Forum Schedule Fall 2018

Modern engineering applications desire materials with significantly improved properties including superior mechanical, chemical and physical properties, enhanced adhesion and surface properties, and superior electronic properties. For example, body armor for soldiers requires lightweight, high strength and high ductility for the materials. First principles based atomistic modeling approaches offer unique opportunities to elucidate the fundamental mechanisms of materials and processes at relevant service conditions, which are essential to designing lighter, stronger, more ductile and more environmental friendly materials. In this talk, we will use following examples to illustrate how to design materials with improved properties by applying first principles based atomistic simulations: (1) Design ductile superhard ceramics; (2) Design mechanical stable thermoelectric materials using nano-scale twins; (3) Improve the catalytic efficiency of Fe in Haber–Bosch process.

The first gravitational wave event due to the merger of two neutron stars detected by humankind, GW170817, was associated with a short GRB 170817A, a “kilonova" AT2017gfo, and a broad-band (from radio to X-rays) afterglow. I will critically review the observational data and various ideas of interpreting this watershed event, and present my understanding of the event regarding the origin of the GRB and its 1.7 seconds delay with respect to the merger, origin of the ``kilonova’’and the broadband afterglow, as well as the nature of the merger product (black hole vs. a long-lived neutron star) and implications for the poorly constrained neutron star equation of state.

In the region close to compact object such as black holes or neutron stars, the extreme conditions created by the strong gravitational field produces copious amounts of energetic radiation (ultra-violet, X-rays, and Gamma-rays). The interaction of this radiation with the surrounding material results in observables that carry important physical information. X-ray spectral and timing techniques provide direct access to the accretion physics on these systems, such as the black hole spin, inner-disk radius, ionization stage, among others. In this talk I will discuss the current state of modern relativistic reflection models that have been recently computed and tailored specifically for (1) supermassive black holes in AGN and black hole binary systems in the hard state; (2) neutron star X-ray binaries; and (3) carbon and oxygen rich ultra-compact X-ray binaries. I will present the implementation of our new models to observational data from several X-ray observatories (RXTE, Swift, Chandra, XMM-Newton, Suzaku, NuSTAR, and NICER), and discuss current outstanding issues, such as the large iron abundances frequently required to fit the reflection spectra, controversies on the disk truncation in BHB, the origin of the soft excess in AGN, and the effects of high density in the observed spectra.

Designing materials to withstand the extreme conditions of fusion requires understanding, at the atomic scale, material evolution at those same conditions. Accelerated molecular dynamics (AMD) methods, which extend the timescale of conventional molecular dynamics while retaining the fidelity of the interatomic interactions, is a key tool in developing this understanding. We have used these methods to examine the behavior of helium bubbles in tungsten with the goal of informing higher level models of basic mechanisms over timescales relevant for fusion conditions. This talk will summarize both past results as well as more recent work, highlighting and contrasting the growth mode of bubbles in bulk tungsten versus at a grain boundary, examining the effect of helium arrival rate on the development and evolution of bubble networks, and determining the rate of migration of embryonic bubbles. Together, these results complement other simulation studies and provide a more comprehensive picture of the dynamics associated with helium bubbles in tungsten.

The number 1.618 known as “The Golden Ratio,” has fascinated scholars since antiquity. Some even considered it to be divine, and others were obsessed with it. In this talk I will describe the incredible history of this number, and its uncanny appearances, true and false, in natural phenomena, in the arts, in mathematics, and in human-created artifacts. This talk is intended for a general audience including enthusiasts of all backgrounds and ages.

Astrophysical discs are often warped, that is, their orbital planes change with radius. Discs can be warped by e.g. Lense-Thirring precession induced by a misaligned spinning black hole, or the gravitational pull of a misaligned companion. I will discuss the dynamics of warped discs, provide some ideas on the role of disc `viscosity' in their evolution, and present simulations of tilt oscillations, Kozai-Lidov oscillations and disc tearing. Finally I will present the results of a stability analysis of the warped disc equations to derive the criteria for the warp to be unstable and the disc to separate into distinct planes.

The first large bodies to form in the Solar System are planetesimals, 1-100 km sized objects that are primordial versions of asteroids and comets. In this talk I will discuss what we know about how planetesimals form, and what the consequences may be for the subsequent stages of growth that yield terrestrial and giant planets. A popular hypothesis is that planetesimal formation occurs due to instabilities in aerodynamically coupled mixtures of solids and gas, and I will present the results of new simulations that allow us to predict the properties of planetesimals that form in this way.

Fast radio bursts (FRBs) are bright millisecond-duration transient events, which were the major unexpected astronomical discovery in decades. There are thousands of bright (> 1 Jy ms) FRBs per day coming from random directions across the entire sky. One source, FRB 121102, was found to generate numerous bursts. This allowed follow-up observations to pin-point its host galaxy, which is at a distance of ~1 Gpc. Recent polarization measurements show that the bursts from FRB 121102 are nearly 100% linearly polarized and that their polarization position angles vary within a small confined range. We show that these two properties can be explained by propagation effects through the magnetosphere of a strongly magnetized neutron star, provided that its rotation period is longer than 0.3 seconds and its magnetic inclination is less than about 20 degrees. In the future, when more bursts from the repeater are detected with polarization measurements, it should be possible to measure the rotation period of the progenitor and test its neutron star nature. I will also present a prediction of the maximum luminosity Lmax for FRBs under the coherent curvature emission model. The existence of this Lmax is because the electric field responsible for accelerating the emitting particles becomes close to the quantum critical strength and is then quickly shielded by Schwinger pairs within a nano-second.

Quasars are among the most luminous objects in the sky. These very energetic regions lie at the center of massive galaxies and are powered by a super-massive black hole. While it has been found that there is a correlation between the mass of these super-massive black holes and the mass of the surrounding galaxies, the co-evolution of galaxies and quasars is barely understood. Outflows launched from the vicinity of super-massive black holes are a key piece in this puzzle, potentially linking the small and the large-scale phenomena. We have discovered that some of this gas is outflowing at very high speeds (up to 216 million km/hr, 20% of the speed of light!). To better characterize the frequency and properties of these outflows, our group is conducting an encompassing multi-wavelength study using data from, among others, the Sloan Digital Sky Survey, Hubble Space Telescope and Chandra X-ray Observatory. All these results have implications for current theoretical models and will help us better understand the quasar phenomenon and their environments.

During the first million years of evolution, nascent planetary systems are embedded in dense disk-shaped clouds of gas. These circumstellar disks are home to a myriad of hydrodynamical processes, which bring about turbulence and the emergence of viscous-like behavior, enabling accretion of gas onto the protostar. Meanwhile, micron-sized dust grains embedded in the disk are growing through coagulation onto pebbles and rocks. Turbulence has a positive effect on these small solids, concentrating them into transient high pressure regions for long enough to achieve gravitational collapse through pebble accretion into km-sized bodies, forming the first planetesimals. Giant storm systems in the disk, similar to Jupiter's Great Red Spot, may exist in quiescent zones of the disk. These are even more prone to collecting solid material, producing the first terrestrial planets and cores of giant planets. In this talk I will discuss the state of the art and recent advances in the field of planet formation, as well as pressing problems such as the structure observed in high resolution sub-millimeter images of circumstellar disks, and how to interpret them.