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Northwestern University

CIERA Special Seminars 2014-2015

*Tech = Technological Institute (2145 Sheridan Road)
**Db = Dearborn Observatory (2131 Tech Drive)

Spring Quarter 2015

  Date/Time      Visitor Host
  May 7
  Tech F210

Jay Strader
    Michigan State University
    Black Holes in Globular Clusters

Hundreds of stellar-mass black holes are expected to form in the early lifetime of a typical globular star cluster, with many predicted to be ejected through close interactions in the cluster core following mass segregation. However, the efficiency of this ejection process in real globular clusters is poorly understood. The existence of black holes in globular clusters would have broad implications for the demography of black holes, gravitational wave sources, and the evolution of globular clusters. Our group has initiated a survey using deep radio continuum, X-ray, and optical data to search for stellar and intermediate-mass black holes in Milky Way globular clusters. I will discuss the current status of black hole candidates in several clusters, planned follow-up observations, and empirical constraints on the relative abundance of neutron stars and black holes in globular clusters.

Fred Rasio and Sourav Chatterjee

  June 3
  Tech F160

Pablo Marchant
   University of Bonn
   The Interplay of Internal Mixing, Tides and Mass Transfer in Massive Binaries

Recent observations have shown that the majority of massive stars reside in close pairs, which will strongly interact through mass transfer. After interaction, a significant amount of these objects will appear as single stars, hiding their former nature to surveys that attempt to exclude binaries using radial velocity variations. Understanding observed populations of massive stars is then intrinsically related to an understanding of the integrated properties of binary systems. In this talk I will present results from simulations of grids of binaries appropriate for comparison to the LMC, including the effects of stellar rotation and tides. I focus on the occurrence of Nitrogen enrichment due to accretion of CNO processed material, and tidal spinup, both tracers of binary interaction, and demonstrate that rotational mixing plays only a minor role in the final post-interaction products. I will also show early results on the modelling of contact systems, allowing for a better assesment of the initial conditions for mergers.

Francesca Valsecchi


Winter Quarter 2015

  Date/Time      Visitor Host
  Jan. 16
  Tech F210

Maria Cunningham
    University of New South Wales
    The probability density function in molecular gas in the G333 and Vela C
    molecular clouds

The probability density function (PDF) is a simple analytical tool for determining the hierarchical spatial structure of molecular clouds. It has been used frequently in recent years with dust continuum emission, such as that from the Herschel space telescope and ALMA. These dust column density PDFs universally show a log-normal distribution in low column density gas, characteristic of unbound turbulent gas, and a power-law tail at high column densities, indicating the presence of gravitationally bound gas. We have recently conducted a PDF analysis of the molecular gas in the G333 and Vela C giant molecular cloud complexes, using transitions of CO, HCN, HNC, HCO+ and N2H+.
The results show that CO and its isotopologues trace mostly the log-normal part of the PDF, while HCN and HCO+ trace both a log-normal part and a power law part to the distribution. On the other hand, HNC and N2H+ mostly trace only the power law tail. The difference between the PDFs of HCN and HNC is surprising, as is the similarity between HNC and the N2H+ PDFs. The most likely explanation for the similar distributions of HNC and N2H+ is that N2H+ is known to be enhanced in cool gas below 20K, where CO is depleted, while the reaction that forms HNC or HCN favours the former at similar low temperatures. The lack of evidence for a power law tail in 13CO and C18O, in conjunction for the results for the N2H+ PDF suggest that depletion of CO in the dense cores of these molecular clouds is significant. In conclusion, the PDF has proved to be a surprisingly useful tool for investigating not only the spatial distribution of molecular gas, but also the wide scale chemistry

Laura Fissel
  Jan. 22
  Tech F160

Frederick Davies
    The Fluctuating Intergalactic Ionizing Background Across Cosmic Time

Understanding the ionizing background of the universe is crucial to interpreting observations of intergalactic gas in the context of large-scale structure. The epochs of H and He reionization, currently under intense observational and theoretical investigation, set the boundary conditions for the propagation of ionizing photons in the universe. Using novel 1D and semi-numerical 3D calculations, we find that fluctuations in the ionizing background due to rare or clustered sources can be very important, in contrast to common assumptions in previous work. We show that fluctuations in the radiation field cause the mean free path of ionizing photons to vary, leading to large-scale correlations that may explain recent observations of the H and He Lyman-alpha forests following their respective reionization epochs.

  Jan. 28
  Tech F160

Nicholas McConnell
    University of Hawaii
    Puzzles in Massive Galaxy Assembly

Giant elliptical galaxies are the most massive stellar systems in the present-day universe. The full story of their origins has eluded astronomers, even while high-redshift surveys have unearthed statistical trends in star formation and galaxy growth across cosmic time. Are giant ellipticals simply the most massive participants in a common sequence of growth and quenching, or are they the relics of exceptional objects that formed violently in the early universe?
I will describe ongoing deep spectroscopic surveys of nearby ellipticals, designed to reveal two of their most extreme components: their low-mass stellar populations and their supermassive black holes. (1) The Black Hole Safari will measure stellar kinematics and central black hole masses (MBH) in over 30 giant elliptical galaxies, spanning a range of cosmic environments. The resulting census of black holes in the local universe will assess whether the linear relation between MBH and stellar spheroid mass arises naturally from hierarchical merging, and it may shed light upon the origins of recently-discovered overmassive black holes. (2) The most massive elliptical galaxies exhibit an extremely bottom-heavy stellar initial mass function (IMF), at odds with galaxy formation models where they are assembled from mergers of lower-mass systems. One possible means of reconciliation would be the detection of radial gradients in the IMF. I will present new measurements of IMF-sensitive spectral features with unprecedented radial coverage, suggesting that massive ellipticals' extreme stellar populations are indeed confined to small radii.

Daniel Angles-Alcazar
  March 4
  Tech F227 A

James Guillochon
    A Dark Year for Tidal Disruption Events

The disruption of a main-sequence star by a supermassive black hole results in the initial production of an extended debris stream that winds repeatedly around the black hole, producing a complex three-dimensional figure. When this stream self-intersects, accretion onto the black hole begins; this process slows down some flares and naturally leads to a "dark period" in which the flare is not observable for months to years. In my talk I'll discuss how this affects which flares are the most likely to be found by present-day surveys, and how they suggest a large fraction of TDEs, perhaps as large as 90%, are not identified. This implies that the true rate of tidal disruptions may be significantly higher than what is suggested by the number in the observed sample.

Fred Rasio



Fall Quarter 2014

  Date/Time      Visitor Host
  Oct. 23
  Tech F210

Rob Ferdman
    McGill University
    Using pulsars in a Galactic-scale gravitational-wave detector

At the forefront of observation astrophysics is the effort to directly detect gravitational waves (GWs), which remains a "holy grail" in validating Einstein's general theory of relativity. Along with ground-based GW detectors such as Advanced LIGO/VIRGO, pulsar timing has become a serious contender for making the first such detection. This will be done using a so-called Pulsar Timing Array (PTA), which uses the distances between Earth and several millisecond-period pulsars (MSPs) as arms of a Galactic-scale GW detector. It aims to measure the common effect of a stochastic GW background on the pulse arrival times of an ensemble of MSPs, thought to be due to coalescing supermassive black holes at the centers of merging galaxies at high redshifts. PTAs are sensitive to the nanohertz frequency region of the GW spectrum, and are thus complementary to the larger frequency ranges probed by ground-based detectors, which will be sensitive to sources such as merging NS pairs.
This is an international undertaking; the North American wing of this effort, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), uses the Green Bank Telescope (GBT) in West Virginia and the Arecibo Telescope in Puerto Rico to regularly observe over 30 MSPs as part of a PTA. In this talk, I will discuss how we will be able to detect GWs with pulsar timing, and describe recent progress. I will also briefly describe ongoing and future instrumentation that will greatly benefit this work, including the 100-m class CHIME telescope, currently being constructed in British Columbia.

Vicky Kalogera and Fred Rasio
  Oct. 27
  Tech F160

Laura Sampson
    Montana State University, Bozeman
    Testing Relativity with the Parameterized Post-Einsteinian Framework

In the very near future, ground-based detectors and pulsar timing arrays will begin to make regular detections of gravitational waves. Among the most interesting science we will be able to accomplish with these detections is in the area of testing General Relativity. In this talk, I give a brief overview of tests of GR that have been performed to date, and then spend the bulk of the time discussing the new tests we will be able to perform using gravitational waves. In particular, I focus on template-based searches for deviations from GR, using a model-independent tool called the parameterized post-Einsteinian framework.

Vicky Kalogera and Tyson Littenberg
  Nov. 3
  Tech F160

Ondrej Pejcha
    Princeton University
    The Rugged Landscape of the Core-Collapse Supernova Explosions

All massive stars end their lives with core collapse and many as supernova explosions. Despite observations of thousands of supernovae, detailed numerical calculations and theoretical efforts, the mechanism of explosion is poorly understood and perhaps even unknown. By parameterizing the systematic uncertainty in the explosion mechanism and by using spherical quasi-static evolutionary sequences for many hundreds of progenitors over a wide range of metallicities, we study how the explosion threshold maps onto observables - fraction of successful explosions, remnant neutron star and black hole mass functions, explosion energies, nickel yields - and their mutual correlations. Successful explosions are intertwined with failures in a complex but well-defined pattern that is not well described by the progenitor initial mass and is tied to the pre-collapse structure of the progenitor star. Other supernova properties show a similar pattern. Finally, to facilitate better comparison of the theory and the data, we present a new method to extract parameters from supernova light curves and expansion velocities in a statistically correct way.

Vicky Kalogera
  Nov. 24
  Tech F160

Titos Matsakos
    University of Chicago
    Classification of Magnetized Star-Planet Interactions:
    Bow Shocks, Comet-like Tails, and In-spiraling Streams

Stellar irradiation is believed to drive outflows from the surface of close-in exoplanets, a phenomenon that is supported by transit observations of Hot Jupiters. Assuming planetary magnetospheres similar to those of our solar system, such outflows are expected to be magnetized. In addition, the environment of short period orbits consists of the sweeping stellar wind plasma that is known to attain super-sonic velocities. This framework suggests the manifestation of complex magnetized star-planet interactions in systems harboring Hot Jupiters. In this work, we perform a series of parameterized 3D magneto-hydrodynamic numerical simulations in order to provide a classification for the different types of interactions that may occur. We incorporate stellar and planetary outflows that are consistent with detailed physical models and investigate case by case the exhibited dynamics.

Francesca Valsecchi
  Dec. 1
  Tech F160

Benedikt Diemer
    University of Chicago
    The (Non-)Universality of Halo Density Profiles

The density profiles of dark matter halos are an essential input for models of galaxy formation, as well as for the interpretation of numerous observations such as weak and strong lensing signals. The profiles are commonly thought to follow a simple, universal shape, and only depend on two parameters, mass and concentration. Using a large suite of cosmological simulations, I will show that the outer halo density profiles depend on an additional parameter, the mass accretion rate, and present an accurate new fitting formula that takes this dependence into account. I will further discuss the question of universality, and show that the definition of the halo boundary plays a crucial role. Similarly, halo concentrations are usually described as a universal function of mass and redshift. Instead, I will present a model in which concentration depends on an additional parameter: the local slope of the matter power spectrum. I will demonstrate that this model accurately (to better than 10-15%) describes simulated concentrations over a large range of redshifts, halo masses and cosmological parameters, and is in excellent agreement with the recent observations of the CLASH cluster survey.

  Dec. 11
  Tech F227A

Konstantin Pavlovskii
    University of Alberta
    A toy model of fast mass transfer from giant donors

Binary population synthesis studies need a quick technique to determine whether a certain giant will initiate a common envelope event in a binary system when it overfills its Roche lobe, that would not require any real-time detailed stellar simulations. I present a toy model that addresses this question with a reasonable accuracy.

Vicky Kalogera

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