Physics Colloquium - Fall 2019

Speaker: Peter Plavchan, George Mason University

Abstract: We will present the discovery of a two-planet system orbiting a nearby young star with a debris disk. One inner planet is discovered using data from NASA’s TESS mission and a second planet with multi-wavelength radial velocities. The two newly identified planets in this system can be used to investigate disk-planet interactions and inform the planet formation and migration process. We will present how this planetary system was found, primarily utilizing the near-infrared radial velocity technique we have developed using the iSHELL spectrometer on the NASA Infrared Telescope Facility. We will also discuss the effects of stellar activity and how multi-wavelength Doppler velocities may allow us to mitigate its effects in our search for planets with the Doppler technique.

Speaker: Jian Du-Caines, University of Louisville, Department of Physics & Astronomy

Abstract: Solar thermal tides are dominant global-scale oscillations in temperature, density, and winds whose periods are integral fractions of a solar day in the middle and upper atmosphere. They can be excited by the absorption of solar radiation from tropospheric water vapor and stratospheric ozone, latent heat release of deep convective system, as well as nonlinear interactions between tides and planetary waves. Tides generated from the lower atmosphere can propagate upwards and deposit their energy and momentum in the upper atmosphere during dissipation process and induce profound effects on the ionosphere and thermosphere. Present estimates are that atmospheric waves (including tides, planetary and gravity waves) from the lower atmosphere contribute on average roughly the same amount of energy to the upper atmosphere as forcing from above does during geomagnetic quiet conditions and medium solar conditions. Accurate forecasting of space weather requires a better understanding of the temporal and spatial variability of these waves. This talk reviews the complex picture of atmospheric tides in our atmosphere, their inter-annual, seasonal, day-to-day variability and how they affect the ionosphere and thermosphere.

Speaker: Karen Collins, Harvard-Smithsonian Center for Astrophysics

Abstract: The Transiting Exoplanet Survey Satellite (TESS) launched on April 18, 2018 atop a Falcon 9 rocket. It started science operations on July 25th, 2018 and is surveying most of the sky over a period of two years, divided into 26 sectors that are each observed for ~27 days. The main mission objective is to search the brightest stars near Earth for transiting exoplanets. The primary science goal is to measure the masses of 50 planets having radius less than four times that of Earth. The TESS data are already producing 50-100 quality transiting planet candidates (PCs) per month, and they are expected to produce thousands over the two-year nominal mission. TESS has large 21 arcsec pixels and photometric apertures with radius ~1 arcmin, which are often contaminated with multiple stars. Therefore, large ground-based photometric, spectroscopic, and high-resolution imaging TESS Follow-up Observing program (TFOP) teams are required to reject false positives and confirm and characterize bona fide planets. An overview of the TESS mission, and an introduction to TFOP will be presented, followed by a mission status update. A description of the publicly available TESS data products will be provided, along with a summary of the most interesting TESS discoveries so far. Finally, we will look ahead to the recently approved extended TESS mission, and other upcoming NASA missions.

Speaker: Bob Williams (Bullitt Lecturer), UCSC & Space Telescope Science Institute

Abstract: Accidentally discovered during a search for quasars, J0608 is a low-mass Wolf-Rayet star on the outskirts of the Large Magellanic Cloud. Unlike other low-excitation [WC] stars it does not appear to be the central star of a planetary nebula. The spectrum is rich in C II emission lines, a surprising number of which originate from doubly excited autoionizing levels, and it originates in a stellar wind that also produces O II and He II absorption features. Numerous spectral features remain unidentified and details of the process by which a spectroscopic analysis is being made will be described.

Speaker: Ming Yu, University of Louisville, Department of Physics & Astronomy

Abstract: Accelerating the discovery of low-dimensional materials having improved properties and advanced capabilities are essential in the 21st century to enable future development of nanotechnology in challenge applications such as flexible electronics, portable sensors, solar panels, photodiodes, phototransistors, tunneling devices, etc. Experimental and computational simulating methodologies, as well as machine learning, developed recently, are mainstream routes to explore nanomaterials. In this talk, I will focus on the computational discovery and design of low-dimensional materials by employing first-principles and semi-empirical methods. The routes in predicting new nanomaterials and methods used in computational simulations will be discussed. I will present our recent theoretical predictions on low dimensional materials as examples including (1) boron structures based on icosahedron B12, (2) SiC nanowires and sheets, and (3) sandwiched two-dimensional phosphide binary compounds sheets. The structural optimizations and analyses of structural properties will be discussed. At the end, I will briefly talk about our ongoing work on the discovery of blue phosphorene with Li intercalation and design of lateral heterostructures formed by two-dimensional SiC and GeC sheets.

Speaker: Benne Holwerda, University of Louisville, Department of Physics & astronomy

Abstract: I will present an overview of the GHOSTS survey, the largest study to date of the resolved stellar populations in the outskirts of disk galaxies. The sample consists of 14 disk galaxies within 17 Mpc, whose outer disks and halos are imaged with the Hubble Space Telescope. Iwill explore the benefits of using resolved stellar populations, and discuss the F606W and F814W photometry. We use artificial star tests to assess completeness and use overlapping regions to estimate photometric uncertainties. The median depth of the survey at 50% completeness is 2.7 mag below the tip of the red giant branch (TRGB). We comprehensively explore and parameterize contamination from unresolved background galaxies and foreground stars using archival fields.

Speaker: Srinivasan Raghunathan, UCLA

Abstract: Galaxy clusters are the most massive gravitationally bound objects in our Universe and are remarkable cosmological probes. Measuring their abundances as a function of mass and redshift can reveal great deal of information about the parameters that influence the geometry and structure growth in the Universe like neutrinos and dark energy. The biases present in the measurement of cluster masses, however, limit their potential as cosmological probes.

In this talk, I will discuss an unbiased method of estimating cluster masses using the weak-gravitational lensing of the cosmic microwave background (CMB). I will then detail the recent efforts in calibrating the mass-richness relation of clusters detected by the Dark Energy Survey using the CMB-cluster lensing measurements from the South Pole Telescope. I will also explain the systematics associated in the measurements and the importance of using CMB polarisation datasets in the future. I will finish by showing the recent first detection of the polarised CMB-cluster lensing signal.

Those attending the talk are warmly invited for informal discussion with the speaker afterward at a local establishment.

Speaker: Meysam Akhtar, University of Illinois, Urbana –Champaign

Abstract: Accurate measurement of the hot spot and combustion temperatures is an essential part of understanding the behavior of insensitive high explosives. These explosives are particularly interesting because they are powerful but insensitive to accidental initiation. In this talk, I will discuss measuring the temperature dynamics of shocked initiated insensitive high explosives with nanosecond time resolution. The explosive temperatures were determined by measuring the spectral radiance in the visible region and fitting the radiance to a graybody model. These measurements are made possible by our shock compression microscope, which looks into materials as they are subjected to controlled high-velocity impacts from laser-launched flyer plates. The tiny plates of metal foil move up to several kilometers per second and can produce a shock wave with up to 200,000 atm pressure jump within one nanosecond while compressing matter up to twice the density. We can fabricate arrays of 1 mm diameter plastic-bonded explosive charges and push them to detonation while we watch. This shock compression microscope lets us see right inside detonating high explosives with high time and space resolution and measure the temperature, pressure, and velocity in real-time. These techniques are not limited to explosives but can find applications in physics, materials science, planetary sciences, biomedicine, chemistry and astronautics.

Speaker: Gerard Williger, University of Louisville, Department of Physics & Astronomy

Abstract: Euclid and LSST are two independent projects which complement each other very well scientifically, but which have had significant problems in working together due to non-scientific reasons. I will first give a head-to-head comparison of the two projects, then discuss several areas where collaboration would significantly improve the results compared to each project working independently. These areas include deblending objects, spectral information for/IR colours of variable sources, galaxy morphologies, improving galaxy ellipticity measurements for weak lensing studies, improved photometric redshifts, more precise cluster mass estimates etc. This talk is based largely on Rhodes et al. (2017, ApJS, 233, 21) but I may add more recent material.

Speaker: B. Bhatia, University of Louisville, Department of Mechanical Engineering

Abstract: Heat transfer processes serve as the backbone of our energy systems. The key to efficient, sustainable energy conversion relies on delivering heat at a higher temperature, rejecting heat at a lower temperature and utilizing renewable energy resources to do so. In this talk, I will present our work on multiscale engineering of thermal processes to achieve high-efficiency energy conversion from ambient sources including waste heat, low-temperature upper atmosphere and the sun. At the nanoscale, I will describe our work enabling high-power-density thermal-to-electric energy conversion using pyroelectric thin films. At the centimeter-scale, I will present a novel directional approach to achieving passive cooling below ambient temperature using commonly available materials by exploiting the high transparency of earth’s atmosphere in mid-infrared wavelengths and the angular confinement of the sun. At the meter-scale, I will demonstrate high-efficiency solar-to-thermal energy conversion using custom-fabricated thermally insulating silica aerogels with record-high solar-transparency. These advances in thermal engineering, from the nanoscale to megascale, offer compelling solutions to the problems of waste heat harvesting, solid-state cooling and solar-thermal energy conversion – significant challenges on our path towards a sustainable energy future.