Center for Relativistic Astrophysics

Public Lecture: Dr Katherine (Katie) Mack (North Carolina State University)
2019-04-23 18:00 - 19:00 (America/New_York) | GA
https://www.physics.ncsu.edu/people/faculty_mack.php

CRA Seminar: Dr Katherine (Katie) Mack (North Carolina State University)
2019-04-23 15:00 - 16:00 | GA

CRA special seminar: Jason Gallicchio and Brian Shuve (Harvey Mudd College)
2019-04-22 15:00 - 16:00 | Bogg 1-44 VizLab
Jason Gallicchio and Brian Shuve from Harvey Mudd College
Title:
Improving Tests of Quantum Entanglement by using Light from Distant Quasars
Abstract:
Quantum entanglement, described by Einstein as “spooky action at a distance,” is both fascinating philosophically and is the basis for quantum computing and quantum encryption. For this reason, it deserves to be tested in experiments that push entanglement to its limits. The inferred power of entanglement rests on several assumptions, including that the measurement chosen for one particle can be made independent of any influence that can affect the other particle. Rather than take this assumption for granted, I devised and helped carry out an experiment on the Canary Islands that used real-time measurements of the color of incoming photons from two quasars on opposite sides of the universe to choose the measurements on each entangled particle. The color of these photons was fixed when the universe was only 10% as old as it is today, so any scheme for one quasar to affect measurements on the opposite particle would needed to have been in place before then.

CRA seminar (Dr. Colleen Wilson-Hodge)
2019-04-18 15:00 - 16:00 | Bogg 1-44 VizLab
Title: “Time Domain Astronomy with the Fermi Gamma-ray Burst Monitor in the Multimessenger Era”
Abstract: On August 17, 2017, a new era in astronomy began with the detection of merging neutron stars in gravitational waves by LIGO/Virgo followed 1.7 seconds later by the detection of a short gamma-ray burst (GRB) with Fermi GBM. This short GRB looked quite ordinary in gamma-rays, but was significantly underluminous when compared with other GRBs with known distances. This event has confirmed that short GRBs are produced by merging neutron stars, but has opened many new questions. I will describe several similar events that we have found in the Fermi GBM data. As LIGO/Virgo begins its 3rd observing run, we look forward to the next exciting joint event. I will end the talk with brief highlights of other transient and variable sources detected with Fermi GBM, such as the first Galactic Ultraluminous X-ray source, Swift J0243.6+6124.

CRA Cosmic Coffee: Chris Evans
2019-04-17 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title: Marginally Trapped Surfaces in Black Hole Mergers

Abstract: For numerical simulations of black hole mergers, typically the apparent horizon that forms is of primary interest and can be used to calculate the mass and angular momentum of the merger remnant. In a generic binary black hole evolution however, the apparent horizon is only one of four marginally outer trapped surfaces that exist at some point during the merger. In this talk I will present a brief overview of horizons in the context of numerical relativity and black hole mergers. I will also discuss the results of some new simulations we have completed to investigate the nature of these surfaces.

CRA Cosmic Coffee: Danielle Skinner
2019-04-10 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title: Where Do Population III Stars Form?

Abstract: We perform a cosmological simulation with a comoving volume of 1 Mpc^3 to study the birth-places of Population III stars, using the adaptive mesh refinement code Enzo. We investigate the distribution of host halo masses and its relationship to the Lyman-Werner background intensity. In our sample of 688 host halos, we find that 84% of them have masses below the Machacek et al. (2001) relation because of the inclusion of H2 self-shielding. In our simulation above a redshift of 11.5, the mean halo mass is time-independent and 10^5.9 M⊙. Afterwards, it steadily rises above the Machacek et al. relation to a mean value of 10^6.6 M⊙. Most of these halos form multiple Population III stars, with a median number of four, up to a maximum of 16. We also find that a few halos do form stars below the Machacek et al. relation but in a high Lyman-Werner radiation field with values up to 50 J21. Our results suggest that Population III star formation may be less affected by Lyman-Werner radiation feedback than previously thought and that Population III multiple systems are common.

CRA seminar: Prof. Konstantin Batygin (Caltech)
2019-04-09 15:00 - 16:00 | Bogg 1-44 VizLab
Title: “I Like to Myself Believe That Planet 5 Turns Slowly”
Abstract:
Within the general framework of the core-nucleated accretion theory of giant planet formation, the conglomeration of massive gaseous envelopes is facilitated by a transient period of rapid accumulation of nebular material. While the concurrent build-up of angular momentum is expected to leave newly formed planets spinning at near-breakup velocities, Jupiter and Saturn, as well as super-Jovian long-period extrasolar planets, are observed to rotate well below criticality.
In this talk, I will argue that the large luminosity of a young giant planet simultaneously leads to the generation of a strong planetary magnetic field, as well as thermal ionization of the circumplanetary disk. The ensuing magnetic coupling between the planetary interior and the quasi-Keplerian motion of the disk results in efficient braking of planetary rotation, with hydrodynamic circulation of gas within the Hill sphere playing the key role of expelling spin angular momentum to the circumstellar nebula. These results place early-stage giant planet and stellar rotation within the same evolutionary framework.

CRA Seminar: Lee Samuel Finn (Penn State University)
2019-04-04 15:00 - 16:00 | GA
Observational tests of general relativity with black hole inspiral, coalescence, and ringdown
Prof. Lee Samuel Finn (Penn State University)
Website: https://www.phys.psu.edu/people/lsf5
Abstract: As far as it has been tested, general relativity “works”; however, tests of general relativity are spotty in their coverage and – especially in the dynamical regime – leave much to be desired in their sophistication. What are the characteristics of sophisticated test of a physical theory? Sophisticated tests identify discrepancies between theory and nature, classify them relative to the theory’s precepts, and quantify the observed discrepancies: i.e., they provide guidance for improved understanding. In this seminar we discuss the use of gravitational wave observations for testing general relativity, paying special attention to inspiral/coalescence/ring-down radiation. What are we assuming, and what are we testing, when we observe ICR waveforms? How do we pose and execute tests that identify, classify, and quantify disagreements between theory and nature? We are preparing to apply these tests to the data in which LIGO and Virgo have reported ICR detections: we end with a report on progress toward carrying out the tests described here.

CRA Seminar: Dr. Juan Calderon Bustillo (Monash University)
2019-04-03 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
https://research.monash.edu/en/persons/juan-calderon-bustillo
Title: Exploring the strong field phenomena of black holes
Abstract: At the event horizon of a black hole, gravity reaches its most extreme behaviour. Studying the dynamics of event horizons is key to understand gravity in is ultra-strong field regime and investigate the most fundamental properties of black holes. Black hole collisions provide a unique scenario to observe event horizons in a highly distorted and violently changing regime, which leads to a vast collection of phenomena that has not yet been detected by Advanced LIGO and Virgo. In this talk I will discuss the imprint that these phenomena leaves in the gravitational-wave emission of black hole collisions and what it can teach us about the properties of black hole horizons.

CRA Seminar: Prof. Huan Yang (Perimeter Institute)
2019-03-28 15:00 - 16:00 | GA
Probing neutron star physics with gravitational wave detections
The recent detection of the binary neutron star merger event (GW170817) has marked the beginning of multi-messenger astronomy with gravitational waves. In this talk I will briefly review current findings associated with GW170817 and introduce new opportunities from future detections. In the first part, I will explain interesting features of gravitational waveform coming from eccentric binary neutron star systems, emphasizing a new type of tidal resonance effect that may show up in the late inspiral stage. I will also show how to use tidal effects to distinguish neutron star/low-mass black hole binaries from neutron star binaries. In the second half of the talk, I will discuss current efforts in detecting post-merger gravitational wave signals. This includes developing a machine learning based post-merger template to perform Matched-Filter search, and a new concept of high-frequency gravitational wave detectors targeting post-merger neutron star physics.
http://www.perimeterinstitute.ca/people/huan-yang

CRA seminar: Prof. Shane Davis (University of Virginia)
2019-03-14 15:00 - 16:00 | GA
Title: How Black Holes Dine above the Eddington “Limit” without Overeating or Excessive Belching
Abstract
The study of super-Eddington accretion is essential to our understanding of the growth of super-massive black holes in the early universe, the accretion of tidally disrupted stars, and the nature of ultraluminous X-ray sources. Unfortunately, this mode of accretion is particularly difficult to model because of the multidimensionality of the flow, the importance magnetohydrodynamic turbulence, and the dominant dynamical role played by radiation forces. However, recent increases in computing power and advances in algorithms are facilitating major improvements in our ability to model radiation in numerical simulations of astrophysical plasmas. I will briefly describe our new radiation transfer modules and discuss our efforts to model super-Eddington accretion flows around stellar mass and supermassive black holes. I will focus on applications to ultraluminous X-ray sources, which must be radiating well above their Eddington luminosity unless they harbor intermediate mass black holes. I will argue that most of these sources can be (and likely are) “normal” ~10 solar mass black holes accreting and radiating with luminosities well above their Eddington “limit”.

CRA Cosmic Coffee: Mathilda Avirett-Mackenzie
2019-03-13 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title: The Evolving X-ray Spectrum of Active Galactic Nuclei

Abstract: The cosmic X-ray background (XRB) spectrum and active galaxy number counts encode essential information about the spectral evolution of active galactic nuclei (AGNs) and have been successfully modeled by XRB synthesis models for many years. Recent measurements of the 8–24 keV AGN number counts by NuSTAR and Swift-BAT are unable to be simultaneously described by existing XRB synthesis models, indicating a fundamental breakdown in our understanding of AGN evolution. Here we show that the 8–24 keV AGN number counts can be successfully modeled with an XRB synthesis model in which the reflection strength (R) in the spectral model increases with redshift. We show that an increase in reflection strength with redshift is a natural outcome of (1) connecting R to the incidence of high column density gas around AGNs, and (2) the growth in the AGN obscured fraction to higher redshifts. In addition to the redshift evolution of R, we also find evidence that an increasing Compton-thick fraction with z is necessary to describe the 8–24 keV AGN number counts determined by NuSTAR. These results show that, in contrast to the traditional orientation-based AGN unification model, the nature and covering factor of the absorbing gas and dust around AGNs evolve over time and must be connected to physical processes in the AGN environment. Future XRB synthesis models must allow for the redshift evolution of AGN spectral parameters which could then be used to constrain fundamental connections between AGNs and galaxies.

CRA Seminar: Mahdi Bagheri (University of New Hampshire)
2019-03-12 15:00 - 16:00 | GA
Monte Carlo Simulation and Balloon-Borne Observation of Energetic Radiations and Electromagnetic Fields from Thunderstorms and Lightning

Mahdi Bagheri
Space Science Center and Physics Department, University of New Hampshire

Abstract:
Lightning is one of the most energetic geophysical phenomena, producing the brightest light and loudest sound commonly occurring on Earth. Because of its high rate of occurrence (~ 8 million per day), one might think lightning is well understood, but that is not the case. Lightning’s seemingly random occurrence in space and time and the wide range of its significant time variation, from tens of nanoseconds for many individual processes to more than a second for the total discharge, and its obscuration by the thundercloud producing it makes lightning particularly difficult to study. In this talk, an overview of energetic radiations from lightning (e.g., Terrestrial Gamma-ray Flashes (TGFs), gamma-ray glows and X-rays) will be given. Then, Monte Carlo simulation of gamma-ray flashes from atmosphere of Venus and linear polarization of TGFs will be discussed. In addition, instrumentation of our balloon campaign for detection of gamma-ray glows will be reviewed. The instrumentation includes one BGO scintillator coupled to a Silicone Photomultiplier (SiPM), two Geiger-Muller tubes, and a low-power lightweight Electric Field-Mill sensor, designed and calibrated to measure both polarity and amplitude of electric field inside the thunderstorm region. Finally, some of the preliminary results of balloon launches from Florida into thunderstorm during the summer of 2017 and 2018 will be discussed.

CRA Seminar: Dr. Stefano Marchesi (Clemson University)
2019-03-07 15:00 - 16:00 | GA
Title: A multi-observatory X-ray approach to characterize heavily obscured AGNs
According to the different models of Cosmic X-ray Background (CXB), the diffuse X-ray emission observed in the 1 to ~200-300 keV band, is mainly caused by accreting supermassive black holes, the so-called active Galactic Nuclei (AGN). Particularly, at the peak of the CXB (~30 keV) a significant fraction of emission (10-25%) is expected to be produced by a numerous population of heavily obscured, Compton thick (CT-) AGN, having intrinsic column density NH>=1E24 cm^(-2). Nonetheless, in the nearby Universe (z<=0.1) the observed fraction of CT-AGN with respect to the total population appears to be lower than the one expected on the basis of the majority of CXB model predictions (~20-30%), being between 5 and 10%. This discrepancy between data and models is one of the open challenges for X-ray astronomers, and needs to be solved to get a complete understanding of the AGN population.
In my talk, I will discuss a multi-observatory X-ray approach to find and characterize heavily obscured AGN. Candidate sources are first selected in the 100-month Swift-BAT catalog, the result of a ~7 years all-sky survey in the 15-150 keV band. These objects are then targeted with snapshot (5-10 ks) observations with Chandra and Swift-XRT, which allow us to constrain the intrinsic absorption value within a 20-30% uncertainty. Finally, deep (25-50 ks) observations with XMM-Newton and NuSTAR allow us to study the physics of these complex and elusive sources.

CRA Cosmic Coffee: Jordi Manyer
2019-03-06 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title: Design of an Expanding Cosmological Background for the Moving Punctures approach to Numerical Relativity

Abstract: Predicted in 1916 by Albert Einstein in his theory of general relativity, gravitational waves are now at the heart of modern astrophysics. Created in some of the most energetic events in the universe, such as binary black hole collisions, neutron star collisions or the big bang, this ripples of space time offer a completely new spectrum for the study of the universe, possibly allowing scientists to get an insight on previously unseen phenomena. In this context, numerical relativity becomes important as a tool to simulate these high-energy systems and calculate the resulting gravitational waves. As we wait for the new generation of laser interferometers, the field has to find ways to keep up with the increasing demand in accuracy. It is therefore important to study and tackle all the possible sources of error that the simulations might have. One of the simplifications numeral relativity codes usually do is to consider the supermassive bodies isolated in an otherwise massless non-expanding universe (a Minkowski background). Although it is a fairly good approximation, it is still physically inaccurate and a potential source of error. This work aims to start exploring the possibility of new cosmological backgrounds for black hole simulations. Specifically, we consider a particular metric, the Friedmnn-Lemaitre-Robertson-Walker cosmology, and provide a full guided development on how to adapt a state-of-the-art numerical relativity code, the Einstein Toolkit, to use that metric as a background for black hole simulations. We then proceed to show results from simulations we performed using that specific theoretical development.

CRA Cosmic Coffee: KwangHo Park
2019-02-27 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title: Intermittent AGN feedback in galaxy clusters driven by cold gas accretion

Abstract: Recent observations of cool core clusters provide evidence that the presence of cold, atomic and molecular gas in their centers is more common than previously expected. This finding has important implications for the fueling of and feedback from the supermassive black holes (SMBHs) located in central cluster galaxies. Motivated by it, we run a suite of 2D and 3D hydrodynamic simulations to study the efficiency of radio-mode feedback driven by intermittent accretion of cold gas on the central SMBH. This type of fueling drives strong and recurrent AGN feedback episodes, whose duty cycle is determined by the presence of cold gas in the vicinity of the central SMBH. We find that intermittent AGN jets, with duty cycles shorter than the gas cooling time scale at the central region, are considerably more effective in suppressing the cooling flow than steady jets, the power of which does not significantly vary over time. Based on this, we suggest that the increased effectiveness of AGN feedback is a natural consequence of intermittency driven by accretion of cold gas, and thus this effect plays an important role in preventing runaway cooling in galaxy clusters.

CRA Cosmic Coffee: Deborah Ferguson
2019-02-20 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title: Gravitational Waves: Using Merger to Understand Ringdown

Abstract: With the advent of gravitational wave astronomy, we have newly found access to a wealth of knowledge in the form of gravitational waves. Scientists are working hard to extract as much information as possible from each of the ten binary black hole coalescences (and one binary neutron star coalescence) detected by LIGO and Virgo to date. From each stage of the coalescence, we can learn different details about the binary system. The gravitational waves emitted during ringdown contain information about the remnant black hole, namely the mass and spin. For the purposes of parameter estimation as well as for tests of general relativity, it is important to obtain a measurement of this spin. However, ringdown is difficult to detect so we discuss a method to determine the final spin using the instantaneous frequency of the gravitational wave during merger.

CRA Cosmic Coffee: Gen Chiaki
2019-02-13 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Title/Abstract: TBA

CRA seminar: Dr. Yan-Fei Jiang (University of California Santa Barbara)
2019-01-31 15:00 - 16:00 | Boggs 1-90 VizLab
3D Radiation Magneto-hydrodynamic Simulations of Accretion Disks around
Supermassive Black Holes
Abstract:
Black hole accretion disks convert the gravitational energy of accreted
gas to photons and produce varies observational phenomena of AGNs. First
principle calculations of black hole accretion disks for a wide range of
accretion rates are not only important for understanding various
observational properties of accreting black hole systems, but can also
improve our understanding on the growth of black holes as well as
connections between the central black holes and the host galaxies. I
will describe our recent 3D radiation MHD simulations of accretion disks
when the accretion rate is below the Eddington limit as observed for
most AGNs. Coronae is self-consistently produced above the photosphere
of the disk. Properties of the coronae as a function of mass accretion
rate are also consistent with observations. Structures of the accretion
disks are different from the standard alpha disk model in many ways.
Additional opacity peaks besides electron scattering play an important
role in AGN disks, which can potentially explain the variability of AGNs
over a few years’ time scale, as recently found for the change-look
AGNs. I will also describe how the structures of disk will change when
the accretion rate is above the Eddington-limit.

CRA Cosmic Coffee: Ziqian Hong
2019-01-23 15:00 - 16:00 | Howey N201/202, 837 State Street, Atlanta, GA, 30332, United States
Could there be an undetected inner planet near the stability limit in Kepler-1647?

Abstract:
Kepler-1647b is the most recently discovered planet that transits two stars, e.g., a circumbinary planet (CBP). Due to its large orbital separation, Kepler-1647b stands out from the rest of the Kepler CBPs, which mostly reside on much tighter orbits near the stability limit. The large separation of Kepler-1647b challenges inward disk migration as a dominant formation pathway, suggested by the other Kepler CBPs. In this paper, we consider the possibility of an undetected planet near the stability limit by examining observational consequences of such a planet. We calculate the transit probability of the putative planet, transit timing variations (TTVs) of the known planet, and eclipsing timing variations (ETVs) of the host binary caused by the putative planet. We find that a low mutual inclination (0.5-1.5 deg.) allows for the inner planet to hide from transit observations, even if it is close to the stability limit; and the presence of a massive (~Saturn mass) inner planet can be ruled out by TTVs. In addition, we provide future TTV and ETV observation windows,which will further constrain possible undetected planets with lower masses near the stability limit.

End of Term
2018-12-15 | GA

CRA Seminar (Prof. Fred Adams, University of Michigan)
2018-11-29 15:00 - 16:00 | GA
The Degree of Fine-Tuning in our Universe — and Possibly Others
Fred C. Adams, University of Michigan
https://lsa.umich.edu/astro/people/jointly-appointed-faculty/fca.html
The fundamental constants of nature must fall within a range of values
in order for the universe to develop structure and ultimately support
life. This talk considers the current constraints on these quantities
and assesses the degree of fine-tuning required for the universe to be
viable. The first step is to determine what parameters are allowed to
vary. In the realm of particle physics, we must specify the strengths
of the fundamental forces and the particle masses. The relevant
cosmological parameters include the density of the universe, the
cosmological constant, the abundance of ordinary matter, the dark
matter contribution, and the amplitude of primordial density
fluctuations. These quantities are constrained by the requirements
that the universe lives for a sufficiently long time, emerges from its
early epochs with an acceptable chemical composition, and can
successfully produce galaxies. On smaller scales, stars and planets
must be able to form and function. The stars must have sufficiently
long lifetimes and hot surface temperatures. The planets must be
large enough to maintain atmospheres, small enough to remain
non-degenerate, and contain enough particles to support a biosphere.
We also consider specific fine-tuning issues in stars, including the
triple alpha reaction that produces carbon, the case of unstable
deuterium, and the possibility of stable diprotons. For all of these
issues, the goal of this enterprise is to delineate the range of
parameter space for which universes can remain habitable.

Thanksgiving Break
2018-11-22 - 2018-11-23 | GA

Student Recess
2018-11-21 | GA
No Classes – Campus Open.

CRA Seminar (Dr. Keith Vanderlinde, University of Toronto)
2018-10-11 15:00 - 16:00 | GA
Title: CHIME – the Canadian Hydrogen Intensity Mapping Experiment
Keith Vanderlinde (University of Toronto)
URL: http://dunlap.utoronto.ca/~vanderlinde/
ABSTRACT:
Among the great surprises of modern cosmology was the discovery of
Dark Energy, which dominates the energy budget of the Universe and
is driving the acceleration of its expansion rate. Decyphering its
properties and nature will require novel measurements spanning vast
swaths of the observable Universe. In this talk, I will introduce
the Canadian Hydrogen Intensity Mapping Experiment (CHIME), an
ambitious project to study Dark Energy by tracing out 4 billion years
of cosmic history, using a purpose-built radio telescope at the
Dominion Radio Astrophysical Observatory (DRAO) in B.C.’s Okanagan
Valley.
Hydrogen Intensity (HI) mapping uses redshifted 21cm emission from
neutral hydrogen as a 3D tracer of Large Scale Structure (LSS) in the
Universe. Imprinted in the LSS is a remnant of acoustic waves which
propagated through the primordial plasma of the nascent cosmos. This
“Baryon Acoustic Oscillation” (BAO) feature, which appears as a
spatial correlation of LSS, can be used as a standard ruler to trace
the expansion history of the Universe, thereby allowing us to constrain
the Dark Energy equation of state.
CHIME is a transit interferometer with no moving parts, which uses a
massive computing backend to image the radio sky from 400-800MHz,
corresponding to 21cm radiation emanating from a redshift range of
0.8 < z < 2.5. Earth rotation sweeps its field of view across the sky,
resulting in complete daily coverage of the northern celestial
hemisphere
and an unprecedented survey sensitivity. I will discuss the motivation,
construction, and status of CHIME, as well as a pair of extensions which
probe the high-cadence time-domain radio sky, monitoring radio pulsars
and
exploring a more recent mystery in radio astronomy, Fast Radio Bursts.

Fall recess
2018-10-08 - 2018-10-09 | GA

CRA Seminar: Dr. Dennis Bodewits (Auburn University)
2018-10-04 15:00 - 16:00 | GA
Title: Physics of comets after the Rosetta mission
Abstract:
Most comet questions revolve around whether observed properties are primordial and representative of conditions during the era of planet formation, or whether they are caused by subsequent processing. The Rosetta mission to 67P/Churyumov-Gerasimenko dramatically changed our understanding of comets. It allowed the most detailed study of a comet’s surface, chemical and physical processes in its atmosphere, and of its interaction with the solar wind. This talk will take attendees on a tour of what we know about comets, what mysteries we need to solve, and how future spacecraft and telescopes could help us answer our questions.

CRA Seminar (Dr. Nicholas Stone, Columbia University)
2018-09-27 15:00 | GA
Title: Dynamical Assembly of Black Hole Binaries in Dense Star Systems
Abstract: The discovery of merging black hole (BH) binaries by the LIGO and Virgo experiments has initiated the era of gravitational wave (GW) astrophysics. Detailed comparisons between theoretical and observed waveforms have already tested classical general relativity, and placed stringent constraints on alternative theories of gravity. However, the zeroth-order astrophysical question implied by these GW detections remains unanswered: what is the astrophysical process, and what is the astronomical environment, responsible for producing the bulk of observed BH binary inspirals? A number of different formation pathways have been proposed, and can be classified as either “isolated binary star evolution” or “dynamical assembly.” After broadly overviewing these two formation channels, I will focus on my recent theoretical work to better understand dynamical assembly processes in dense stellar clusters or disks. The best-studied such process is binary-single scatterings in dense star clusters, where repeated three-body interactions create metastable triples that lead to partner swaps and the hardening of binary orbits. The chaotic evolution of these three-body systems is usually studied numerically, but I will present a new analytic formalism that employs the ergodic hypothesis to derive general, closed form statistical solutions for the non-hierarchical three-body problem. This statistical mechanics formalism appears to give reasonable agreement with ensembles of numerical scattering experiments, and I will discuss applications to GW source formation. I will also briefly introduce a novel BH binary formation scenario, the “AGN channel” my collaborators and I have recently proposed. In this scenario, hydrodynamic torques in the gas disks of active galactic nuclei may accelerate the inspiral of wide BH binaries, or pair up singleton BHs into tight orbits. Given ~100 detections, the fractional contribution of this scenario to the total population of GW sources can be deduced from LIGO sky localization volumes alone.

CRA Seminar: Dr Jane Pratt
2018-09-13 15:00 - 16:00 | GA
Title: Stellar Physics: from Plasma Dynamics to Stellar Evolution
Abstract:
New data available from recent space-missions Kepler and GAIA, as well
as up-coming missions TESS and PLATO, has introduced a new era for
stellar physics. New theoretical/computational models are needed to
interpret the wealth of detailed new data. This includes fundamental
theories of stellar convection, overshooting and pulsation, differential
rotation, chemical mixing, and the dynamo. It also includes
instabilities that occur in binary stars evolving close to a companion.
The forefront of theoretical modeling is to expand models of the stellar
interior into the stellar atmosphere, to include mass accretion and mass
loss from stellar winds, as well as stellar spots and stellar flares. I
will describe the multi-dimensional, time implicit, fully compressible,
hydrodynamic, implicit large eddy simulation code MUSIC, and outline the
ongoing development of this computational tool designed for realistic
stellar hydrodynamics and magnetohydrodynamics. I will discuss how I
have used MUSIC to study convection during an early stage in the
evolution of our sun where the convection zone covers approximately half
of the solar radius. I interpret simulation data using extreme value
theory and derive a new model for convective overshooting and
penetration, targeted for use in one-dimensional stellar evolution
calculations. My model provides a scenario that can explain the observed
lithium abundance on the surface of the Sun and other solar-like stars
at a range of ages. The motivation for this program of work is to derive
new models based on realistic fluid simulations, to be implemented in
stellar evolution codes and broadly used for stellar and galactic
astrophysics.

Labor Day
2018-09-03 | GA
