Forum Schedule Spring 2016

Fridays 3:45pm - 4:45pm BPB-217

Date Speaker Topic (click down-arrow to see abstract)
Jan 22 David Radice
California Institute of Technology
host: Bing Zhang
Turbulent Lives: Tales of Neutron Stars

Neutron stars live interesting and mysterious lives: from the moment they are created in core-collapse supernovae, to the moment some of them eventually collapse to black-holes. In this talk, I will cover some aspects of their lives. I will discuss their birth in core-collapse supernovae, focusing on the key role of hydrodynamic turbulence in triggering the explosion, and their death in neutron star mergers. Finally, I will present recent results on the outcome of the dynamical ejection of neutron-star matter that might take place during mergers.

Jan 29 Joseph M. Zaug
Lawrence Livermore National Laboratory
host: Pamela Burnley & Andrew Cornelius
Ultrafast Shock Compression Experiments to Rapidly Test Extreme Condition Materials Predictions

In this talk, we will discuss recent results from ultrafast tabletop laser compression experiments on fluids, polymers, and high energy density organic molecules including single crystals. Previous work on ultrafast shocked metals will be summarized and serve as an introduction to our technical approach. Extreme material theories benefit from this research through a growing understanding of how ultrahigh strain rate (108 -1011 s-1) loading processes affect later-time high-strain rate (104 -106 s-1) phenomena occurring on macroscale dimensions.

Larger scale gun-based compression platforms nominally generate 106 s-1 maximum equilibrated strain rate loads; however, initial rising transient strain rates —not measured— may actually reach ultrahigh values. At present, the ultrafast shock community currently utilizes diagnostics that measure hydrodynamic flow and UV/VIS absorption; however, these methods tell us nothing directly about structure or chemical states. (We can consider perspectives on the viability of potential solutions to this long-standing challenge.) Nonetheless, when we’ve matched —on identical temporal and spatial scales— hydrodynamic data with commensurate molecular dynamics or crystal mechanics simulation results, more comprehensive pictures materialize that further illuminate the progression of early-time shock induced phenomena, such as high-strain rate induced elastic to plastic wave transitions preceding chemical initiation. Definitive knowledge gaps are also discovered.

We will conclude this presentation with an example of how one may use ultrafast compression-quench experiments to freeze metastable intermediate products. Shockwave compression states normally release to high-temperature thermodynamic states governed by the heat capacity of the starting material; however, by stopping (at an early-stage) shock induced chemical decomposition, i.e., bond breaking, one can trap or even consider synthesizing previously inaccessible transient species. For example, diamond formation from shocked TATB (1,3,5-triamino-2,4,6-trinitrobenzene) had been predicted for decades and was finally proved correct using this novel experimental approach.

Dr. Joseph (Joe) Zaug - Founding member (1997) and leader of the High Pressure Chemistry Group within the Materials Sciences Division at Lawrence Livermore National Laboratory. (Ph.D. in Physical Chemistry, University of Washington, Seattle, 1994; B.S. in Chemistry, Illinois Institute of Technology, 1988) He has twenty-five years of experience developing tools that quasi-statically and/or dynamically compress materials and engineering new approaches to characterize extreme condition material response using primarily laser-based systems. Numerous grand-challenge science issues have been met by these innovations resulting in high-profile publications in disciplines such as geophysics, high-pressure physics and chemistry including chemical synthesis, and materials science. Joe and his group actively collaborate with international and U.S. collaborators. His current research focus is on measuring equations of state and physical or chemical phase transitions of single crystals, polymers, and composite materials subjected to quasi-static and ultrahigh strain rate loads. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Feb 5 Rebekah Dawson
UC Berkeley
host: Jason Steffen
All Planets Great and Small

Discoveries of exoplanets so different from those in our Solar System have called in question conventional theories for how planetary systems form and evolve. I will present recent progress in our understanding of the physical processes that drive the assembly of planetary systems and result in the surprising variety of orbital architectures we observe today. I will discuss orbital evolution in both the large and small planet regime and physical processes that link planetary orbits to their physical properties and properties of their host stars.

Feb 11 (Thursday) Norbert Werner
Stanford University
host: George Rhee
Sculpting the Visible Universe

In the course of structure formation, only a small fraction of the baryons turned into stars - most remain in a diffuse intergalactic medium. The growth and evolution of galaxies is controlled by feedback processes, such as energy and momentum input from supernovae, and from the jets and winds of accreting supermassive black holes. I will start my talk by presenting observational results on the role of supermassive black holes in suppressing star formation in the most massive galaxies, keeping them 'red and dead'. Then, I will show how deep observations of extreme clusters of galaxies inform us about the microphysics of the intergalactic medium, which determines how the energy from accreting black holes couples with the diffuse gas. Then, I will 'zoom out' to the outskirts of galaxy clusters where we also find hints that supermassive black holes played an important role in the distant past. X-ray observations reveal a remarkably homogeneous distribution of iron out to the virial radius of the nearby Perseus Cluster, requiring that most of the metal enrichment of the intergalactic medium occurred before the cluster formed, probably more than ten billion years ago, during the period of maximal star formation and black hole activity. Finally, I will talk about the upcoming ASTRO-H satellite which will revolutionize X-ray spectroscopy and our understanding of the physics of galactic feedback.

Feb 12 Shanti Deemyad
University of Utah
host: Ashkan Salamat
Lithium under pressure

Even at zero temperature lattice of lithium remains far from static. In periodic table lithium is the first element immediately after helium and the lightest metal. While fascinating quantum nature of condensed helium is suppressed at high densities, because of the presence of long range interactions in metallic systems, lithium is expected to adapt more quantum solid behavior under compression. Physics of dense lithium offers a rich playground to look for new emergent quantum phenomena in condensed matter. In this talk I will discuss the physics of ultra-light materials under extreme pressures and will present some of our studies on unraveling the physics of dense lithium.

Feb 19

Feb 22 (Monday) Daniel Perley
Neils Bohr Institute
host: George Rhee
The Environments of the Universe's Most Extreme Explosions Across Cosmic History

Massive stars have been known to end their lives violently for almost a century, but the extremes of this process have become appreciated only recently: rare classes of "superluminous" supernovae are hundreds of times more luminous than other SNe, and long-duration gamma-ray bursts fleetingly outshine the brightest quasars by orders of magnitude. Their immense luminosities make these events easily detectable from great distance, from which they can serve as probes of the high-redshifts IGM, ISM, and rate and sites of cosmic star-formation. However, employing them as tools in this way requires a thorough understanding of how varying conditions such as metallicity may favor or disfavor their production in different environments. I will discuss two large surveys I am leading to study the connection between extreme transients and their galaxy environments: SHOALS, a multi-observatory effort to examine the impact of galaxy evolution on the GRB rate and host population across cosmic history, as well as the PTF superluminous-supernova host project at Keck and Palomar. Extreme transients will be discovered at a much wider range of distances and greater numbers in the coming era of all-sky synoptic surveys, and studies of these events and their environments with existing and upcoming facilities will prove invaluable for understanding the composition and evolution of dwarf galaxies, the history of the early universe, and theories of massive-stellar evolution and variations in the IMF.

Feb 25 (Thursday) Ivan Ramirez
University of Texas
host: George Rhee
High-precision spectroscopy: from exoplanets to Galactic chemical evolution

Stellar spectra contain a wealth of information, but they can be difficult to interpret due to modeling limitations. Over the past few years, a new technique that circumvents these limitations and allows astronomers to determine atmospheric parameters and elemental abundances of stars with unprecedented precision (~1%) has been developed by me and my collaborators. Using this technique, we have discovered that the Sun has a peculiar chemical composition, one which can be interpreted as a signature of the formation of the solar system. We can use this technique to infer the presence and bulk composition of planets around other stars. Moreover, it allows us to peer into the formation histories of stars and how they could affect planet formation. In addition to being useful for exoplanet research, these data sets are also well-suited for studies of nucleosynthesis and the chemical evolution of our Milky Way galaxy. In this talk I will describe the methods and key results that we have obtained using high-precision stellar spectroscopy.

Feb 26 Anna Barnacka
Harvard-Smithsonian Center for Astrophysics
host: Daniel Proga
Resolving the High Energy Universe with Strong Gravitational Lensing

Gravitational lensing is a powerful tool for elucidating the origin of gamma-ray emission from distant sources. Cosmic lenses magnify the emission and produce time delays between mirage images. Gravitationally-induced time delays depend on the position of the emitting regions in the source plane. Temporal resolution at gamma-ray energies can be used to measure these time delays, which, in turn, can be used to resolve the origin of the gamma-ray flares spatially. As a prototypical example of the power of lensing combined with long, uniformly sampled light curves provided by the Fermi satellite, we investigated the spatial origin of gamma-ray flares from two known gravitationally lensed sources: PKS 1830-211 and B2 0218+35.

Feb 29 (Monday) Zhaohuan Zhu
Princeton University
host: Rebecca Martin
From Protoplanetary Disks to Exoplanets: Theory Confronts Observations

We are entering a golden era of study in the field of planet formation. Recently commissioned telescopes and instruments (e.g., Subaru, GPI, VLA, ALMA, EVLA) are now finally able to resolve the protoplanetary disk down to the scale of a planet's immediate assembly zone, and a rich variety of disk features have been revealed: gaps, large scale disk asymmetry, and spiral arms. Despite this progress on the observational front, theoretical models have yet to be developed that can reveal what these observations are telling us about the physics of disk structure and planet formation. In this talk, I will present my work on numerical simulations of planet-disk interaction, with an emphasis on understanding current observations. My simulations have not only successfully reproduced observed spiral arms, gaps and asymmetric features, but also constrained protoplanetary disk properties and revealed potential planets in these disks. To directly find young planets, I will suggest that disks around these forming planets, so-called circumplanetary disks, could be the key and we may have already found some circumplanetary disk candidates. Finally, I will discuss how the new generation of numerical codes and simulations I am working on are important for not only interpreting upcoming observations but also revealing fundamental physical processes in protoplanetary disks. Combining the new generation of observations and simulations, we may finally unveil the mystery of planet formation in the next decade.

Mar 4

Mar 11 Elissaios Stavrou
Lawrence Livermore National Lab
host: Ashkan Salamat
Extreme thermodynamic conditions: novel stoichiometries, violations of textbook chemistry, and intriguing possibilities for the synthesis of new materials.

As evidenced by numerous experimental and theoretical studies, application of high pressure can dramatically modify the atomic arrangement and electronic structures of both elements and compounds. However, the great majority of research has been focused on the effect of pressure on compounds with constant stoichiometries (typically those stable under ambient conditions). Recent theoretical predictions, using advanced search algorithms, suggest that composition is another important variable in the search for stable compounds, i.e. that the more stable stoichiometry at elevated pressures is not a priory the same as that at ambient pressure. Indeed, thermodynamically stable compounds with novel compositions were theoretically predicted and experimentally verified even in relatively simple chemical systems including: Na-Cl, C-N, Li-H, Na-H, Cs-N, H-N, Na-He, Xe-Fe. These materials are stable due to the formation of novel chemical bonds that are absent, or even forbidden, at ambient conditions.

Tuning the composition of the system thus represents another important, but poorly explored approach to the synthesis of novel materials. By varying the stoichiometry one can design novel materials with enhanced properties (e.g. high energy density, hardness, superconductivity etc.), that are metastable at ambient conditions and synthesized at thermodynamic conditions less extreme than that those required for known stoichiometries. Moreover, current outstanding questions, “anomalies” and “paradoxes” in geo- and planetary science (e.g. the Xenon paradox) could be addressed based on the stability of surprising, stoichiometries that challenge our traditional “textbook” picture. In this talk, I will briefly present recent results and highlight the need of close synergy between experimental and theoretical efforts to understand the challenging and complex field of variable stoichiometry under pressure. Finally, possible new routes for the synthesis of novel materials will be discussed.

This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Security, LLC under Contract DE-AC52-07NA27344.

Mar 18 Scott Tremaine
Institute for Advanced Study, Princeton, NJ
host: Daniel Proga
Statistical mechanics of self-gravitating N-body systems

There have been many attempts to apply the powerful tools of statistical mechanics to self-gravitating N-body systems such as star clusters, galaxies, and planetary systems. I will describe why this is difficult, some notable failures and successes, and recent work on two arenas where these tools may offer new insight: the distribution of young stars in the central parsec of our Galaxy, and the distribution of orbits of exoplanets.

Mar 25 Spring Break

Mar 3 Jeffery H. Nguyen
Lawrence Livermore National Laboratory
host: HiPSEC
Phase Transitions at Extreme Conditions

Accurate determination of phase boundaries of materials at extreme pressure and temperature conditions is of great interest to the high-pressure community. Our research at the light gas gun at Livermore has been focused on accurate measurement of equations of state and phase transitions of metals at high pressure, both on and off-Hugoniot. In this presentation, we will highlight our results on the melting pressure of Fe, Mo, and Ta. From the measure longitudinal sound velocities, we were able to simplify previously known Fe and mo phase diagrams and confirm existing Ta phase diagram. Implications from these results will be discussed. To achieve off-Hugoniot states, we developed Graded Density Impactors (GDI), which allow us to tailor our compressions paths. By using the GDIs, we were able to study new phenomena such as liquid-solid transition and its kinetics. We will highlight experiments on freezing of the molten tin, iron and water on compression. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Apr 1 Krzystof Gofryk
Idaho National Lab
host: Andrew Cornelius
Spin-phonon interaction in uranium dioxide

Uranium dioxide (UO2) is a Mott-Hubbard insulator with well-localized 5f-electrons and its crystal structure is the face-centered-cubic fluorite. It experiences a first-order antiferromagnetic phase transition at 30.8 K to a non-collinear antiferromagnetic structure that remains a topic of debate. Despite extensive experimental and theoretical efforts the nature of the competing degrees of freedom and their couplings (such as spin-lattice coupling) are still unclear. In this talk I will present results of our extensive transport and thermodynamic investigations, on well-characterized and oriented single crystals of UO2, performed at low temperatures and high magnetic fields up to 100 T. We were able to elucidate some important questions such as the detailed nature of the low temperature multidomain 3k-AFM state and its importance for linear coupling between the system's magnetic polarization and mechanical strain, and the reasons behind unusual lattice properties that severely hinder the ability of this important nuclear material to transport heat. I will discuss implications of these results.

Apr 8 Richard de Grijs
Kavli Institute for Astronomy and Astrophysics
Peking University
host: Bing Zhang
Not-so-simple stellar populations in nearby, resolved massive star clusters

Until about a decade ago, star clusters were considered "simple" stellar populations: all stars in a cluster were thought to have similar ages and the same metallicity. Only the individual stellar masses were thought to vary, in essence conforming to a "universal" initial mass function. Over the past decade, this situation has changed dramatically. Yet, at the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. I will discuss my group's recent progress in this context, with particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Local Group star clusters across the full age range. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate- age (~2 Gyr-old) star clusters, which opens up exciting avenues for future research efforts.

Apr 15 Timothy Strobel
Geophysical Laboratory
host: Ashkan Salamat
Beyond thermodynamic stability: synthetic pathways to new materials with exceptional properties

Multiple allotropes and/or chemical compounds can be formed under various pressure / temperature conditions, and some of these could remain metastable under standard conditions for time scales as long as the age of the universe (in fact it is estimated that 50% of all known inorganic compounds are metastable ones!). But the number of known allotropes/compounds pales in comparison with the number of hypothetical ones with energetic feasibility. For any given thermodynamic state, thousands of energetically competitive structures are plausible, a subset of which will exhibit mechanical stability. Further subsets of these structures offer enticing physical properties that differ from those of thermodynamic ground states. Here we delineate thermodynamic and kinetic synthesis methods and discuss strategies and examples for accessing these states experimentally. In particular, we discuss successful experimental realization of new forms of silicon and carbon.

Apr 22 Niel Brandt
The Pennsylvania State University
host: Daniel Proga
A Good Hard Look at Growing Supermassive Black Holes in the Distant Universe

Sensitive cosmic X-ray surveys with the Chandra, XMM-Newton, and NuSTAR observatories have revolutionized our ability to find and study distant active galactic nuclei (AGNs), the main sites of supermassive black hole growth in the Universe. I will describe some of the resulting discoveries about the demographics, physics, and ecology of AGNs. Topics covered will include the utility of deep X-ray plus multiwavelength surveys for investigating distant AGNs; evolution constraints for the typical AGNs of the distant Universe; the cosmic balance of power between supermassive black holes and stars; interactions between AGNs and their hosting galaxies; and the AGN content of newly forming galaxies. I will end by discussing some key outstanding questions and new observations and missions that aim to answer them.

Apr 27 (Wednesday 12:00PM, BPB-250.) Rolf Kuiper
University of Tubingen, Germany
host: Bing Zhang
The Formation and Feedbak of the Most Massive Stars

The formation of massive stars yields strong feedback effects onto its host core and stellar cluster environment via protostellar outflows, radiation heating, radiation pressure, ionization, stellar winds and mass loss, and supernova (in chronological order). From a theoretical point of view, the question arises, how a massive protostar is able to accrete its mass up to the observed upper mass limit despite its strong radiation pressure feedback. In this talk, I present results of several series of self-gravitating radiation-hydrodynamics simulations of core collapse towards high-mass star formation, including the effects of radiation heating, radiation pressure, protostellar outflows, and ionization. We propose a solution to the radiation pressure problem in the formation of massive stars via the so-called flashlight effect: due to the formation of an optically thick accretion disk, the thermal radiative flux becomes strongly anisotropic and preferentially escapes through the disk's atmosphere, i.e. perpendicular to the sustained accretion through the disk's mid-plane. Furthermore, including feedback of early protostellar outflows yields a large scale anisotropy, which extends the disk's flashlight effect from the few hundred AU scale of the circumstellar disk to a core's flashlight effect up to a 0.1-parsec scale. This core’s flashlight effect allows core gas to accrete on the disk for longer, in the same way that the disk’s flashlight effect allows disk gas to accrete on the star for longer. In summary, I will demonstrate straight-forward mechanisms, which allows the formation of the most massive stars known in the present-day universe despite of their strong feedback via outflows and radiation. The basic theory of star formation herein is just a scaled-up version of low-mass star formation, i.e. based on accretion from core to disk to protostellar scales.

Apr 29 Alexandre Lazarian
University of Wisconsin
host: Bing Zhang
Turbulent reconnection and particle acceleration

I shall explain how turbulence makes magnetic reconnection fast and will compare this approach with other ideas in the field, e.g. related to tearing instability. I shall discuss both non-relativistic and relativistic reconnection. I shall demonstrate that turbulent reconnection inevitably leads to the particle acceleration. Finally, I shall discuss the implications of the process for the famous ICMART model by Zhang & Yan.

May 6 Dipanjan Mitra
Tata Institute of Fundamental Research
India / University of Vermont
host: Bing Zhang
On the nature of coherent radio emission from pulsars

Pulsars, are rotating and radiating neutron stars and are superb astrophysical laboratories of extreme physics. A typical neutron star has radius of ∼ 10 km, magnetic field of ∼ 1012 Gauss, density of ∼ 1017 kg/m3, rotating at a frequency ∼ 1 Hz and has a surface gravity of ∼ 1012m/s2. We observe pulsars as a sequence of periodic pulses mostly in the radio wavelength. What is mind-boggling is that the radio emission arises from a kilometer-sized emission patch which is at a distance of ∼ 1019 meter from us, and yet we see it!! The equivalent blackbody temperature of this radio emission is in the range 1025--1030 K, which exceeds the limit for any incoherent emission process. The physical mechanism of how this emission is generated is considered as one of the most challenging problems in astrophysics. In this talk I will discuss the wide spectrum of physical phenomena (like the QED phenomenon of magnetic pair creation, effect on the structure of neutron star surface, relativistic plasma dynamics etc) that takes place around the fast rotating, highly magnetized neutron star. These processes lead to generation of a highly relativistic flow of electron positron plasma in which we believe the radio emission is excited by a process called coherent curvature radiation, where charged ``bunches'' are accelerated in curved magnetic field. I will mention how evidence from high quality radio and X-ray observations of pulsars is putting stringent constraints to these ideas.

May 13 Final Examinations

Past forums: Fall 2015   Spring 2015   Fall 2014   Spring 2014   Fall 2013   Spring 2013   Fall 2012 Spring 2012   Fall 2011   Spring 2011   Fall 2010   Spring 2010   Fall 2009   Spring 2009   Fall 2008