Forum Schedule Spring 2019

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

Date Speaker Topic (click down-arrow to see abstract)
Jan 25 Jennifer Ciezak
Army Research Lab
host: Michael Pravica
The Promise and Challenge of Extended Solids of Nitrogen

The response of nitrogen to extreme conditions has attracted great interest since the predictions that nitrogen would transform into a nonmolecular phase at pressures less than 1 Mbar. As a result, the experimental phase behavior of nitrogen has been well characterized. Of particular interest was the identification of the cubic-gauche single bonded polymeric structure at high pressures and temperatures (>150 GPa, 2000K), which was found to be metastable at room temperature to pressures near 40 GPa. Nitrogen is of particular interest due to its promising potential as a high-energy-density material and it has been suggested its energy release may be roughly three times that of a traditional energetic materials. However, considerable challenges exist before such a capability could be realized, such as addressing the significant pressure-temperature barrier to transition from the molecular to non-molecular form, the conditions of which are not amenable to larger scale production technique; additionally, methods must be developed to increase the metastability to ambient conditions. In an effort to increase the metastability of the extended solid, recent studies have focused on mixing, or doping, the nitrogen with small amounts of secondary gases, such as hydrogen or carbon monoxide. It was been postulated the secondary gas would passivate the terminal ends thus increasing the stability of the nitrogen extended solid. Our group was the first to demonstrate such an approach could be used successfully to decrease the transition pressure for the formation of the nitrogen extended solid through doping with hydrogen. Although recent studies on nitrogen/hydrogen mixtures by other research groups have also observed several non-molecular nitrogen/hydrogen structures, recovery of these materials to ambient conditions has not yet been demonstrated. In this talk, I will describe our progress in the study of the synthesis, characterization, and recovery of extended solids of nitrogen from high pressure conditions from nitrogen/carbon monoxide mixtures. I will also detail results from our closely coupled modeling and simulation efforts and discuss how these results help guide our experimental efforts. New opportunities and challenges that have arisen in the course of our studies that will be pursued in the future will also be presented.

Feb 1

Feb 8 Mathilde Jauzac
Durham University, UK
host: Jason Steffen
From Lensing to Multi-Wavelength : (Some of) What you can do with massive galaxy clusters …

The outskirts of clusters make the best, and most efficient locations to observe and trace the mass assembly processes of the Cosmic Web. Residing at the vertices of this Cosmic Web (Bond et al. 1996), galaxy clusters grow by steady accretion of matter from the surroundings, as well as by discrete mergers with nearby groups and clusters. Supported by simulations, this scenario predictions regarding the total mass content and distribution in filaments themselves remain largely untested. Filaments are vital elements of the cosmic census, containing up to half the baryonic mass of the Universe as a ‘warm hot intergalactic medium’ but also the majority of the dark matter.

Recently, some of the most massive and disturbed clusters have been the centre of attention thanks to the Hubble Frontier Fields (HFF) initiative, which constitutes the largest commitment ever of Hubble Space Telescope (HST) time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. These clusters were chosen for their strong lens properties, and are all highly disturbed objects, showing major and minor merging on-going processes, making them ideal target to trace the Cosmic Web assembly.

While combining strong- and weak-lensing regimes to map the total mass with X-rays observations of the hot gas and spectroscopy of cluster galaxies to look at their direction of motion, we can thus study the dynamical scenarios in place within these massive galaxy clusters, and trace the substructures engaged in these processes. I will present the latest results we obtained on the HFF clusters, and discuss the different caveats present on both the observing and simulation sides. Finally, I will present the upcoming BUFFALO large HST programme, the ’spatial extension’ of the HFF that has just started back in July 2018.

Feb 15 Sarah Stewart
UC Davis
host: Jason Steffen
The Rosetta Stones of Planet Formation: Linking meteorites to giant planets

Meteorites are time capsules of planet formation. The most abundant meteorites types originate from primitive bodies that never heated to the point of differentiation and contain chondrules, which were transiently molten silicate spherules. These primitive planetesimals formed contemporaneously with planets over the first few million years of the solar system. The physical processes that formed chondrules and assembled them into planetesimals is one of the biggest unsolved problems in planetary science. I will present a new physical model for the formation of chondrules and planetesimals that links their origin to the dynamical excitation by the giant planets. I propose that the uncertain history of our giant planets, their formation locations and migration distances, was recorded by planetesimals and preserved in the asteroid belt. Meteorites are the Rosetta stones of planet formation that can translate the history of plantesimals to the history of the giant planets.

Feb 22 Lee Hartman
University of Michigan
host: Zhaohuan Zhu
Numerical experiments on star formation: mass functions and angular momenta

The initial mass function of stars - the number of stars N formed per interval of mass M - has long been known to be a power law, with power index of -1.3. Many explanations of this distribution have been advanced over the years, including fractal dimensions of star-forming molecular clouds and/or self-similar grouping of clouds. I will discuss simplified numerical simulations of star formation which show that gravitational accretion onto initial seed masses generates a power law mass distribution approaching power index of -1 asymptotically, irrespective of complications of cloud structure. This mechanism can also explain the mass function of young star clusters, which also has a power law distribution with the index of -1. Finally, I will describe preliminary results concerning the origin of protostellar cloud angular momenta, with implications for the properties of protoplanetary disks that result from cloud collapse.

Mar 1

Mar 8 Pierre Sokolsky
University of Utah
host: Bing Zhang
Joules from the sky: the search for the origins of Ultra-high Energy Cosmic Rays

Ultra-high energy cosmic rays represent the most energetic particles in Nature. Their origin is extragalactic, but remains mysterious. I will describe two experiments dedicated to resolving this mystery: the Telescope Array project in Utah and the Auger observatory in Argentina. Our current understanding of these particles will be reviewed as well as future efforts to deepen our understanding.

Mar 15 Anna Lia Longinotti
Instituto Nacional de Astrofísica
host: Daniel Proga
Fast X-ray winds and their connection to AGN feedback

The growing evidence for energy-conserving outflows in powerful and luminous AGN supports the idea that high-velocity winds launched from the accretion disc evolve after undergoing a shock with the ambient medium, and that they are capable to expel enough mass and energy so as to impact the host galaxy and produce the so-called AGN feedback often invoked in galaxy formation and evolution.

This talk will give an overview of AGN Ultra Fast Outflows with focus on recent results obtained from grating X-ray spectra of bright sources, that are unveiling a more complex structure of these winds than previously thought. I will review how UFO work, their observational properties and their relation with AGN outflows in other bands, what is their impact on the host galaxies and their role in feedback processes.

In particular, I will present the case for a multi-phase outflow in the Narrow Line Seyfert 1 Galaxy IRAS17020+4544 spanning from accretion disk to galaxy-scale, which has been targeted by an unprecedented multi-wavelength campaign including XMM-Newton, Chandra/LETG,VLBI, LMT, NOEMA, HST/COS.

Mar 22 Spring Break

Mar 29 Feng Liu
University of Utah
host: Qiang Zhu

Apr 5 Kengo Tomida
Osaka University
host: Zhaohuan Zhu

Apr 12 Dimitri Veras
University of Warwick
host: Jason Steffen
Transformative advances in post-main-sequence planetary system science

Connecting planetary systems at different stages of stellar evolution helps us understand their formation, evolution, and fate, and provides us with exclusive and crucial insights about their dynamics and chemistry. Post-main-sequence white dwarf and giant branch stars host planetary systems which include a variety of observed objects and phenomena, such as planetary debris discs, disintegrating and embedded asteroids, and photospheric metal pollution. Here, I provide a review of both our current knowledge of these systems and models which have been used to explain them. I also highlight the transformative advances expected in upcoming years with the current and next generation of ground-based and space-based initiatives. Looming orders-of-magnitude increases in available data must be accompanied by novel theories and simulations in order to understand the results from this interdisciplinary and expanding research field.

Apr 19 Kamal Choudhary
National Institute of Standards and Technology (NIST)
host: Qiang Zhu

Apr 26 Michael Brown
University of Washington
host: Ashkan Salamat

May 3 Alice Harding
NASA Goddard Space Flight Center
host: Bing Zhang

May 10 Study Week

May 17 Monika Moscibrodzka
Radboud University Nijmegen, Netherlands
host: Daniel Proga

Future forums: Fall 2019 

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