Forum Schedule Fall 2016

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

Date Speaker Topic (click down-arrow to see abstract)
Aug 25 Maosheng Miao
California State University Northridge
host: Michael Pravica
Novel Chemistry Under High Pressure

The chemistry at ambient condition has implicit boundaries rooted in the atomic shell struc- ture: the inner-shell electrons and the unoccupied outer-shell orbitals do not involve as major component in chemical reactions and in chemical bonds. The chemical properties of atoms are determined by the electrons in the outermost shell; hence, these electrons are called valence electrons. These general rules govern our understanding of chemical structures and reactions. Using first principles calculations, we demonstrate that under high pressure, the above doctrines can be broken. We show that both the inner shell electrons and the outer shell empty orbitals of Cs and other elements can involve in chemical reactions. In the presence of fluorine and under pressure, the formation of CsFn (n > 1) compounds containing neutral or ionic molecules is predicted.[1] Their geometry and bonding resemble that of isoelectronic XeFn molecules, showing a caesium atom that behaves chemically like a p-block element under these conditions. Furthermore, we find that under high pressure Hg in Hg-F compounds transfers charge from the d orbitals to the F, thus behaving as a transition metal. Oxidizing Hg to + 4 and + 3 yielded the thermodynamically stable compounds HgF4 and HgF3.[2] The former consists of HgF4 planar molecules. HgF3 is metallic and ferromagnetic, with a large gap between its partially occupied and unoccupied bands under high pressure. In other works, we find that Xe, Kr, and
 Ar can form thermodynamically stable compounds with Mg at high.[3] The resulting
compounds are metallic and the noble gas atoms are negatively charged,
suggesting that chemical species with a completely filled shell can gain
electrons, filling their outermost shell(s).
 Similarly, we predicted that pressure can cause large electron transfer from light alkali metals such as Li to Cs, causing Cs to become anionic with a formal charge much beyond -1.[4] Furthermore, we show that the quantized orbitals of the enclosed interstitial space may play the same role as atomic orbitals, an unprecedented view that led us to a unified theory for the recently observed high-pressure electride phenomenon [5].

Aug 26 Charles Steinhardt
host: Jason Steffen
Is Hierarchical Merging Broken?

I will describe two sharp puzzles suggesting a possible mismatch between the current hierarchical merging paradigm and observations of evolving galaxies. Current models predict that the z ~ 4-8 universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. Instead, massive galaxies appear to exist impossibly early, before their halos should even have been able to assemble. Further, small halos should have generally formed earlier than larger halos, yet the observed hierarchy in galaxy formation is inverted, with more massive galaxies completing their growth earlier. After describing these puzzles, we will consider hierarchical merging is truly inconsistent with observation or whether other explanations might be more likely.

Sep 2

Sep 9 Jonathan Fortney
University of California, Santa Cruz
host: Jason Steffen
Toward an Understanding of Exoplanetary Composition

Nearly all of the narratives about understanding the composition of planets come from our understanding of the solar system. However, this is a terribly small sample size. The era of exoplanet characterization allows us to understand planets with a tremendously larger scope. In this talk I will cover several topics that shed light on planetary composition. In the realm of transiting planets, planetary structure models, compared to observations from the Kepler telescope and ground-based surveys, show our solar system’s population of rocky planets is potentially unusual since they lack massive hydrogen atmospheres. However, our two gas giants, Jupiter and Saturn, fit well within the population of transiting giant exoplanets, in that all these planets show similar enrichments in "metals" (elements heavier than He) compared to their parent star’s composition. Next, we are obtaining spectra of transiting planet atmospheres with Hubble to constrain atmospheric molecular abundances, which also point toward metal-enrichment for gas giants. Finally, we are developing new theoretical tools to extract constraints on atmospheric abundances from the spectra of brown dwarfs ("failed stars" with planetary temperatures) and giant planets to look for composition clues that distinguish these two populations. As a whole these advances will yield fundamentally new insights on planetary formation and evolution.

Sep 16 Esen Alp
Argonne National Laboratory
host: Barbara Lavina
Nuclear resonant and inelastic x-ray scattering studies under high pressure

Nuclear resonant scattering (NRS) and inelastic x-ray scattering (IXS) studies under high pressure continues to be very popular among geophysics and mineral physics researchers. At present, Advanced Phonon Source has two dedicated beamlines for high-energy resolution (1-2 meV) IXS studies (Sector 3 and 30). Additional capabilities exist at Sector 16 for NRS studies. I will present new results on Fe, Sn, Eu and Dy based nuclear resonant studies, including isotope fractionation measurements in iron and tin compounds, kinetics of phase transformations under varying temperature and pressure in iron, europium and dysprosium metals. I will highlight the use of APS Hybrid mode for synchrotron Mössbauer Spectroscopy and I will point out some of the expected changes in the near future.

Sep 23 Ken Nagamine
Osaka University
host: Bing Zhang
WHERE ARE THE BARYONS? — cosmological distribution of gas, metals, and dust in the Universe

It has been one of the triumphs for cosmological hydrodynamic simulations to predict the distribution of baryons in the Universe since the 80’s, when large-scale observational surveys of galaxies and quasars were still unavailable. Within the framework of standard Lambda CDM cosmology, we now understand the distribution of dark matter and baryons in the Universe relatively well, but not quite for the metals and dust yet. The reason lies in the complexity of feedback mechanisms, by which gas, metals and dust are ejected from galaxies into the intergalactic medium (IGM). In this talk, I will summarize the history and recent progress on this subject, and describe our future research plans.

Sep 30 Antonio Moreira Dos Santos
Oak Ridge National Laboratory
host: Barbara Lavina
The Reach of High Pressure Research in the Spallation Neutron Source

The intrinsic properties of neutrons and the way these interact with matter, make neutron scattering an exceptional tool in materials research, allowing studies on problems mostly inaccessible through other techniques. These include structural studies of compounds combining heavy and light elements, the determination of the magnetic structure of materials, the non-destructive testing of engineering parts and the probing of crystal dynamics, both structural and magnetic. The SNAP instrument (Spallation Neutrons and Pressure) a neutron diffractometer dedicated to the study of materials under high pressure that is part of the SNS’s suite of instruments has pioneered the revival of high pressure neutron diffraction. Since it began operating, a broad range of materials systems have been investigated, in the form of powders, glasses and single crystals. Here we will present some recent scientific results of research performed at SNAP, along with ongoing improvements and additions to the SNAP capabilities. Finally, some examples will be used to illustrate how other neutron techniques can provide valuable insight in the context of physics and materials science.

Oct 7 Mario Livio
UNLV Physics & Astronomy
host: Rebecca Martin

I will review the status of the physical existence of intelligent life on Earth, in view of the latest findings in astronomy, cosmology, and particle physics. I will discuss fine-tuning arguments, that ask whether our entire universe (or our solar system) are somewhat special, when considering Dark Energy, the Higgs particle, or other exoplanetary systems. I will also discuss some philosophical considerations concerning the importance of human intelligence in the grand cosmic scheme. Are we but a speck of star dust or are we in some sense central in the cosmic landscape?

Oct 14

Oct 21 Adrian Lenardic
Rice University
host: Jason Steffen, Askan Salamat
Tectonics, Climate, and Planetary Life Potential.

Volcanic and tectonic activity affects the climate evolution of terrestrial planets and, by association, the potential that a planet could maintain liquid water at its surface over geological time scales. This connects the volcanic-tectonic state of a planet to the potential that it could allow for life as we know it. As we have found more and more planets orbiting stars beyond our own, the question of what we can say about the volcanic and tectonic state of a planet, based on remote observations, has generated some interest based on its inferred link to the bigger issue of life potential within our galaxy. I will review competing ideas about what we can and, crucially from my own point of view, what we cannot say about the probable tectonic states of exo-planets given current observations. As well as bridling in some unrealistic expectations, highlighting some “limits of knowledge” can also suggest alternate modeling strategies that can be adopted under the assumption that the number of observations we have will increase. The exercise will also isolate critical factors that have not received as much consideration as they may well merit, e.g., the potential that planets can transition between different tectonics modes over their geologically active lifetimes and the potential that the specifics of planetary formation and the early years of evolution of a terrestrial planet can cast a long time shadow (longer than has previously been assumed). Finally, I will discuss recent models of climate-tectonic coupling that explore the hypothesis that plate tectonics, as it operates on the geologically modern day Earth, may not be the only tectonic mode that allows a planet to maintain livable surface conditions over time scales that allow for biological evolution.

Oct 28 Nevada Day Recess

Nov 4 Chris Hayward
host: George Rhee
Starbursts, outflows, and the emergence of disk galaxies

A complete theory of galaxy formation requires understanding the details of how gas is converted into stars over cosmic time, which is affected by gas supply, star formation, and feedback-driven outflows. Based on the results of state-of-the-art cosmological zoom simulations, I will argue that galaxy formation is a violent process: at high redshift, stellar feedback causes all star-forming galaxies to undergo rapid fluctuations in their star formation rates on ~10-Myr timescales. Bursts of star formation are followed by strong outflows, which cause the star formation rate to drop precipitously. Fresh gas supply from galactic fountains rejuvenates star formation and restarts the cycle. At z ~ 1, simulations of massive galaxies exhibit a qualitative transition: outflows are no longer driven effectively, and the galaxies transition to steadily star-forming, well-order disk galaxies. I will present an analytic theory that potentially explains the reasons for this transition.

Nov 11 Veterans Day Recess

Nov 18 Andrew Youdin
University of Arizona
host: Rebecca Martin
New Solutions to Old Problems in Protoplanetary Disks and Exoplanets

The origin of planets is one of the oldest scientific questions, while the study of exoplanets is one of the newest disciplines in astronomy. New discoveries provide an inspiration to reconsider long-standing problems and to adapt existing models to new phenomena. As examples, I will discuss (1) how protoplanetary disks accrete onto their stars (2) how solid planetesimals form and (3) why highly irradiated hot Jupiters are so large. All of these problems have multiple possible solutions, but outstanding issues remain. I will discuss the role of two specific mechanisms, the vertical shear instability to drive protoplanetary disk transport and the streaming instability to facilitate planetesimal formation. For hot Jupiters, I will show how shallow surface heating suffices to explain bloated radii.

Nov 25 Thanksgiving Day Recess

Dec 2 Michail Petaev
Harvard University
host: Jason Steffen
Physico‐chemical modeling of condensation in the early Solar system

Why are condensation models needed? The planet‐forming inner region in a physical model of solar nebula introduced by Cameron (1963) was totally vaporized. Therefore, before making planets nebular gas had to condense and accumulate into planetary building blocks or planetesimals (meteorite parent bodies) made up of chondritic meteorites. While now it is clear that neither of these postulates is correct, modeling nebular condensation still remains a powerful tool for understanding and, in some cases, predicting both mineralogical and chemical evolution of nebular reservoirs in which the Solar System objects have formed. Another important application of such models is to evaluate mineralogy and chemistry of extrasolar planets orbiting stars chemically different from our Sun. The purpose of this presentation is to overview existing condensation models and to discuss their strengths and weaknesses using several examples. First, I will introduce you in mineralogy and chemistry of chondritic meteorites. Then describe equilibrium and non-equilibrium condensation models based on my own work. Specifically discuss the concept of 50% condensation temperatures that is widely used in planetary studies, including exoplanets. And describe application of condensation models to understanding origin and evolution of meteoritic objects and Moon.

Dec 9 Enwei Liang
Guangxi University
host: Bing Zhang
Observations for Cosmic Gamma-Ray Bursts: Progress, Opportunity and Challenge

Gamma-ray bursts (GRBs) originate from the deaths of massive stars or mergers of compact stars. My talk focuses on our understanding of the recent observational data of GRBs with the Swift and Fermi missions as well as our own optical telescopes. With precise localizations of GRBs, it may be possible to observe relevant signals of these catastrophic events in the very early stages. I will also talk our search for relevant signals associated with GRBs prior to the GRB triggers and during the very early afterglow stages. Opportunity and Challenge of GRB observations and our ongoing monitoring project are also briefly discussed.

Dec 16 Final Examinations

Past forums: Spring 2016  Fall 2015   Spring 2015   Fall '14   Spring '14   Fall '13   Spring '13   Fall '12 Spring '12   Fall '11   Spring '11   Fall '10   Spring '10   Fall '09   Spring '09   Fall '08