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Northwestern University

CIERA Special Seminars 2015-2016

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

Summer Quarter 2016

  Date/Time      Visitor Host
  Aug. 5
  Tech F227A

Evan Schneider
    University of Arizona
    Simulating Galactic Winds on Supercomputers Using Cholla

In an era of increasingly complex computer architectures, codes that take full advantage of available computational power are essential. Leveraging the power of today's supercomputers will allow numerical simulations to continue to provide new theoretical insights across a broad range of topics in astrophysics, including the formation and evolution of galaxies. In this talk, I will discuss the code Cholla, and the particular problem within galaxy evolution that I am tackling with it. Cholla is a new hydrodynamics code that I have written and developed to harness the power of the graphics processing units (GPUs) responsible for much of the speed of modern supercomputers. Using Cholla, I have run a set of extremely high resolution simulations of the interaction between cool and hot gas in multiphase galactic winds. The results of these simulations suggest that mass-loading of the warm and hot phases of galactic winds is a highly efficient process. Additionally, I find that similar amounts of mass and momentum are incorporated into wind phases across a broad range of densities and temperatures. Both findings have broad implications for our current theories regarding wind expulsion and recycling that ideally can be leveraged in the future to improve cosmological simulations.

Claude-André Faucher-Giguére


Spring Quarter 2016

  Date/Time      Visitor Host
  Apr. 13
  Tech F210

Ethan Vishniac
    Johns Hopkins University
    Editor-in-Chief of The Astrophysical Journal

    Dynamos: How Do They Work?

Magnetic fields are ubiquitous in the universe, with significant power on the size of the object that contains them. This includes planets, stars, and the disks of the galaxies. Conventional treatment, using mean field dynamo theory in which magnetic field growth is driven by kinematic helicity, "twistiness", of the underlying fluid turbulence, fails to explain observations of stellar magnetic fields. There are also strong theoretical arguments against its application to realistic systems. I will show how the transport of magnetic helicity in turbulent rotating systems provides a natural mechanism for the rapid growth of large magnetic fields. This leads to a generic prediction for the strength of stellar magnetic fields as a function of rotation rate which matches observations.

Fred Rasio
  May 12
   Tech F160

B-G Andersson
    Status of, and Science With, the Stratospheric Observatory For Infrared Astronomy (SOFIA)

The SOFIA project flies a significantly modified Boeing 747SP, carrying a 2.7m telescope, into the stratosphere 3-4 times a week to perform astronomical observations, primarily in the mid- to far-infrared. The observatory is now in the middle of executing Cycle 4 of its general user observations, employing all the first generation instruments, and covering all areas of astronomy from Solar System studies to extra-galactic astronomy. The Cycle 5 Call for Proposals was released at the end of April, offering all first and second generation instruments, including the Far-IR polarimeter HAWC+. I will review the status of the project and provide some science highlights from the first few observing cycles.

Giles Novak
  May 23
   Tech F160

Jonathan Stern
    Max Planck Institute for Astronomy (MPIA)
    New Constraints on the Acceleration of AGN-driven Galactic Winds and on the Density
    Structure of the CGM

A vital ingredient in our theoretical understanding of AGN-driven galactic winds is the nature of the physical mechanism which accelerates them, such as radiation pressure on dust grains or the ram pressure of an expanding hot gas bubble. I will demonstrate that the nature of the acceleration mechanism can be constrained from the ratios of emission lines which originate from the cool (T~10^4K) gas illuminated by the AGN. I will then show that observed line ratios in UV-selected quasars suggest that radiation pressure dominates at all scales (0.1 pc -- 10 kpc). I will demonstrate that this result is apparently in conflict with the large momentum outflow rates measured by many previous studies, and discuss how this conflict might be resolved.

In the second part of my talk I will argue for a new phenomenological model for the T~10^4K circum-galactic medium (CGM), where the cool CGM spans a large range of gas densities, and small high-density clouds are hierarchically embedded in large low- density clouds. I will then constrain the relation between gas density and physical size, or the `density structure', using ionic column measurements from the COS-Halos survey of low-redshift ~L* galaxies. I will show that this density structure is inconsistent with self-gravity, and thus its physical origin is unclear. I will also present a 3D model of the cool CGM based on our results, which can provide a benchmark for the CGM structure in hydrodynamic simulations.

Claude-André Faucher-Giguère
  Jun. 2
   Dearborn 23

Juan Diego Soler Pulido
    Service d’Astrophysique, CEA. France
    Magneto is Not a Hero: The Galactic Magnetic Field and the Formation of Molecular     Clouds.

ESA's Planck satellite has produced the first all-sky map of the polarized emission from dust at submillimetre wavelengths. Compared with earlier ground-based and balloon-borne observations this survey is an immense step forward in sensitivity, coverage, and statistics. It provides new insight into the structure of the Galactic magnetic field, as well as the first statistical characterization of one of the main foregrounds to CMB polarization.

I will summarize the main results of the Galactic magnetic field studies by the Planck Consortium, particularly focusing on the correlation between the magnetic field and the column density structures, its relation to previous studies based on the observation of starlight polarization and the Zeeman effect, and its implications for our understanding of the formation of density structures in the interstellar medium.

Giles Novak


Winter Quarter 2016

  Date/Time      Visitor Host
  Jan. 11
  Tech F160

Tim Brandt
    Institute for Advanced Study
    Discovering and Characterizing Exoplanets with High-Contrast Spectroscopy

Advances in adaptive optics and infrared instrumentation now enable us to see young exoplanets millions of times fainter than their host stars. By collecting photons emitted by these worlds, imaging allows us to measure the chemistry and physical states of their atmospheres. Now, a new generation of experiments is combining upgraded adaptive optics with integral-field spectrographs (IFSs) to discover and characterize fainter worlds closer to their host stars. I will present the results of recent high-contrast surveys and the scientific promise of this new generation of instruments, with a particular focus on the CHARIS IFS for the Subaru telescope. CHARIS will be the only instrument of its class in the northern hemisphere and will have the broadest spectral coverage of any high-contrast IFS; it will provide unique sensitivity to close-in exoplanets and present new data analysis challenges. CHARIS is now being built and will begin operations this summer. It will commence its first two year, 20 night survey in early 2017, taking spectra of giant exoplanets and searching about 100 stars for new companions.

Vicky Kalogera
  Jan. 14
  Tech F160

Ian Stephens
    Boston University
    Uncovering the Link Between Magnetic Fields and Star Formation

The role of magnetic fields during star formation — at all scales — has been a heated topic for decades. The magnetic field morphology can be discerned from polarized light emitted from dust grains. In this talk I will discuss the current observational constraints on the magnetic field morphology at the scales of star-forming filaments and at the scales of protostellar outflows and disks. The current constraints are very limited, but large polarimetric surveys will finally be possible within the next couple years. Specifically, ALMA, SOFIA, and BLAST have just been upgraded to probe the magnetic field morphology. These new results will answer fundamental questions about the role of magnetic fields during the star formation process from scales of molecular clouds (10s of pc) to the protostellar disks (10s of AU).

Giles Novak and Farhad Yusef-Zadeh
  Jan. 19
  Tech F160

Jennifer Yee
    Harvard-Smithsonian Center for Astrophysics
    Probing Small Planets at Several AU with Microlensing

Small, rocky planets at a few AU are noticeably absent from the census of exoplanets taken by radial velocity and transit surveys (e.g. Kepler). Unlike other techniques, microlensing has the unique ability to find such planets and provides a vital component of a complete exoplanet census. Over the next decade, we will see microlensing planet discoveries reach a scale comparable to results from radial velocity and transits. I will discuss microlensing’s potential for revolutionizing our understanding of exoplanets, with a particular focus on the Spitzer microlensing parallax program, the KMTNet project, and the upcoming WFIRST microlensing mission.

Vicky Kalogera
  Jan. 22
  Tech L324
  Joint Seminar
  with EECS

Greg Mosby
    University of Wisconsin-Madison
    Reliable and Accurate Stellar Populations of Quasar Host Galaxy Spectra and the
    RSS-NIR Detector System

To study the effect of supermassive black holes (SMBHs) on their host galaxies it is important to study the hosts when the SMBH is near its peak activity. This can be done by investigating the host galaxies of high luminosity quasars by obtaining optical spectra at positions offset from the nucleus where the relative contribution of the quasar and host are comparable. However, at these extended radii the galaxy surface brightness is low and the resulting spectrum has such low S/N that it hinders analysis with standard stellar population modeling techniques. To address this problem, we have developed a method that can recover galaxy star formation histories (SFHs) from rest frame optical spectra with S/N ~ 5 Å^-1. We use the machine learning technique diffusion k-means to tailor the stellar population basis set, composed of 4 broad age bins, and it is successful in recovering a range of galaxy SFHs. We use synthetic data to compare results of our novel method with previous techniques. Our new method has the advantage in recovering information from QHGs and could also be applied to the analysis of other low S/N galaxy spectra such as that typically obtained for high redshift objects and integral field spectroscopic surveys. To extend our work done in the optical, the use of the next generation NIR spectrographs will be crucial. I introduce the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT) and its NIR arm being built at the University of Wisconsin-Madison. We have made significant progress in the optimization of the detector system using a H2RG HgCdTe array with the SIDECAR ASIC and the IUCAA ISDEC controller card. Specifically, we have tuned the SIDECAR ASIC to achieve a CDS read noise of < 20 e- for the detector system in our test dewar for a readout speed of 200 kHz. Testing our science grade detector in the instrument dewar is nearing completion and includes measurement and characterization of persistence.

Mel Ulmer and
Hooman Mohseni
  Jan. 25
  Tech F160

Branimir Sesar
    Max Planck Institute for Astronomy
    From Big Data Catalogs to Astrophysical Insights

In the next five years the Galactic astronomy is poised for significant breakthroughs, fueled by an unprecedented increase in the quality and quantity of astrophysical data provided by surveys such as Pan-STARRS1, Palomar Transient Factory, Zwicky Transient Facility, and GAIA. However, it is important to keep in mind that these surveys provide catalogs, not answers. I will show how time-domain data sets, machine learning techniques, and forward-modeling of data can be used to provide valuable insights on (1) the accretion history of the Milky Way, (2) the number of faintest Milky Way satellites, (3) the Galactic potential, and (4) the nature of a mysterious stellar stream.

Vicky Kalogera
  Jan. 27
  Tech F160

Ryan Foley
    University of Illinois
    Continuing the Legacy of Supernova Cosmology

Type Ia supernovae (SNe Ia) are superb distance indicators and are used to map the expansion history of the Universe. In the last millennium, astronomers used observations of SNe Ia to find that the Universe's expansion is currently accelerating. This discovery resulted in the Nobel Prize for Physics in 2011. Since this initial discovery, we have used SNe Ia to loosely constrain the nature of "dark energy," which drives the accelerated expansion. To improve our dark energy constraints beyond our current basic understanding, we must design new and better SN surveys and techniques. I will present the Foundation Supernova survey, a new high-fidelity, low-redshift (z < 0.1) SN survey started in 2015 that will replace the current heterogenous low-redshift sample and reduce the (currently) largest uncertainties for SN cosmology. I will describe the survey, our implementation, and first results. I will present the "velocity-color relation" and show how we can significantly improve the distance precision of SNe Ia. I will also discuss the next major leap in SN cosmology, WFIRST, which will launch in about a decade. I will show the first simulations of the WFIRST SN survey and make some suggestions for how to further improve this mission. With the combination of the Foundation survey, WFIRST, and new physical understanding like the velocity-color relation, SNe Ia will continue to be a premier cosmological probe, continuing the legacy started decades earlier.

Vicky Kalogera
  Feb. 1
  Tech F160

Raffaella Margutti
    Harvard University
    Exploring the unknown: Massive Stars at the very end of their lives

Observations are drawing a complex picture of the latest stages of massive stars evolution and their explosions. In this seminar I concentrate on one among the least understood aspects of stellar evolution, adopting an observational perspective: How do massive stars loose a significant fraction of their mass in the years preceding the explosion? I address this question by taking advantage from panchromatic observations of two remarkable explosions: (i) the puzzling, double explosion of SN2009ip in 2012; (ii) and the normal envelope-stripped SN2014C, which experienced a dramatic metamorphosis and evolved from Type I into Type II supernova over a timescale of a few months, thus violating the supernova classification scheme that hat has existed for decades.

Vicky Kalogera
  Feb. 25
  Tech L324/348

Jamil A. Shariff
    Case Western Reserve University
    Searching for the Echoes of Inflation from Antarctica

The standard Big Bang cosmological model + inflation is able to explain nearly all of the features of our observable universe with remarkable precision. However, direct evidence of inflation remains elusive, and much about the inflationary paradigm is still a mystery. What physical mechanism drove inflation, and at what energy scales did this occur? In this talk I will relate the story of Spider, an ambitious balloon-borne experiment designed to detect B-modes of polarization in the Cosmic Microwave Background on degree angular scales. If present, these features would be a signature of the primordial gravitational waves predicted to have been produced by inflation in the early universe. Spider had its first long-duration balloon flight around Antarctica in January 2015. Using vivid visuals, I will illustrate the Antarctic adventure and the vagaries of ballooning. I will conclude by describing the progress of the analysis of data from the first flight, which is well underway.

Giles Novak


Fall Quarter 2015

  Date/Time      Visitor Host
  Sept. 15
  Tech F160

Adrian Hamers
    Leiden Observatory
    Secular dynamics of high-order multiple systems
    — applications to circumbinary planets and Hot Jupiters

We present a new efficient method to study the long-term dynamics of hierarchical multiple systems such as high-multiplicity stellar systems and multiplanet systems, without restrictions on the number of bodies nor on the hierarchical configuration. We apply this method to hierarchical quadruple systems consisting of a hierarchical triple orbited by a fourth body, in different contexts. First, we consider the general dynamics, and we characterise the behavior of the system in terms of a ratio of Kozai-Lidov (KL) time-scales applied to different binary pairs. Second, we apply the method to explain the currently observed lack of transiting circumbinary planets around short-period binaries. Finally, we present preliminary integrations of multiplanet systems, in which secular chaos gives rise to high eccentricities and tidal shrinkage in the innermost planet's orbit, giving a pathway for producing Hot Jupiters. Our new method allows for much faster and longer integrations compared to previous direct N-body integrations, making it an excellent tool for better understanding the conditions for producing Hot Jupiters in multiplanet systems through secular chaos.

Fabio Antonini
  Oct. 8
  Tech F160

Andrew W. Smith
    University of Maryland, College Park and NASA/GSFC
    Revealing The Violent Universe: Current Results and The Future of Gamma-ray Astronomy

The last 10 years have seen a dramatic explosion of important science results from space based instruments (~0.3-300 GeV) such as Fermi-LAT and AGILE, as well as ground based instruments (~100-5000 GeV) such as VERITAS, HESS, and MAGIC. These instruments have provided an in-depth exploration of particle acceleration in the the most energetic known cosmic sources (supernova remnants, gamma-ray bursts, pulsars, AGN, and the center of our own Galaxy), as well as providing important clues to the mystery of high energy cosmic ray origins. In this talk I will review the most important recent results from these instruments as well as providing a preview of the next generation of both ground and space based gamma-ray observatories. The future for ground based instruments is already being cemented with the completion of the HAWC array in Mexico, as well as the upcoming CTA array planned for both the Northern and Southern Hemispheres. The successor to Fermi-LAT is also in the works, with several mature mission concepts progressing forward already. I will review some of these concepts and provide an outlook for the future of this exciting and dynamic field.

Farhad Yusef-Zadeh
  Nov. 11
  Tech F160

Tuguldur Sukhbold
   University of California Santa Cruz
    Core-Collapse Supernovae from Neutrino-driven Explosions

Nucleosynthesis, light curves, explosion energies, and remnant masses are calculated for a grid of supernovae resulting from massive stars with solar metallicity and masses between 9-120 solar masses. The full evolution is followed using an adaptive reaction network of up to 2000 nuclei. A novel aspect of the survey is the use of a one-dimensional neutrino transport model for the explosion. This explosion model has been calibrated to give the observed energy for SN 1987A and Crab supernova. As a result of using a calibrated central engine, the final kinetic energy of the supernova is variable and sensitive to the structure of the presupernova star. Many progenitors with extended core structures do not explode, but become black holes, and the masses of exploding stars do not form a simply connected set. The resulting nucleosynthesis agrees reasonably well with the sun provided that a reasonable contribution from Type Ia supernovae is also allowed, but with a deficiency of light s-process isotopes. The resulting neutron star IMF has a mean gravitational mass near 1.4 solar masses. The average black hole mass is about 9 solar masses if only the helium core implodes, and 14 solar masses if the entire presupernova star collapses. Only ~10% of supernovae come from stars over 20 solar masses and some of these are Type Ib or Ic. Some useful systematics of Type IIp light curves are explored.

Ron Taam

  Nov. 16
  Tech F160

John Tobin
   Leiden Observatory
    Toward Understanding the Formation of Proto-planetary Disks and Multiple Star Systems

Disks and multiple star systems are thought to form early in the star formation process. Conservation of angular momentum enables the formation of protostellar disks; protostellar companions can also form if the disk is gravitationally unstable. However, theory and simulations have suggested that large, massive disk formation may be difficult due to removal of angular momentum by magnetic fields. Observations are now sensitive enough to begin testing these predictions, and Keplerian disks have been discovered around newly formed star systems. However, the numbers of Keplerian disks remain small and the frequency of companion protostars on scales < 150 AU is poorly known. To greatly expand our knowledge on the frequency of disks and multiplicity, I am leading a 264 hour VLA large program to observe all protostars in a single star forming region, the Perseus molecular cloud (d 230 pc, N 80), with a spatial resolution of 15 AU. With these data, we are identifying new protostellar disk candidates and closer multiple systems than have ever been previously identified. We have found evidence for a bimodal distribution of protostar separations, suggesting that multiple formation mechanisms are acting on different scales. Finally, we also see indications of evolution in the separation distribution for younger protostars relative to those that are more-evolved. These results open the door to expanded multiplicity and disk surveys to determine if the trends found in Perseus are applicable to other star forming regions.

Farhad Yusef-Zadeh

  Nov. 23
  Tech F160

Morgan MacLeod
    University of California, Santa Cruz
    Common Envelope and the Formation of Close Neutron Star Binaries

How do close neutron star binaries form? Many aspects of our understanding of the complex channels through which compact binaries form remain uncertain. This talk focuses on a crucial phase in the formation of close binary pairs of neutron stars: a common envelope episode. During the common envelope phase, one star evolves to engulf its companion in a shared gaseous envelope. Before this envelope is ejected, drag forces pull the two stellar cores into a tighter orbit. But the neutron star can also accrete from the surrounding gas. This talk explores the hydrodynamics of the common envelope phase in order to model the metamorphosis of the neutron star and the binary orbit.

Fabio Antonini

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Past CIERA Special Seminars