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Northwestern University

Theory Group Meetings 2014-2015

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


Fall Quarter 2014

  Date Speaker / Seminar
  Dec. 5

Aaron Geller
    AMUSE-ing Planets in Star Clusters

Meagan Morscher
    Stellar Black Holes in Globular Clusters

  Nov. 21

Sam Hadden
    Measuring Kepler Planet Masses and Eccentricities with
    Markov Chain Monte Carlo

Niharika Sravan
    Ultra-Violet Emission from the Circum-Galactic Medium of
    High-Redshfit Galaxies

  Nov. 14

Fabio Antonini
    Dynamical Friction in Galactic Centers

Jason Hwang
    The Architecture of Kepler Multis.

  Oct. 31   Douglas Heggie
      University of Edinburgh
      Dynamical Modeling of the Globular Cluster M4


  Gongjie Li
      Harvard-Smithsonian CfA
      How to Flip a Binary Without a Spatula --- Tales of Hierarchical Three-body Systems


The secular dynamical evolution of a hierarchical three body system, in which a distant third object orbits around a binary has been studied extensively, demonstrating that the inner orbit can undergo large eccentricity and inclination oscillations. It had been shown before that starting with a circular inner orbit, large mutual inclination (40 - 140 degree) can produce long timescale modulations that drive the eccentricity to extremely large values and can flip the orbit. Here, we demonstrate that starting with an almost coplanar configuration, for eccentric inner and outer orbits, the eccentricity of the inner orbit can still be excited to high values, and the orbit can flip by ~180 degree, rolling over its major axis. The ~180 degree flip criterion and the flip timescale are described by simple analytic expressions that depend on the initial orbital parameters. With tidal dissipation, this mechanism can produce counter-orbiting exo-planetary systems. In addition, we also show that this mechanism has the potential to change the stellar distribution for binary black hole systems. Furthermore, we explore the entire eccentricity and inclination parameter space to identify the underlying resonances, the chaotic regions and the regions that can excite the eccentricity and flip the orbit.

Hosts: Fred Rasio and Francesca Valsecchi

  Oct. 24

Tyson Littenberg
    Detecting and characterizing gravitational waves

Fani Dosopoulou
    Orbital evolution of eccentric binaries due to mass transfer

  Oct. 17

Adam Dempsey
    Planet-induced Gap Formation in Protoplanetary Disks

Carl Rodriguez
    Hybrid Techniques in Globular Cluster Simulation

  Oct. 10

Josh Fixelle
    Resolving Mass Transfer from Hot Jupiters at Roche Lobe

Daniel Angles Alcazar
    Supermassive Black Holes in Cosmological Simulations

  Oct. 3

Sourav Chatterjee
    Inside-Out Planet Formation

Shane Larson
    Discovering Eccentric Binaries in Gravitational Wave Data


Winter Quarter 2015

  Date Speaker / Seminar
  Jan. 16

  Jing Luan
      Historical heat responsible for Enceladus's plume


Enceladusís plume indicates a subsurface ocean, but its current heating rate is far less than enough to maintain water liquid. We propose that it was heated up when it was deep in mean motion resonance with Dione. Although it has already retreated from large depth in resonance, its historical heat still supports a subsurface ocean, producing plumes on its south pole. Enceladus underwent many heating-cooling cycles, which is made possible by co-evolution of its internal structure and orbital dynamics.

Host: Yoram Lithwick


  Alex Richings
      Leiden Observatory
      Non-equilibrium interstellar chemistry in simulations of galaxy formation


I will present a method to follow the chemical evolution of interstellar gas in hydrodynamic simulations of galaxy formation. This model follows the abundances of 157 chemical species in total, including all ionization states of the 11 elements that are most important for cooling, and 20 molecular species including H2 and CO. This allows us to evolve the galaxy using gas cooling rates that are calculated using non-equilibrium abundances, and it allows us to predict the observable emission from individual species, such as CII and CO, without needing to assume chemical equilibrium. To explore how non-equilibrium chemistry can affect the gas cooling rate, we have applied this chemical model to idealized cases of a uniform gas cloud that is cooling at either constant density or constant pressure. We find that, at temperatures below 10^4 K, the cooling rate calculated using non-equilibrium abundances is enhanced by up to two orders of magnitude. Finally, I will present simulations of isolated disk galaxies that we have run using this chemical model. We find that the abundance of molecular hydrogen at low metallicities tends to be further from chemical equilibrium than at high metallicities. We also compute the emission from CO and we find that there is more scatter in the relation between H2 column density and CO intensity when we use non-equilibrium abundances.

Host: Claude-André Faucher-Giguère

  Jan. 23

  Monica Turner
      Leiden Observatory
      The properties of the gas around galaxies from metal-line absorption


We use quasar absorption lines to study the physical conditions in the circumgalactic medium of z~2.3 star-forming galaxies taken from the Keck Baryonic Structure Survey (KBSS). The pixel optical depth technique is applied to 15 background QSOs and used to construct the first galaxy-centered 2-D maps of the median absorption by OVI, NV, CIV, CIII, and SiIV. We find that metal-line absorption is strongly enhanced within 240 km/s from 21 galaxies with impact parameters <180 proper kpc from the sight lines to the background quasars. Furthermore, the optical depth of OVI at fixed HI, CIV and SiIV is observed to be enhanced with respect to the full pixel sample. We use ionization models to show that the observed strength of OVI as a function of HI would lead to implausibly high metallicities at low densities if the gas were photoionized by the background radiation. We conclude that the galaxies are surrounded by gas that is sufficiently hot to be collisionally ionized (T>10^5 K) and that a substantial fraction of the hot gas has a metallicity >10^(-1) of solar.

Host: Claude-André Faucher-Giguère

  Jungyeon Cho
      Chungnam National University, Korea
      The origin of magnetic fields in the intracluster medium

Host: Yoram Lithwick

  Jan. 30

  Serena Repetto
      Radbound University
      Natal kicks received by stellar-mass black holes


The formation of stellar mass black-holes (BHs) is still very uncertain. Two main uncertainties are the amount of mass ejected in the supernova event (if any) and the magnitude of the natal kick the black hole receives at birth (if any). Repetto et al. 2012, studying the position of Galactic X-ray binaries containing black holes, found evidence for black holes receiving high natal kicks at birth. In this work, we extend that previous study, taking into account the binary evolution of the sources as well. We study the observed Galactic sample of short-period black-hole low-mass X-ray binaries in order to put constraints on the natal kick and mass ejected at BH formation. Short-period BH-LMXBs are compact binaries consisting of a low-mass star orbiting a black hole in a period less than 1 day. We trace their binary evolution backwards in time, from the current state of mass-transfer to the moment the BH was formed, and we add the extra information on the kinematics of the binaries.

Host: Vicky Kalogera

  Feb. 6

  Jeremy Webb
      McMaster University
      The Orbital Anisotropy and Tidal Filling Profiles of Globular Clusters in Giant Galaxies


We compare the relationship between globular cluster effective radii rh and projected galactocentric distance Rgc for the cluster populations of M87, NGC 1399, and NGC 5128. Observations of all three galaxies find a shallower increase in rh with Rgc then predicted by basic tidal theory. To reproduce the relationship, we first model a globular cluster population in each galaxy that has an isotropic distribution of orbits and is tidally filling at all Rgc. We then explore the effects of either cluster orbits becoming more radial or clusters becoming less tidally filling with Rgc. Finally, we determine the combination of orbital anisotropy and tidal filling profiles that best reproduce the observed relationship between rh and Rgc in each galaxy and compare the results. We also explore the effects of orbital anisotropy and tidal filling on the metal-rich and metal-poor sub-populations of each galaxy.

Host: Fred Rasio

  Feb. 13

David Rice
    The role of chaos in the distribution of multi-planet
     system instability times

Jason Hwang
    Planet-planet collisions in compact multi-planet systems

Lauren Barmore
    Instabilities and the distribution of orbital period
    in Kepler multis

  Feb. 27

Jason Steffen


  March 6

  Edo Berger

Vicky Kalogera


Spring Quarter 2015

  Date Speaker / Seminar
  April 3

Carl Haster

Josh Fixelle

  April 10

  Scott Tremaine
      Institute for Advanced Study

Yoram Lithwick

  April 17

“No Faculty Allowed” Session
Organized by: Daniel Anglés-Alcázar


Niharika Sravan
    Tutorial on MESA

Sam Hadden
    A Tutorial on EMCEE

  April 24

Katie Breivik
    Probing the Milky Way Electron Density with
        Multi-messenger Astronomy

Sourav Chatterjee
    Inside-out Planet Formation and its Implications

  May 1

Aaron Geller

Meagan Morscher

  May 8

  Pranab Ghosh
      Tata Institute of Fundamental
        Collective Properties of X-ray Binaries in Normal Galaxies

Collective properties of X-ray binaries, i.e., the distributions of their essential properties like X-ray luminosity and binary period are well-established observationally in Milky Way and other galaxies. We describe a theoretical framework for understanding these distributions both outside and inside globular star clusters in these galaxies. Outside globular clusters, the essential picture is the evolution of the distribution of primordial binaries into that of X-ray binaries along the evolutionary path followed by an individual binary. High-mass and low-mass X-ray binaries behave differently, as expected. We compare the calculated X-ray luminosity distribution (also known as X-ray Luminosity Function) with the observed one for both massive and low-mass x-ray binaries. Inside globular clusters, X-ray binary formation and evolution are dominated by stellar encounter processes, which include tidal capture, exchange processes, and dissociation. We construct a Boltzmann scheme for describing the evolution of X-ray binary populations of globular clusters, focusing first on the dependence of the number of X-ray binaries in a cluster on the essential parameters of that cluster. We compare the calculated dependence with the observed one.

  David Merritt
      Rochester Institute of Technology
        New Results on the Final-Parsec Problem

  May 15
 Tech L250

Francesca Valsecchi

Niharika Sravan

  May 22

Zach Hafen

Fani Dosopoulou

  June 5

Sam Hadden

Adam Dempsey

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Past Theory Group Meetings