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Northwestern University

Weekly Astrophysics Seminars 2017-2018

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 2017

  Date      Speaker / Seminar Host
  Sep. 19

Greg Taylor
   University of New Mexico
   Science at Low Frequencies with the Long Wavelength Array

The Long Wavelength Array (LWA) is a new multi-purpose radio telescope operating in the frequency range 5-88 MHz with many different operational modes. Scientific programs include (1) the high-z universe, including distant radio galaxies and clusters - tools for understanding the earliest black holes and the cosmological evolution of Dark Matter and Dark Energy; (2) planetary, solar, and space science, including space-weather prediction and extra-solar planet searches; (3) studies of the Earth's ionosphere; and (4) the radio transient universe including the recent discovery of self-emission from meteors. The first station of the LWA, called LWA1, is located near the center of the VLA and has been operating for 6 years. A new station has begun operating at the Owens Valley Radio Observatory in California and a second station in New Mexcio is being commissioned. We are now combining these stations with the new, wide-band, low frequency capability on the VLA. I will discuss a number of scientific results and future prospects.

Farhad Yusef-Zadeh

  Sep. 26

Fabienne Bastien
   Pennsylvania State University
    Convection in Cool Stars, as Revealed Through Stellar Brightness Variations

As a result of the high precision and cadence of surveys like MOST, CoRoT, and Kepler, we may now directly observe the very low-level light variations arising from stellar granulation in cool stars. Here, we discuss how this enables us to more accurately determine the physical properties of Sun-like stars, to understand the nature of surface convection and its connection to magnetic activity, and to better determine the properties of planets around cool stars. Indeed, such sensitive photometric "flicker" variations are now within reach for thousands of stars, and we estimate that upcoming missions like TESS will enable such measurements for ~100 000 stars. We present recent results that tie “flicker” to granulation and enable a simple measurement of stellar surface gravity with a precision of ~0.1 dex. We use this, together and solely with two other simple ways of characterizing the stellar photometric variations in a high quality light curve, to construct an evolutionary diagram for Sun-like stars from the Main Sequence on towards the red giant branch. We discuss further work that correlates “flicker” with stellar density, allowing the application of astrodensity profiling techniques used in exoplanet characterization to many more stars. We also present results suggesting that the granulation of F stars must be magnetically suppressed in order to fit observations. Finally, we show that we may quantitatively predict a star's radial velocity jitter from its brightness variations, permitting the use of discovery light curves to help prioritize follow-up observations of transiting exoplanets.

Aaron Geller

  Oct. 10

Hanno Rein
   University of Toronto
   Formation and Stability of Planetary Systems

The diverse population of extrasolar planets keeps challenging theories of planet formation. Multi-planetary systems are of particular interest as their dynamical architectures allow us to constrain an otherwise unobservable formation phase.

I'll show how a simple stability requirement together with machine learning tools can be used to constrain orbital parameters of planetary systems such as HL-Tau and Trappist-1. Some of the dynamical properties we find in the these systems can be explained by a turbulent protoplanetary disk and stochastic planet migration. Saturn's rings can be thought of as a small scale version and test bed of the early Solar System. I'll show evidence that stochastic migration can be directly observed in moonlets around Saturn.

I'll finally talk about some of the numerical challenges when running accurate long term simulations of planetary systems. Even though we are solving differential equations that have been known since Newton's time, several breakthroughs were made only very recently with the help of clever numerical algorithms. Among these is the discovery that one percent of all realization of the Solar System lead to collisions between planets within the life-time of the Sun.

Ben Nelson

  Oct. 17

Shane Larson
   Northwestern University
   GRAVITY & LIGHT: GW170817 and the Birth of Gravitational-wave Multi-messenger Astronomy

The first binary neutron star merger ever observed, GW170817, has expanded the scope of gravitational wave astronomy dramatically. Combined with the nearly coincident detection of GRB170817A and a multi-week long electromagnetic follow-up campaign, this discovery has been a watershed event for multi-messenger astronomy with gravitational waves.

In this talk, we'll conduct a brief survey of the discovery and cover highlights of the science that has emerged from the gravitational wave observations.


  Oct. 24

Susan Clark
   Institute for Advanced Study
   Magnetism and Morphology in the Interstellar Medium

The interstellar medium is suffused with magnetic fields. Sensitive, high-resolution observations of Galactic neutral hydrogen (HI) reveal an intricate network of slender linear features. Across the high Galactic latitude sky, this HI is aligned with the magnetic field as traced by both starlight polarization and polarized dust emission. I will discuss this link between the neutral gas and the ambient magnetic field, and other novel ways to exploit the information encoded in the morphology of the ISM. In particular I will present a new probe of line-of-sight magnetic field tangling: the dispersion in the orientation of linear features in neutral hydrogen gas as a function of gas velocity traces the polarization fraction of thermal dust emission. This constitutes a new link between gas morphology and the line-of-sight magnetic field geometry.

Giles Novak

  Oct. 31

Frank van den Bosch
   Yale University
   Dark Matter Substructure: Cosmological Treasure Trove or a Pandora's Box?

Hierarchical structure formation in a LCDM cosmology gives rise to virialized dark matter halos that contain a wealth of subtructure. Being able to accurately predict the abundance and demographics of dark matter subhaloes is of paramount importance for many fields of astrophysics: gravitational lensing, galaxy evolution, and even constraining the nature of dark matter. Dark matter substructure is subject to tidal stripping and tidal heating, which are highly non-linear processes and therefore best studied using numerical N-body simulations. Unfortunately, as I will demonstrate, state-of-the-art cosmological simulations are unable to adequately resolve the dynamical evolution of dark matter substructure. They suffer from a dramatic amount of artificial subhalo disruption as a consequence of both inadequate force softening and discreteness noise amplification in the presence of a tidal field. I present criteria that can be used to assess the reliability of subhaloes in numerical simulations, discuss implications for a variety of astrophysical applications, and briefly suggest potential ways forward.

Sarah Wellons

  Nov. 7

Gregg Hallinan

Deanne Coppejans

  Nov. 9
  Joint Astro/CMP

Vanessa Graber
   McGill University

Vicky Kalogera,
Jim Sauls (CMP)

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Past Astrophysics Seminars