Events: Departmental Colloquia

• Computational tools for physics education

  Guest: Joan Adler, Technion
  Thursday, February 20, 2014 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

  iCal

  While computer use has revolutionized our lives and physics research, the extent of a "Computer Revolution" in physics education is much less clear. One may ask if computers have really made a difference to physics education over and above convenient administration and dissemination and testing of information. None of the latter is trivial, but it is somewhat obvious and common to all disciplines.

  - can computer use make a substantial difference in presentation of concepts?

  - do younger faculty use more computer based examples?

  - should and can computers replace hands on demonstrations and experiments? - what can realistically be accomplished?

  All physicist have opinions on these issues and during my talk I will describe some approaches of others and present examples from areas such as modern/quantum physics and condensed matter where computational tools can greatly benefit student comprehension.

  Thursday February 20, 2014

  Listeners are welcome to bring smartphones or tablets, (resisting the temptation to email or check the news) so they can experience some interactive websites in the last part of the talk.

• Quantum fine-structure of the Black Hole central singularity

  Guest: David Finkelstein, Georgia Tech
  Thursday, February 6, 2014 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  I review the curious reasoning that carried me through the outer Schwarzschild singularity and present my current attempt to resolve the inner one. Both are based on quantum theory.

• 3D X-ray imaging with Digitome®

  Guest: Prof. Dan Boye, Physics Department, Davidson College
  Thursday, January 23, 2014 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  Volumetric x-ray imaging offers exciting possibilities for multidisciplinary investigations. Davidson College is the first non-governmental entity to possess the Digitome® software, although this software has matured in development and has been applied in significant ways for 30 years. Unlike Computed Tomography (CT), the Digitome® software employs digital tomosynthesis through a series of conventional 2D x-ray images of an object taken from different non-planar perspectives. Using calibration information with regard to geometry and distances, the user can view the image from any direction and easily make measurements with 25-50 micrometer accuracy. The Digitome® user can even detect features obscured by intervening physical features within the object of study. A major benefit is the immediate scrolling through the object along any axis. Because of the small amount of data that is used, scrolling through 2D planes can viewed in near real time with just a typical desktop computer after taking the image, about a 15-minute process. Incorporation of this software into our liberal arts research and teaching program, including topics in archaeology, animal physiology and metrology, will be presented.

• Chemical Evolution and Gamma-Ray Astronomy

  Guest: Prof. Dieter Hartmann, Department of Physics and Astronomy, Clemson University
  Thursday, November 14, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  One of the grand themes of modern astrophysics is the progressive chemical enrichment of the Universe. Since the hot beginnings with little more than hydrogen and helium, the 4% mass fraction of the mostly metal free baryon component has been enriched to about 2%, by mass, in elements beyond H and He. The underlying cycle of star formation, stellar evolution, static/dynamic (explosive) nucleosynthesis and feedback of freshly processed material to the interstellar medium is also critical for our understanding of galactic evolution, and thus our general understanding of the building blocks of the Universe. This colloquium will discuss how gamma-ray astronomy is used to trace some of the relevant quantities involved in local chemical evolution, through "Astronomy with Radioactivities", and how observations in the gamma-ray regime shed light on chemical evolution on cosmic scales.

• Non-thermal Processes on Lunar Surfaces and at Buried Ice: Graphite Grain Interfaces

  Guest: Prof. Thomas Orlando, School of Chemistry and Biochemistry and School of Physics Georgia Institute of Technology
  Thursday, November 7, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium Rm. 202

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  Photodesorption and photodissociation of H2O following 157-nm irradiation of amorphous solid water deposited on an impact melt breccia from the Apollo 16 mission has been studied. The desorbing H2O and O(3PJ=2,1,0) products were detected with resonance-enhanced multiphoton ionization (REMPI). Vibrationally excited water was also detected with non-resonant ionization. Direct desorption involves exciton decay which is in competition with molecular dissociation events on the surface. Cross sections for H2O (v = 0) removal were found to increase with decreasing coverage and are considerably higher than photodesorption from common metal oxides. This work represents the first measurement of an absolute photodesorption cross section from an actual lunar sample. The vacuum ultraviolet (121.6 nm) synthesis of carbon dioxide on ice-coated graphite and isotopic labeled 13C graphite has also been examined. The results show that CO2 can be formed at the buried ice:graphite interface with Lyman-α photon irradiation via reaction of radicals (O and OH) produced by direct photodissociation and dissociative electron attachment (DEA) of the interfacial water molecules. The synthesized CO2 molecules can desorb in hot photon dominated regions (PDRs) and lost to space when ice coated carbonaceous dust grains cycle within the protoplanetary disks. Thus, non-thermal formation of CO2 at the buried ice:grain interface by VUV photons may regulate the carbon inventory during the early stage of planet formation. This may help explain the carbon deficits in our solar system, and suggests that a universal carbon deficit gradient may be expected within astrophysical bodies surrounding center stars.

• The PhD Skills --- a personal view

  Guest: Prof. Yiping Zhao, UGA Department of Physics and Astronomy
  Thursday, October 24, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  For some college students, the pursuit of a PhD degree is a very important goal for their future career. This period of time could be one of the most precious/critical moments in their life for their skill development. Facing this changing and competitive modern world, what are the necessary skills that a graduate student needs to develop? I will give some of my personal thoughts on this subject based on my own experience, and hope they are useful for some of the graduate and undergraduate students.

• Mechanosensitive Fluorescent Dyes -- Molecular Rotors: Fundamentals and Applications

  Guest: Prof. Mark Haidekker, UGA College of Engineering
  Thursday, October 17, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  Many fluorescent dyes change their photophysical properties with changes in their environment. Examples include fluorescent dyes that are sensitive to the environment's polarity, pH, presence of ions, temperature or electrostatic potential. A group of fluorophores whose photophysical mechanism is excited- state intamolecular charge transfer (ICT) have been termed "molecular rotors" because of their additional capability to form twisted conformations. Conformational changes are associated with changes in the ICT dipole, and twisting during the excited state changes the dye's fluorescence emission.

  Intramolecular twisting depends on the environment, most notably, microviscosity. Molecular rotors have therefore been established as nanoscale, real-time viscosity reporters with the potential to exceed mechanical viscometers in precision, however, confounding factors that influence measured intensity need to be eliminated. Lifetime spectroscopy and engineered ratiometric dyes present possible solutions.

  Molecular rotors have also been reported to show sensitivity towards fluid shear stress. The underlying photophysical mechanism is not yet understood, but two competing possible explanations involve polar-polar interaction and the formation of photoisomers. Data in support of both hypotheses exist, and it is likely that no single mechanism fully explains shear-sensitivity. From the application side, however, molecular rotors provide unsurpassed sensitivity at low flow rates combined with high spatial resolution.

• Kondo Phenomenon at Steady-State Nonequilibrium

  Guest: Dr. Hong, POSTECH & Asia Pacific Center for Theoretical Physics, Korea
  Thursday, October 10, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  Nonequilibrium Kondo problem is one of long standing problems in condensed matter physics. For this reason, all of the experimental observations for the tunneling conductance of various Kondo-involved mesoscopic systems are not properly explained. In this colloquium, I will introduce basic Kondo physics in equilibrium and extend the discussion to steady-state nonequilibrium in which entangled Kondo singlets perform coherent tunneling depending on the amount of bias. Then, I clarify that the Kondo phenomenon at steady-state nonequilibrium is the side peaks appearing in dI/dV vs. V curve, and I reproduce the dI/dV line shapes of various mesoscopic Kondo systems such as quantum dot single-electron transistor, quantum point contact, and adsorbed magnetized atom on a metallic substrate. I will show that multilayer graphene and high-Tc superconductor can be treated in the same category. I will introduce the theoretical method to obtain the on-site Green’s function for the two-reservoir Anderson impurity model under bias, which gives the tunneling conductance mentioned above.

• The strong interaction: status, strategies, and perspectives.

  Guest: Prof. S. Krewald, Research Center Juelich, Germany
  Thursday, October 3, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  Nuclear physics investigates the origin and structure of strongly interacting matter: the atomic nucleus, the proton and the neutron, and the quarks. The interaction between quarks has a remarkable property called asymptotic freedom: it decreases for small distances between the quarks. It is widely believed that quarks do not exist as free particles because a separation is prevented by the growing forces. Unfortunately, the growing coupling strengths make calculations for larger distances virtually impossible. There are two strategies to make progress. Theory has developed lattice simulations of the strong interaction. Experimentally, one studies meson production in nuclear reactions. The Thomas Jefferson National Laboratory has taken a leading role in this field using photonuclear reactions as a probe to produce Baryon resonances and to study their decays in a large energy region ranging from the reaction threshold up to approximately 3 GeV. The extraction of resonance properties from the data at large energies is non-trivial, as one has to take into account the final state interactions between the produced hadrons. Nuclear theory has made progress during the last decade by developing effective theories of meson-baryon reactions that can describe the data. 

• Lightwave Neuromorphic Signal Processing

  Guest: Prof. Mable Fok, UGA College of Engineering, Electrical and Electronics Engineering
  Thursday, September 26, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  In this talk, a new computational paradigm is presented using photonics to mimic the functionality of a spiking neuron. Spike processing is both computationally efficient and scalable, adopting the best features of both analog and digital computing. Whereas biological neurons implement spike processing using electrochemical spikes on a millisecond time scale, our “photonic neuron” performs spike processing using optoelectronic technology in picoseconds. Like it’s physiological counterpart, the photonic neuron consists of a reconfigurable FIR filter at it’s front end followed by integration and thresholding stages. A single photonic neuron circuit accomplishing feature recognition and imitating the escape response of a crayfish will be presented. One of the most powerful capabilities of neurons is their ability to learn and adapt to the environment based on experience. This is accomplished by adjusted the strength of synaptic connections based on correlations between pre-synaptic and post-synaptic activity. The first demonstration of learning using photonic technology will be presented, potentially laying the foundation for learning at speeds a billion times faster than biological neurons.

• Ultrashort pulse propagation and ionization in dielectrics

  Guest: Prof. Jeremy Gulley, Department of Biology and Physics, Kennesaw State University
  Thursday, September 19, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  The numerous applications of femtosecond lasers during the last 15 years has necessitated a detailed understanding of pulse propagation coupled with ultrafast laser-material interactions. Current ultrashort pulse propagation models describe the evolution of fields with broad spectra while models of laser-induced ionization and laser-plasma interactions typically assume monochromatic laser fields. In this seminar I address the inherent contradiction of combining multi- chromatic propagation models with monochromatic laser-material descriptions. Recently published experimental and simulation results are presented that show how this contradiction leads to order-of- magnitude errors in calculating the ionization yield. Further simulation results suggest that multi-chromatic effects will alter the shape and severity of laser-induced modifications to the bulk of dielectric solids.

• Exploring Spin Glasses and other Complex Energy Landscapes with Extremal Dynamics

  Guest: Stefan Boettcher, Emory University
  Thursday, September 12, 2013 4:00 pm - 12:00 am
  Location: Physics Auditorium Rm. 202

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  I describe Extremal Optimization (EO), a general-purpose, local search heuristic for hard combinatorial (and physical) problems  such as bi-partitioning (ie, model-B ferromagnets), coloring (ie, Potts antiferromagnets), and max-cut (ie, Ising Spin Glasses). EO is motivated by the Bak-Sneppen model of self-organized criticality (SOC). SOC provides a general dynamics of driven dissipative systems that operate far from equilibrium and exhibit many emergent properties,  such as scale-free fluctuations, memory and learning, and persistent returns to untypical (here: ground-state) configurations. The generic properties of EO are explored, which explain the efficiency of EO in searching many 'complex energy-landscapes'. Numerical results of EO are discussed especially for the Edwards-Anderson spin-glass problem.

• Atomic and Molecular Collisions using a Time-Dependent Close-Coupling Method

  Guest: Mitch Pindzola, Auburn University
  Thursday, September 5, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium Rm. 202

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  We review the recent progress made in applying the time-dependent close-coupling approach to ionizing collisions of electrons, photons, and bare ions with small atoms and molecules.

• Massive Star Formation Through The Universe

  Guest: Jonathan Tan, University of Florida
  Thursday, August 29, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium Rm. 202

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  Massive stars have played a dominant role in shaping our universe since its earliest times, but there is no consensus on the mechanism by which they form. I review the physical processes thought to be important in massive star formation, concentrating on a particular theoretical model, Turbulent Core Accretion. This assumes the initial conditions are massive, turbulent, magnetized cloud cores of gas and dust that are reasonably close to virial equilibrium. We test this via theoretical simulations of the physics and chemistry of the interstellar medium and observational searches for these cores. We next consider the protostellar collapse phase as a massive star grows from the core. Various forms of feedback become important in reducing the efficiency of accretion, although it is not clear if one particular mechanism operates to set a fundamental limit on the maximum stellar mass. Again, these theoretical ideas can be tested by observations of massive stars forming in our Galaxy today. Finally, I discuss an application of massive star formation theory to the early universe: how massive were the first stars and could they have been the progenitors of supermassive black holes?

• Nanostructured Surface Coatings and Light

  Guest: George Larsen, Kirkpatrick Award Winner, UGA Physics and Astronomy
  Thursday, August 22, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium, Rm. 202

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  Due to their small size and tailorable properties, nanostructures can exhibit unique interactions with light. These interactions have the potential to impact all areas of technology. In this talk I discuss some aspects of designing and making nanostructures using glancing angle deposition (GLAD), a simple and scalable method, for specific applications that harness light. In particular, I will focus on our work in fabricating photocatalytic materials, which can use ambient lighting to split water into oxygen and hydrogen fuel, break down harmful organic pollutants, and even kill bacteria. Additionally, I will discuss our more recent work using GLAD to fabricate metamaterials. Metamaterials have engineered structures that allow them to exhibit unique properties not typically found in nature, the most famous of which is a negative index of refraction. 

• Undergraduate Awards Day

  Guest: Steve Compton
  Thursday, April 25, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  TBD

• Graduate Awards Day

  Guest: TBD
  Thursday, April 18, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  TBD

• Hot Gas in the Galactic Halo

  Guest: David Henley, UGA Physics and Astronomy
  Thursday, April 11, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  The Galactic halo, above and below the Galactic disk, is filled with gas
  at ~1-3 million K. This gas is a major contributor to the diffuse soft
  X-ray background (SXRB) in directions looking out of the Galactic disk. 
  Despite many years of study, the origin of the hot halo gas remains
  uncertain. X-ray spectroscopy of the SXRB emission enables us to
  determine the physical conditions in the hot gas, providing clues to its
  origin and evolution.
  
  We have recently completed a survey of the SXRB using data from the  
  orbiting XMM-Newton X-ray observatory. I will describe how we use the 
  data from this survey to measure the halo's X-ray emission, and how we  
  are using these results to test physical models for the origin of the
  hot gas.

• The Fascinating Structure of Hadrons: What have we learned about excited protons?

  Guest: Volker Crede, Florida State University, Physics Department
  Thursday, April 4, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  Hadrons are composed of quarks and gluons held together by the strong force. They can be categorized into two families: baryons, three-quark states such as the proton and the neutron, as well as mesons, quark-antiquark states such as the pion. Other types of hadrons may exist and much effort is invested in state-of-the art experiments around the world to search for new forms of matter, but no current evidence conclusively suggests the existence of unconventional or exotic hadrons. The composite nature of these particles manifests itself in the existence of a rich spectrum of excited states. The properties of these resonances can be identified by systematic investigations using electromagnetic and strong probes, primarily with beams of electrons, photons, and pions. In the spectroscopy of baryon resonances, after decades of research, the fundamental degrees of freedom underlying the baryon excitation spectrum are still poorly understood. The search for hitherto undiscovered bu
  t predicted resonances continues at many laboratories worldwide. Recent results from photo- and electro-production experiments provide intriguing indications for new states and shed light on the structure of some of the known nucleon excitations. The continuing study of available data sets with consideration of new observables and improved analysis tools have also called into question some of the earlier findings in the field. I will present the hadron spectroscopy program at Jefferson Laboratory and will discuss recent progress toward understanding baryon resonances. 

• The Outcome of the LHC Search for the Higgs Boson

  Guest: Tom LeCompte, Argonne National Lab
  Thursday, March 28, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  The idea that the universe might be filled with scalar fields to explain why fundamental particles are not all massless was born, and incorporated into the highly successful Standard Model of fundamental particles and interactions, nearly 50 years ago. Until this summer there remained one unconfirmed prediction of this model, the existence of a massive particle with the same quantum numbers as the vacuum: the Higgs boson. The confirmation or falsification of this prediction is one of the highlights of the physics program of the Large Hadron Collider (LHC) at CERN. I discuss the recent experimental evidence for the existence of the Higgs Boson, concentrating on results from the ATLAS experiment.

• Unraveling the complexity of simple models: From knotted proteins to snowy landscapes

  Guest: Thomas Wuest, WSL Switzerland
  Thursday, March 7, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  With the continuous increase in computer power, computational scientists are able to devise models of the physical world with ever increasing complexity and detail. Despite the accuracy of such sophisticated models though, the "forest" of underlying basic principles often gets lost within the "trees" of details of the model. In this talk I will illustrate the benefits of the reverse direction: With examples ranging from materials science, protein folding and snow physics, I will show that intricate scientific questions can often better be understood by simplified models where unimportant details are systematically peeled off (reductionism). In particular, the investigated topics are: (i) Growth-induced polarity formation in molecular crystals; (ii) statistical physics questions in proteins and polymers; (iii) snow depth distribution and smoothing in complex terrain. Besides giving insight into fundamental behavior of complex physical phenomena, I will demonstrate that simplified models surprisingly often pose also a considerable computational challenge asking for sophisticated algorithms. 

• Density and Diffusion Anomalies in Lattice Model for Water

  Guest: Prof. Marcia Barbosa, Instituto de Física, Universidade Federal do Rio Grande do Sul
  Thursday, February 28, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

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  Water is one of the most abundant substance on the planet, however its thermodynamic and dynamic properties are away from being fully understood. Unlike other liquids, its specific volume at ambient pressure increases when cooled below T = 4oC. Besides, the isothermal compressibility, κT and the specific heat at constant pressure, CP , have a minimum at T = Tmin. For temperatures below Tmin, κT and CP increase with temperature decrease and above Tmin, κT and CP increase with temperature increase.

  In the last years the interest the supercooled region of the pressure temperature phase diagram has increased. In this region water is forced to be in liquid state due to fast freezing of the system. Different from normal liquids, the self diffusion, D, of the supercooled water increases with the compression up to maximum value Dmax(T) at p = pDmax. Beyond this maximum value, for higher pressures, the ”normal” behaviors is restored, and diffusion decreases with pressure. This results are supported by numerical simulation using the SPC/E water model where the supercooled region is easily accessed.

  In this talk we present a lattice model in which the directionality present in the hydrogen bonds of water is introduced. This model is capable to reproduce qualitatively the density and diffusion anomalies observed in liquid water. In addition it provides a new scenario for the liquid-liquid phase transition proposed for liquid water. 

• Nitric Oxide and its Van der Waals Complexes: A Fresh Look at a Tiny Molecule and its Impact

  Guest: Henning Meyer, UGA Physics and Astronomy
  Thursday, February 21, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

  iCal

  Nitric oxide is a persistent radical which results from the direct combination of nitrogen and oxygen. While it has a long standing history, it is still of considerable chemical and biological importance. More recently, research efforts have been reinvigorated by efforts to control NO emissions from internal combustion engines (especially with the advent of alternative fuels) and to understand its functional role in biology. NO has indeed been identified as a key physiological regulator involved in a variety of biological processes. An important factor in the chemistry of NO and NO2 is the strength and prevalence of van der Waals complex formation, especially their self and cross dimers.

• Chemical Evolution and Gamma-Ray Astronomy

  Guest: Dieter Hartmann, Clemson, Department of Physics and Astronomy
  Thursday, February 14, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

  iCal

  One of the grand themes of modern astrophysics is the progressive chemical enrichment of the Universe. Since the hot beginnings with little more than hydrogen and helium, the 4% mass fraction of the mostly metal free baryon component has been enriched to about 2%, by mass, in elements beyond H and He. The underlying cycle of star formation, stellar evolution, static/dynamic (explosive) nucleosynthesis and feedback of freshly processed material to the interstellar medium is also critical for our understanding of galactic evolution, and thus our general understanding of the building blocks of the Universe. This colloquium will discuss how gamma-ray astronomy is used to trace some of the relevant quantities involved in local chemical evolution, through “Astronomy with Radioactivities”, and how observations in the gamma-ray regime shed light on chemical evolution on cosmic scales. 

• Dynamic UC HII regions in Sgr B2: Flickering and Ionized Flows

  Guest: Chris DePree, Agnes Scott, Department of Physics and Astronomy
  Thursday, February 7, 2013 4:00 pm - 5:00 pm
  Location: Physics Auditorium (202)

  iCal

  The Sgr B2 star forming region in our Galaxy contains a large sample of HII regions, and the diversity of morphologies and number of unusual broad line sources make it an ideal laboratory for testing theories of ultracompact (UC) HII region evolution. Recent high-resolution, radiation-hydrodynamic simulations show that the dense, rotating, accretion flows required to form massive stars quickly become gravitationally unstable. The resulting HII region flickers between hypercompact (HC) and UC sizes throughout the main accretion phase, rather than monotonically expanding. Imaged at 1.3 cm, the Sgr B2 region contains 49 regions, 25 of which are hypercompact (HC), with physical diameters < 5000 AU. We have re-observed this large sample of UC HII regions in the Sgr B2 region after 23 years in the three hybrid arrays in the 1.3 continuum and H66a and H68a lines with a resolution of 0.25². Preliminary results from these new 1.3 cm EVLA observations of Sgr B2 have revealed that 4 of the 49 sources have significant changes in flux density (>10 sigma).

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