high resolution. I will present the methodology and results of these calculations and will describe how Doppler shifted exoplanet spectra can be used to diagnose key properties of an exoplanet's dynamical atmosphere -- its longitudinal temperature and wind structure and its rotation rate.

  Oct. 21

Fabio Antonini
   Nuclear Clusters and (Supermassive) Black Holes


Daniel Angles-Alcazar
   What Drives the Co-evolution of Supermassive Black Holes and Galaxies?

  Oct. 28

Eve Lee
   University of California - Berkeley
   The Late-Time Formation and Dynamical Signatures of Small Planets

The Kepler mission has established that approximately half of all Sun-like stars harbor planets. Of these, close-in super-Earths are the most common. Understanding the origin of super-Earths can lend us insight into the default pathway of planet formation. The riddle posed by super-Earths is that they are not Jupiters: their core masses are large enough to trigger runaway gas accretion, yet somehow super-Earths accreted atmospheres that weigh only a few percent of their total mass. I will show that this puzzle is solved if super-Earths formed late, in the inner cavities of transitional disks. Super-puffs present the inverse problem of being too voluminous for their small masses. I will show that super-puffs most easily acquire their thick atmospheres as dust-free, rapidly cooling worlds outside 1 AU, and then migrate in just after super-Earths appear. Small planets may remain ubiquitous out to large orbital distances. I will demonstrate that the variety of debris disk morphologies revealed by scattered light images can be explained by viewing an eccentric disk, secularly forced by a planet of just a few Earth masses, from different observing angles. The farthest reaches of planetary systems may be perturbed by eccentric super-Earths.

  Nov. 4

Jacob Simon
   University of Colorado - JILA
   Probing the Nature of Accretion and Planet Formation in Protoplanetary Disks:
   Connecting Theory with ALMA Observations

Protoplanetary disks play a key role in star and planet formation processes. Turbulence in these disks, which arises from the magnetorotational instability (MRI), not only causes accretion of mass onto the central star, but also sets the conditions for processes such as dust settling, planetesimal formation, and planet migration. However, the exact nature of this turbulence is still not very well constrained in these systems.

In this talk, I will describe new work, utilizing both state-of-the-art numerical simulations and powerful new radio observations, to directly link numerical predictions for the turbulent velocity structure of protoplanetary disks to observations by the Atacama Large Millimeter Array (ALMA). ALMA’s unprecedented resolution and sensitivity allow us to generate a three-dimensional map of disk turbulence by measuring the turbulent broadening component of molecular lines at different disk heights (i.e., optical depths) and radii. Using a series of high-resolution numerical MHD simulations and a Monte-Carlo radiative transfer code, we have made detailed predictions for the strength and distribution of turbulent molecular line broadening in the disk around HD 163296. We have found that at large distances from the central star, the upper layers of the disk should be Doppler broadened by ~50% of the sound speed. However, our follow-up ALMA observations suggest that turbulence within these regions is extremely weak, bringing into question our understanding of accretion processes. I will conclude by describing a combination of theoretical and observational work to understand these observations.

  Dec. 2

Alexander Rasskazov
   Rochester Institute of Technology
   Dynamical Evolution of Supermassive Black Hole Binaries


Deanne Coppejans
   Multi-Wavelength Accretion Studies of Cataclysmic Variable Stars

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Past Theory Group Meetings