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Northwestern University

CIERA Special Seminars 2016-2017

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

Spring Quarter 2017

  Date/Time      Visitor Host
  April 21
  Tech F160

Wei Zhu
    Ohio State University
    Detection of a 1 Earth-mass Planet at 1 AU around a Brown Dwarf

We report the discovery of a 1 Earth-mass planet in a 1 AU orbit around a brown dwarf host using the microlensing parallax method (arXiv:1703.08548). This is the third published planet from the on-going Spitzer microlensing program. I will first explain the principle behind the microlensing parallax method and, in particular, why it can measure the masses in a model independent way. Then I will present some significant and unique discoveries from the Spitzer microlensing program, and discuss our future plans. The status of the K2 microlensing program (K2 campaign 9) and its scientific potential will also be discussed.

Fred Rasio
  May 15
  Tech F160

Stephen Zepf
    Michigan State University
    The IMF in Early-Type Galaxies and also their UV Upturn

I will discuss several programs aimed at understanding how early-type galaxies and their globular clusters form. The stellar initial mass function (IMF) in early type galaxies is a critical parameter for many aspects of galaxy evolution, yet it is difficult to constrain because all of the massive stars in the galaxies are long dead, and the lowest mass stars contribute very little light. I will present results from our study of the number of low-mass X-ray binaries in the field of eight early-type galaxies. We show that the data are consistent with an IMF in which the relative number of massive stars does not vary with early-type galaxies properties. I will also discuss a project aimed at understanding the far-ultraviolet emission from globular clusters. We show that metal-rich globular clusters in a number of galaxies are much brighter in the FUV than expected from simple stellar populations models, but are well fit by models with extensive "second-generation" populations enhanced in helium. This demonstrates the ubiquity of the second generation phenomena in globular clusters, and also suggests that the FUV upturn in elliptical galaxies may originate in globular clusters.

Raffaella Margutti,
Vicky Kalogera
  Jun. 6
  Tech F160

Anja Feldmeier-Krause
    University of Chicago
    The Milky Way’s Nuclear Star Cluster: Stellar Populations and Kinematics

Nuclear star clusters are common objects and are detected in ~75% of nearby galaxies, including the Milky Way. Because the Milky Way nuclear star cluster (MWNSC) is at a distance of only 8 kpc, we can spatially resolve its stellar populations and kinematics much better than in external galaxies.

The MWNSC contains mostly old (>5 Gyr) stars, but also a young (<10 Myr) population. I will discuss the spatial and kinematic distribution of the different populations, and present the metallicity distribution of the old stars. In addition, I will present a dynamical orbit-based model for the MWNSC, to derive its mass and constrain the central Galactic potential.

Farhad Yusef-Zadeh


Winter Quarter 2017

  Date/Time      Visitor Host
  Feb. 6
  Tech F160

Cullen Blake
    University of Pennsylvania
    MINERVA-Red: An Intensive Survey for Planets Orbiting the Nearest
    Low-mass Stars to the Sun

Recent results from Kepler and ground-based exoplanet surveys suggest that low-mass stars are host to numerous small planets. Since low-mass stars are intrinsically faint at optical wavelengths, obtaining the Doppler precision necessary to detect these companions remains a challenge for existing instruments. I will describe MINERVA-Red, a project to use a dedicated, robotic, near-infrared optimized 0.7 meter telescope and a specialized Doppler spectrometer to carry out an intensive, multi-year campaign designed to reveal the planetary systems orbiting some of the closest stars to the Sun. The MINERVA-Red cross-dispersed echelle spectrograph is optimized for the “deep red”, between 800 nm and 900 nm, where the stars that will be targeted are relatively bright. The instrument is very compact and designed for the ultimate in Doppler precision – it uses a single-mode fiber input. I will describe the spectrometer and the status of the MINERVA-Red project, which is expected to begin routine operations at Whipple Observatory on Mt Hopkins, Arizona, in 2017.

Fred Rasio


Fall Quarter 2016

  Date/Time      Visitor Host
  Oct. 31
   Tech F160

Elad Steinberg
    Hebrew University of Jerusalem
    New Insights on the G2 Gas Cloud and the Connection Between Asteroids and
    Terrestrial Planet Formation

Recent observations have shown that a gas cloud, called G2, is in the midst of being tidally disrupted in our galactic center. I will present novel simulations of the G2 gas cloud and discuss it's agreements and disagreements with observations. The Kepler mission has shown that terrestrial planets are extremely common in our galaxy. However, there is still ongoing debate regarding their last stage of formation. I will try and shed some light on this issue utilizing observations from asteroids in our solar system and relating their spin magnitude to terrestrial planet formation theories.

Yoram Lithwick
  Nov. 11

Benjamin Montet
    University of Chicago
    Making the Most of Exoplanet Searches

The Kepler mission has revolutionized our understanding of the number and diversity of planetary systems in our galaxy, and TESS will soon follow in its footsteps. By observing hundreds of thousands of stars to search for subtle brightness variations induced by transiting planets, Kepler has also enabled major advances in stellar and galactic astronomy well beyond the core mission requirements. In this seminar, I will discuss some of the lagniappe science results enabled by both Kepler and future missions which significantly add to their legacies. Specifically, I will describe how the large number of stars observed by Kepler has allowed us to understand spacecraft systematics, enabling the success of the extended Kepler mission, K2. I will also outline ongoing work to search for long-term brightness variations of stars in the Kepler field caused by stellar magnetic cycles. I will conclude with a look to the future, describing how the WFIRST microlensing mission will continue the transiting planet revolution.

Fred Rasio
  Nov. 14

Ondrej Pejcha
    Princeton University
    Cool and Luminous Outbursts from Merging Binary Stars

Many binary stars pass through a phase of dramatic energy, mass, and angular momentum loss. The existing theoretical uncertainties in this short-lived interaction phase significantly affect our understanding of the binary evolution pathways and their rates, including formation of close binaries composed of black holes, neutron stars and white dwarfs. The discovery of V1309 Sco, a contact binary with rapidly decreasing orbital period followed by an outburst, recently established a connection between these astrophysically critical, catastrophic interactions and a group of astronomical transients characterized by their red color and the luminosity in the gap between novae and supernovae. I will present an exploration of the dynamics of outflows from mass-losing binary stars and the associated menagerie of transients. I will interpret the unprecedentedly detailed pre-merger data on V1309 Sco and argue that these transients can provide a fresh observational input into the open problem of violent binary interactions.

Fred Rasio
  Nov. 17
  Db 23

Siyao Xu
    Peking University, Beijing
    The Power Law in the Radio Sky

Turn on the radio light, and you will see the elephant in the room.

As shown by overwhelming observational evidence, the interstellar medium (ISM) is turbulent and magnetized. The ubiquitous magnetized turbulence in the ISM connects the physical processes happening on local small scales (~10^7 cm) to global large scales (~ 100 pc), through the power-law spectrum of turbulence. The turbulent spectrum has been measured in both the warm ionized medium, known as “the big power law in the sky”, and cold neutral medium. The power-law nature of the interstellar turbulence is widely imprinted in fluctuating observables in the ISM. An in-depth understanding of the power-law statistics of magnetized turbulence is the key to unlocking many mysteries in radio observations that have been puzzling for decades. These observables in turn provide valuable information on interstellar turbulence and density distribution in a straightforward way. In the light of radio observations of Galactic pulsars, I will show you what I have seen in our room, i.e., the Galactic ISM, and in the fast radio burst’s room, extragalactic ISM.

Giles Novak,
Farhad Yusef-Zadeh
  Nov. 18

Wen-fai Fong
    University of Arizona
    Unfolding the Dynamic and Colorful Lives of Compact Object Mergers

The mergers of two compact objects (neutron stars: NS and/or black holes: BH) in a binary system serve as signposts of gravitational wave emission, potential sites of heavy element nucleosynthesis, and laboratories for high-energy astrophysical processes not seen elsewhere in the universe. With the recent onset of the advanced era of gravitational wave detectors, there will soon be an unprecedented rise in the detected rate of these systems. At the end of the decade, the arrival of the Large Synoptic Survey Telescope will also lead the community to unexplored territory regarding the timescales, luminosities, and rates at which we discover explosive transients. In this talk, I will present observational challenges for this upcoming revolution in transient astrophysics, with a particular focus on the ground-breaking detections of electromagnetic counterparts to gravitational wave sources. I will also describe ongoing and future efforts across the electromagnetic spectrum to confront these challenges.

Fred Rasio
  Nov. 21

François Foucart
    Lawrence Berkeley National Laboratory
    Electromagnetic and Gravitational Wave Signals from Merging Black Holes
    and Neutron Stars

Black hole-neutron star and neutron star-neutron star mergers are powerful emitters of gravitational waves, and can produce a wide range of bright electromagnetic transients. They also provide us with a remarkable environment to study nuclear interactions in the cold, dense core of neutron stars, and may significantly contribute to the production of many heavy elements whose origin remains unexplained. Given the increasing sensitivity of existing gravitational wave detectors, we expect gravitational waves from neutron star mergers to be observed in the coming years. In order to extract as much information as possible from these observations, reliable models of the gravitational wave and electromagnetic signals powered by mergers are necessary. In this talk, I will discuss our efforts to study binary mergers using general relativistic simulations. I will in particular focus on recently developed gravitational wave models which account for tidal effects in neutron stars, on the electromagnetic transients powered by radioactive decays in the matter ejected by neutron star mergers, and on the outcome of nucleosynthesis in that ejecta. Tidal effects in neutron star mergers can provide us with information about the size of neutron stars, an important observable in nuclear physics, while radioactively powered transients can offer both improved localization of the merger events and additional information about the properties of the merging objects.

Fred Rasio
  Nov. 28

Jennifer van Saders
    Carnegie Observatories
    Broken Clocks and Fields in Flux: Making Sense of Stellar Rotation Observed with Kepler

Gyrochronology utilizes the spin-down of stars as a function of time as an indicator of stellar age. This technique has the potential to yield precise ages for large samples of stars, providing unprecedented chronological information for studies of the Milky Way and extrasolar planets. However, gyrochronology is in its adolescence: it has been tested under limited scenarios, but its weaknesses and limitations have hitherto been largely unexplored. With data from the Kepler mission we can address these gaps: we now have access to datasets of rotation periods for tens of thousands of stars, as well as independent asteroseismic ages and rotation periods for a few hundred old (main sequence) stars. I will discuss my comparisons of theoretical rotation models to these Kepler data, which have yielded unexpected insights into the rotational and magnetic lives of stars (and the Sun!), as well as a better understanding of the power and peril of gyrochronology as a tool.

Fred Rasio
  Nov. 30

Sasha Tchekhovskoy
    University of California - Berkeley
    New Frontiers in Simulating Black Hole Accretion and Jets

Black holes are responsible for a wide variety of astrophysical phenomena. They devour stars, eject relativistic jets, affect star formation and galaxy evolution, and enrich the Universe with heavy elements. In the next several years, the Event Horizon Telescope will produce resolved images of infalling gas and jets on the event horizon scale that promise to revolutionize our understanding of black hole physics. However, until recently, no first-principles models to quantitatively interpret these observations existed. I will present the first such models, the simulated spectra and images, and the constraints on the near event horizon physics coming from the comparison to the observations of the supermassive black hole at the center of our galaxy. I will then use simulations to constrain black hole physics in several other astrophysical contexts. I will finish by making connections to my future research plans.

Fred Rasio
  Nov. 30

Rebecca McElroy
    University of Sydney
    The Close AGN Reference Survey: Mrk 1018 Returns to the Shadows After 30 Years
    as a Seyfert 1

We report the discovery of an AGN that has changed spectral type not once, but twice. So called ‘changing look’ AGN are an uncommon phenomenon, but twice changed AGN are much rarer. This AGN first transitioned from a narrow line AGN (type 2) to a broad line AGN (type 1) in the 1980s. It was recently observed as part of The Close AGN Reference Survey (CARS). CARS aims to provide a detailed multi-wavelength view of 40 nearby (0.01 < z < 0.06) unobscured AGN to study the link between AGN and their host galaxies. The primary CARS observations come from the MUSE integral field unit on the VLT, and complementary multi-wavelength observations have been approved from a wide array of sources (SOFIA, Chandra, VLA, HST, and others). Examination of the MUSE data for this particular source showed that it no longer had the spectral features typical of a type 1 AGN. The continuum emission from the accretion disk was no longer visible and the broad lines were dramatically diminished. In this talk we describe the possible reasons for this change, supported by analysis of multi-epoch optical photometry and spectroscopy, alongside data obtained through director’s discretionary time from Chandra, HST, and the VLA. We then conclude by discussing the implications of this discovery on our understanding of AGN timescales and the physics behind AGN spectral types.


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Past CIERA Special Seminars