Research Events
For the Public
Northwestern University

Weekly Astrophysics Seminars 2013-2014

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


Fall Quarter 2014

  Date      Speaker / Seminar Host
  Sept. 23

Mike McCourt
   UC Berkeley
    Do Galaxy Clusters Boil?

The hot plasma filling galaxy clusters is susceptible to a convective MHD instability known as the MTI. Understanding the implications of this convection has been surprisingly difficult, however; it depends on both the large-scale evolution of the cluster and on the plasma physics of the gas. I will describe a new set of "semi-cosmological" simulations which capture enough of the cosmic evolution of galaxy clusters to reproduce their large-scale properties but still provide an idealized environment in which to study cluster convection and its implications. I will also describe some future applications of these simulations to other problems in galaxy formation.

Claude-André Faucher-Giguère

  Sept. 30

Francesca Valsecchi
   Northwestern University
    Investigating the Role of Stellar Tides in Hot Jupiters’ Origin and Fate

Two formation models have been proposed to explain hot Jupiters’ tight orbits. These could have migrated inward in a disk (disk migration), or they could have formed via tidal circularization of an orbit made highly eccentric following gravitational interactions with a companion (high-eccentricity migration). Disk migration drives hot Jupiters down to their Roche limit separations a_R, in orbits where the stellar spin and orbital angular momentum vectors are nearly aligned. High-eccentricity migration results in an inner cutoff at 2a_R and in a broad range of misalignments. Using state-of-the-art stellar models and a detailed treatment of tidal dissipation, we show that currently observed systems are consistent with high-eccentricity migration. In this scenario, stellar tides shaped the observed distribution of misalignments, and brought inward from beyond 2a_R the currently known hot Jupiters that lie within 2a_R. Interestingly, this population potentially provides direct empirical constraints on tidal dissipation theories.
Eventually, stellar tides will cause the orbits of many hot Jupiters to decay down to a_R. Using a standard binary mass transfer model we show how a hot Jupiter undergoing a phase of Roche-lobe overflow (RLO) leads to a hot super-Earth in an orbit of few hours to several days. This model predicts planets with intermediate masses (``hot Neptunes'') that should be found in the process of losing mass through RLO. The observed excess of small single-planet candidate systems observed by Kepler may also be the result of this process. If so, the number of systems that produced hot Jupiters could be 2-3 times larger than one would infer from contemporary observations.


  Oct. 7

Andrew MacFadyen
   New York University
    The Dynamics, Stability and Radiation of GRB Jets

Gamma-rays bursts and their afterglows involve the dynamics of highly relativistic plasma as it is accelerated at the central engine and expands over more than ten orders of magnitude in length scale. I will discuss high resolution studies of GRB jets using a powerful new code (JET) which employs a moving numerical mesh, thus allowing for highly accurate Lagrangian multi-dimensional simulations of relativistic jet dynamics. I will present the first multidimensional simulations of a collapsar jet starting from the center of a massive star, breaking out of the stellar surface, coasting and producing internal then external shocks. Synchrotron light curves computed from the simulation data naturally produce the early steep decay and extended plateau observed in early X-ray light curves. During the deceleration phase I will demonstrate that GRB jets are unstable to the Rayleigh-Taylor instability. This makes the jets turbulent, thus amplifying magnetic fields via small-scale dynamo to values sufficient to explain the synchrotron emission. The Rayleigh-Taylor fingers can also impact the forward shock thus corrugating it with possible implications for the afterglow emission. I will demonstrate fits of theoretical light curves computed from numerical simulations to the observational data.

Shane Larson

  Oct. 14

Ryan Foley
    University of Illinois at Urbana-Champaign
    The Most Common "Peculiar" Supernova

In the last decade, transient surveys have identified several new types of supernovae (SNe). These new events represent astrophysical phenomena that are either less luminous or rarer than the more prevalent classes of SNe Ia, II, Ib, and Ic. I will discuss a relatively new class, Type Iax supernovae (SNe Iax). These events are observationally similar to SNe Ia, but are physically distinct being less luminous and having lower kinetic energy. To date, ~40 clear members of the class have been identified, making them the most common (by number) peculiar class of supernova. After accounting for their luminosity, there are roughly 30 SNe Iax for every 100 SNe Ia in a given volume, also making SNe Iax the most common peculiar SN by rate and more common than SNe Ib. I will describe observations of individual members of the class and those of the entire class. Taken together, we can constrain their progenitor systems much better than we have for normal SNe Ia. The progenitors are likely a C/O white dwarf that accretes material from a non-degenerate helium star. The explosion is likely a sub-sonic deflagration, and at least some of the time the white dwarf does not completely disrupt, leaving a remnant with particular observational signatures.
I will also present Hubble Space Telescope observations of two SNe Iax. For one SN, in pre-explosion images, we have detected its progenitor system, which is most consistent with being the predicted C/O WD-He-star system. This is the first detection of a thermonuclear SN progenitor system. For the other SN, in images taken 4 years after explosion, we detect a source consistent with being a puffed up remnant star. If true, this would represent a new class of objects, of which there may be a handful in the Milky Way.

Vicky Kalogera

  Oct. 21

Rachel Friesen
   Dunlap Institute, University of Toronto
    Tracing the Mass Flow in Clustered Star Forming Regions

Most stars in our galaxy do not form in isolation. Instead, stars are born in groups and clusters embedded within dense filaments and clumps in molecular clouds. Many clustered star-forming regions share similar morphologies, where the greatest star formation rates are found within a central ‘hub’ of dense molecular gas, that is connected to streams or filaments of additional material. To understand how stars form in clusters, we need to understand how these filaments accrete mass from the surrounding environment, funnel mass to star-forming ‘hubs’, and fragment to form dense star-forming cores. I will present observational evidence of ongoing accretion of material onto dense filaments in a nearby young cluster, with derived mass accretion rates that are sufficient to trigger additional fragmentation and gravitational collapse. In particular, I will show how combining observations of gas dynamics and chemistry in star forming regions can be used to answer these questions.

Laura Fissel

  Nov. 4

Gwen Rudie
   The Carnegie Observatories

Daniel Anglés-Alcázar

  Nov. 11

Saul Rappaport

Fred Rasio and Francesca Valsecchi


Winter Quarter 2015

  Date      Speaker / Seminar Host
  Jan. 13

Michael Meyer
   ETH - Zurich

Fred Rasio

  Jan. 20

Mariska Kriek
   UC Berkeley


  Jan. 27

Tom Abel
   Stanford University
    Dark Matter Dynamics

Computational Physics allows us to study extremely non-linear systems with fidelity. In astrophysical hydrodynamics and studies of galaxy formation much of the last two decades we have explored various discretization techniques and found subtle differences in some applications. Interestingly numerical studies of collisionless fluids such as e.g. the collapse of cold dark matter to form the large scale structure of the Universe has only been studied meaningfully with one approach; N-body Monte Carlo techniques. I will introduce a novel simulation approach, and demonstrate its feasibility, that for the first time can study a collisionless system in the continuum limit in multi-dimensions. I will also show this new technique opens a new window in making sense of structure formation as well as plasma physics. In this context we have developed a novel rasterization/voxelization algorithm applicable in computational geometry, computational physics, CAD design and other fields. I show how these approaches allow also for much improved predictions for gravitational lensing, dark matter annihilation, properties of cosmic velocity fields , and many other applications.

Fred Rasio

  Feb. 17

Lucianne Walkowicz
   Adler Planetarium and LSST

Aaron Geller

  Feb. 24

Sarah Gallagher
   University of Western Ontario

John Everett

  March 3

Mark Reid
   Harvard CfA

Farhad Zadeh

For more information, contact: Janet Howe (janet.howe@northwestern.edu)

Past Astrophysics Seminars