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Northwestern University


Weekly Astrophysics Seminars 2011-2012

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



Spring Quarter 2012

  Date   Visitor / SeminarHost
  March 27

Hanno Rein
    Institute for Advanced Study at Princeton
     Multi-planetary systems, Saturn's Rings and the collisional N-body code REBOUND

The increasing number of discovered extra-solar planets opens a new opportunity for studying the formation of planetary systems. Resonant systems are of particular interest because their dynamical configuration provides very strong constraints on the otherwise unobservable phase of formation and migration. I will illustrate the main effects of planetary migration in multi-planetary systems by discussing several examples from past and ongoing studies. By observing a specific resonance in a planetary system, one can constrain the properties of the proto-planetary disc. Close, first order resonances require a fast migration speed for capture, which can be attributed to a massive disc. Furthermore, proto-planets are exposed to stochastic forces, generated by density fluctuations in the proto-planetary disc. Systems with massive planets are usually stable. However, systems with smaller planets such as Super-Earths, similar to those that have been discovered by the Kepler spacecraft, can get easily disrupted. Interestingly, these systems are dynamically very similar to Saturn's rings. The stochastic migration of small bodies in Saturn's rings can be described using the same equations. I will show new results from our current work with direct N-body simulations which aim to be directly comparable to the observations of moonlets that show signs of non-Keplerian motion. In the last part of the talk will be devoted to advertising a new freely available collisional N-body code, REBOUND. All of the numerical work presented in this talk has been performed with REBOUND. I will show how you can easily reproduce these calculations yourself.

Adrian Barker
  April 3

Philip Chang
    University of Wisconsin-Milwaukee
     The Physics and Cosmology of TeV Blazars

The universe is teeming with very high energy gamma ray sources (> 100 GeV), but it is generally thought that their impact on the universe is minor at best. On energetic grounds, this assumption seems well-founded because the energy density in TeV photons is 0.2% of that of ionizing photons from quasars. However, as I hope to show in this talk, this is not the case. Rather, the greater efficiency by which TeV photons can be converted to heating in the intergalactic medium (IGM) allows TeV blazars dominate the heating of the IGM at low redshift. I will discuss the nature of this conversion via beam instabilities. I will then discuss how the resultant heating from these TeV sources makes dramatic differences in the formation of structure in the universe. In particular, I will discuss how it gives rise to the inverted temperature-density profile of the IGM, the bimodality of galaxy clusters, and the paucity of dwarf galaxies in galactic halos and voids.

Nate Bode
  April 10

David Merritt
    Rochester Institute of Technology
     Relativistic Stellar Dynamics

Encounters between stars and stellar remnants at the centers of galaxies drive many important processes, including generation of gravitational waves via extreme-mass-ratio inspirals (EMRIs). The fact that these encounters take place near a supermassive black hole (SMBH) turns out to be important for two reasons: (1) The orbital motion is quasi-Keplerian, so that correlations are maintained for much longer than in purely random encounters. (2) Relativity affects the motion, through mechanisms like precession of the periapsis, frame-dragging, and quadrupole torques. The interplay between these processes is just now beginning to be understood, based on N-body simulations that contain a post-Newtonian representation of relativistic dynamics. A key result is that relativity can be crucially important even for orbits that extend outward to a substantial fraction of the SMBH influence radius, by destroying the long-term correlations that would otherwise drive the evolution. I will discuss this work and its implications for the EMRI problem, for the evolution of SMBH spins, for experimental tests of theories of gravity, and for the long-term evolution of galactic nuclei.

Yoram Lithwick
  April 17

Philip Marcus
    University of California, Berkeley
     Vortices in Protoplanetary Disks

Coherent, long-lived vortices in protoplanetary disks (PPDs) may be important in the late-stages of star formation due to their ability to transport angular momentum and energy radially. They may also play a role in planet formation due to their ability to accumulate dust in both two and three dimensions. We show that off-mid-plane, anticyclonic vortices in Keplerian disks are robust, but mid-plane vortices, unless very weak and elongated, are not likely to be stable. We also show that in numerical simulations of protoplanetary disks that Poincaré waves, those neutrally stable waves due to rotation (inertial waves) and/or density stratification (internal gravity waves), are almost always present (and are, in fact, difficult to avoid). Small perturbations, including oscillating vortices, produce Poincaré waves. As the waves propagate from the mid-plane of the disk, their kinetic energy flux ??3/2 remains approximately constant, so as the density ? decreases away from the mid-plane (approximately as a Gaussian) the velocity becomes large and the waves break. The breaking is shown to produce robust vortices, which obey scaling laws that we have derived and review. The shear in PPDs produces critical layers where neutral eigenmodes of the disk have logarithmic singularities. Weak dissipation blunts the singularities, but the layers grow to large amplitude, deriving their kinetic energy from the Keplerian differential rotation. Thus, the growth of these layers is an example of finite-amplitude instability in a linearly, neutrally-stable disk that derives its energy from the Keplerian motion. With sufficiently large amplitude, critical layers can spawn off-midplane vortices. Our calculations are done with spectral methods, and we discuss the pitfalls of the shearing-sheet approximation, computational domains that are too small, and calculations with insufficient spatial resolution.

Elizabeth Hicks
  April 24

Sam Finn
    Penn State
    Caelum Incognita, Astronomy, and Gravitational Waves

Gravitational waves may be the first truly unique probe of the heavenly firmament since Galileo first turned a telescope toward the sky. Indeed, firmament is a particular apt description of the gravitational wave sky: gravitational waves are a manifestation of the spacetimes texture, and spacetime is the foundation upon which the Nature's pageant unfolds.
Setting metaphor aside, the interpretation of gravitational wave observations and their integration other observational modalities pos a set of as yet unmet challenges. The conventional description of gravitational waves equates an unambiguous physical observable (an antenna response) to an unphysical quantity (the "metric perturbation"), confusing the question of how gravitational wave antennas work and what they measure. The conventional means of analyzing gravitational wave observations focuses almost exclusively on fitting those observations to specific, well-modeled sources, limiting our ability to "discover" anything beyond that which we have presupposed. Here I will describe recent research whose goal is to provide a physical description and characterization of gravitational wave antennas, analogous to the corresponding description of radio antennas, and a means of analyzing and interpreting gravitational wave observations that is simultaneously independent and indicative of a source or source model.

Vicky Kalogera
  May 1

Fred Adams
    University of Michigan
    Constraints on the Birth Environment of the Solar System

Most stars -- and hence most solar systems -- form within groups and clusters. The first objective of this talk is to explore how these star forming environments affect solar systems forming within them. The discussion starts with the dynamical evolution of young clusters with N = 100 - 3000 members. We use N-body simulations to study how evolution depends on system size and initial conditions. Multiple realizations of equivalent cases are used to build up a robust statistical description of these systems, e.g., distributions of closest approaches and radial locations. These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models determine the efficacy of radiation in removing disk gas and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine probabilities for solar system disruption. The result of this work is a quantitative determination of the effects of clusters on forming solar systems. The second objective of this talk is to use these results to place constraints on the possible birth environments for our solar system.

Will Farr
  May 8

Margaret Hanson
    University of Cincinnati
    Improving mass and age estimates of stellar clusters, near and far

Stellar clusters equip astronomers with powerful benchmarks to derive the history and evolution of the galaxies they reside in, provided accurate values of cluster mass, age and distribution can be obtained. However, traditional methods suffer severe challenges in locating and then deriving mass and age estimates of stellar clusters within our own Milky Way galaxy, as well as more distant, unresolved clusters in galaxies beyond our Local Group. We have spent several years developing a robust suite of programs that combine the power of Monte Carlo methods with sophisticated statistical inference. I will describe how our novel analysis will successfully overcome many of the historical challenges found in traditional methods for locating and deriving characteristics of stellar clusters, near and far.

Farhad Zadeh
  May 15

Tamara Rogers
    The University of Arizona
    Internal Gravity waves in Stellar Interiors

Internal Gravity Waves (IGW), such as those observed in our own atmosphere surely exist in the solar radiative interior and in the radiative envelopes of massive stars. In the Sun, these waves have received a great deal of attention recently, both because of their ability to transport angular momentum and because of their observational potential. In more massive stars these waves could affect the orbital properties of extra-solar giant planets. In this talk I will present work I have done on IGW both in the solar radiative interior and in massive star envelopes; comparing and contrasting the spectra and amplitudes of the waves and commenting on their dynamical consequences.

Adrian Barker
  May 21

Don Figer
    Rochester Institute of Technology
    Single-photon Array Detectors

Single-photon array detectors promise the ultimate in sensitivity by eliminating read noise. These devices could provide extraordinary benefits for photon-starved applications, e.g., seeing the first stars in the Universe, imaging exoplanets, fast wavefront sensing, and probing the human body through optical transilluminescence. Recent implementations are often in the form of sparse arrays that have less-than-unity fill factor. For imaging, fill factor is typically enhanced by using microlenses, at the expense of photometric and spatial information loss near the edges and corners of the pixels. Other challenges include afterpulsing and the potential for photon self-retriggering. Both effects produce spurious signal that can degrade the signal-to-noise ratio. This talk reviews development and potential application of single-photon-counting array detectors, including highlights of initiatives in the Center for Detectors at the Rochester Institute of Technology. Current projects include single-photon-counting imaging detectors for the Thirty Meter Telescope, a future NASA terrestrial exoplanet mission, and imaging LIDAR detectors for planetary and Earth science space missions.

CIERA Colloquium
Vicky Kalogera
  May 29

Simona Vegetti
    Massachusetts Institute of Technology
    Quantifying mass substructure in early-type galaxies

The cold dark matter (CDM) paradigm predicts that a significant number of substructures, with a steeply rising mass function towards lower masses, populates the dark halo of galaxies. In the Milky Way, however, of order 104 substructures are predicted inside the virial radius, whereas only about 20 have been so far observed. This poses a major challenge to the CDM paradigm. New and independent methods are, therefore, required to assess the level of mass substructure in galaxies in the Local Universe and beyond. One such method will be discussed in this talk, which consists of three parts.
First, I will present a newly developed fully Bayesian adaptive-grid method, that uses all the information contained in the surface brightness distribution of highly magnified Einstein rings and arcs, to detect and precisely quantify mass substructure in single lens galaxies, even in case of very high mass-to-light ratios. The method has been successfully tested on simulated data and is able to detect mass substructures with Msub>= 108??, when positioned on a highly-magnified Einstein ring.
Second, I will discuss how to combine, in a statistical sense, the detections of mass substructures from multiple lens galaxies to constrain the substructure mass fraction ƒsub and the slope of the mass function ?. Third, I will present new results on the level of mass substructure.

Daryl Haggard
  June 5
  *3:00pm
 

Bill Cotton
    National Radio Astronomy Observatory (NRAO)
    Faint Radio Source Counts: Deeper Understanding Through Confusion

The extragalactic radio sky is dominated by two distinct populations of objects. The brighter sources are predominately radio loud Active Galactic Nuclei (AGNs), the brightest of which are visible anywhere in the universe. Below about 1 milliJansky, the population becomes dominated by sources powered by massive star formation in galaxies. Source counts of these populations can constrain their evolution. Recent theoretical developments suggest that the cosmic evolution of AGNs and star formation are coupled through AGN feedback.
A recent balloon borne CMB experiment, ARCADE 2 (Fixen et al., 2011) , measured an excess brightness near 3.5 GHz which they attributed to a hitherto unknown population of very faint sources. We have recently completed very deep observations using the newly refurbished Jansky Very Large Array to test the ARCADE 2 results and to constrain the evolutionary laws for both star forming galaxies and AGNs. These observations make use of the distribution of "confusion" when multiple sources contribute to each resolution element. Our results directly demonstrate for the first time the similarities between the evolution of AGNs and star formation out to a z~1 and show that the ARCADE 2 results are unlikely to be from sources associated with galaxies (or even stars).

Farhad Zadeh


Past Astrophysics Seminars