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Northwestern University


Weekly Astrophysics Seminars 2015-2016

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.


 

Winter Quarter 2016

  Date      Speaker / Seminar Host
  Feb. 16

Ben Bar-Or
   Institute for Advanced Study
   Stellar Dynamics around a Massive Black Hole

Galactic nuclei and the Massive black holes (MBHs) in their centers are rich environments, where the complex stellar dynamics plays an important role in many physical phenomena. We show that the standard description of the two-body relaxation, around an MBH, as a diffusion process, is incomplete. We derived a non-perturbative solution for energy relaxation as an anomalous diffusion process, and a robust estimation technique to measure it in N-body simulations. We also show how the dynamics of a nearly-Keplerian N-body system, where the angular momentum evolution is accelerated by resonant relaxation, can be described and studied in a formal statistical mechanics framework. In this framework the background potential is described as a correlated Gaussian noise. We obtained the leading order, phase-averaged stochastic equations of motion and derived an effective Fokker-Plank equation for a general correlated Gaussian noise. We combine the Fokker-Plank description of two-body relaxation and resonant relaxation to study the loss-cone dynamics in Galactic nuclei.

Fabio Antonini

  Feb. 23

Leslie Looney
   University of Illinois at Urbana-Champaign
   Setting the Initial Conditions for Planets: The Young Circumstellar Disk

The origin and evolution of the circumstellar disk around protostars informs the origin and evolution of planets. Only recently, have we even detected the youngest circumstellar disks, but their frequency and typical properties (i.e. mass and size) are still not well known. With our new high-resolution millimeter surveys of protostars, we now have about 20 young disks with radii > 15 AU. However, these only represent less than of the population in our surveys. One possible explanation for small disks (< 15AU) in the youngest sources is magnetic braking of material at the earliest stages. In this talk, I explore the early evolution of the disk, implications for dust growth, and discuss future surveys.

Laura Fissel

  Mar. 1

Julie Hlavacek-Larrondo
   Université de Montréal
   AGN Feedback in Brightest Cluster Galaxies

The last few decades have shown us that radio jets originating from supermassive black holes can interact strongly with their surrounding medium. During this talk, I will review the current status of this field, known as AGN feedback, while focusing on the most massive black holes in the Universe, those that lie in Brightest Cluster Galaxies, the central dominant galaxies in galaxy clusters. I will review the physics behind these interactions and then focus on the evolution of radio feedback over cosmic time while presenting new results from clusters discovered via the South Pole Telescope.

Claude-André Faucher-Giguère

  Mar. 8

Tsevi Mazeh
   Tel Aviv University
   Stellar rotation and exoplanets obliquity:
   What can we learn from the Kepler space mission data gold mine?

Spots on the stellar rotating surface can produce periodic brightness modulation that varies with the stellar rotational period. The Kepler space mission observed more than 150,000 stars continuously for more than four years, producing unprecedentedly precise light curves. We used this data gold mine to derive the rotational periods of more than 30,000 stars. About a thousand of the Kepler stars with derived rotational periods have been found to have planets orbiting around them. We use this sample and the derived amplitudes of the stellar modulation in particular, to infer some evidence for the star-planet obliquity - the angle between the stellar spin axis and the orbital planetary angular momentum of a planet. Previous studies found two populations of hot Jupiters - one around cool stars with orbits well-aligned with the stellar rotational axes, and the other around hot stars with isotropic distribution of obliquities, including planets with retrograde motion. We will show surprising statistical evidence that supports these findings. We will also discuss whether the amplitudes of the cool stars with planets depend on the planetary orbital periods, a dependence that might have some implications for the formation of short-period planetary formation.

Fred Rasio

 


Past Astrophysics Seminars