Friday, February 26th, 2016

Katelyn Allers – Garcia, E. V.; Dupuy, Trent J.; Allers, Katelyn N.; Liu, Michael C.; and Deacon, Niall R. “On the Binary Frequency of the Lowest Mass Members of the Pleiades with Hubble Space Telescope Wide Field Camera 3.” Astrophysical Journal 804, no. 1 (2015) : 65.

Katelyn Allers, Associate Professor of Physics & Astronomy

We present the results of a Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging survey of 11 of the lowest mass brown dwarfs in the Pleiades known (25-40M(Jup)). These objects represent the predecessors to T dwarfs in the field. Using a semi-empirical binary point-spread function (PSF)-fitting technique, we are able to probe to 0 ”.03 (0.75 pixel), better than 2x the WFC3/UVIS diffraction limit. We did not find any companions to our targets. From extensive testing of our PSF-fitting method on simulated binaries, we compute detection limits which rule out companions to our targets with mass ratios of greater than or similar to 0.7 and separations greater than or similar to 4 AU. Thus, our survey is the first to attain the high angular resolution needed to resolve brown dwarf binaries in the Pleiades at separations that are most common in the field population. We constrain the binary frequency over this range of separation and mass ratio of 25-40M(Jup) Pleiades brown dwarfs to be < 11% for 1 sigma (< 26% at 2s). This binary frequency is consistent with both younger and older brown dwarfs in this mass range.

Garcia, E. V.; Dupuy, Trent J.; Allers, Katelyn N.; Liu, Michael C.; and Deacon, Niall R. “On the Binary Frequency of the Lowest Mass Members of the Pleiades with Hubble Space Telescope Wide Field Camera 3.” Astrophysical Journal 804, no. 1 (2015) : 65.

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Friday, February 26th, 2016

Katelyn Allers – Petigura, Erik A.; Schlieder, Joshua E.; Crossfield, Ian J.M.; Howard, Andrew W.; Deck, Katherine M.; Ciardi, David R.; Sinukoff, Evan; Allers, Katelyn N.; Best, William M.J.; Liu, Michael C.; Beichman, Charles A.; Isaacson, Howard; Hansen, Brad M.S.; and Lepine, Sebastien. “Two Transiting Earth-Size Planets Near Resonance Orbiting a Nearby Cool Star.” Astrophysical Journal 811, no. 2 (2015) : 102.

Katelyn Allers, Associate Professor of Physics & Astronomy

Discoveries from the prime Kepler mission demonstrated that small planets (<3R(circle plus)) are common outcomes of planet formation. While Kepler detected many such planets, all but a handful orbit faint, distant stars and are not amenable to precise follow up measurements. Here, we report the discovery of two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located 65 +/- 6 pc from Earth. We detected the transiting planets in photometry collected during Campaign 3 of NASA's K2 mission. Analysis of transit light curves reveals that the planets have small radii compared to their host star, R-P/R-star = 2.60 +/- 0.14% and 3.15 +/- 0.20%, respectively. We obtained follow up NIR spectroscopy of K2-21 to constrain host star properties, which imply planet sizes of 1.59 +/- 0.43 R-circle plus and 1.92 +/- 0.53 R-circle plus, respectively, straddling the boundary between high-density, rocky planets and low-density planets with thick gaseous envelopes. The planets have orbital periods of 9.32414 days and 15.50120 days, respectively, and a period ratio P-c/P-b = 1.6624, very near to the 5: 3 mean motion resonance, which may be a record of the system's formation history. Transit timing variations due to gravitational interactions between the planets may be detectable using ground-based telescopes. Finally, this system offers a convenient laboratory for studying the bulk composition and atmospheric properties of small planets with low equilibrium temperatures.

Petigura, Erik A.; Schlieder, Joshua E.; Crossfield, Ian J.M.; Howard, Andrew W.; Deck, Katherine M.; Ciardi, David R.; Sinukoff, Evan; Allers, Katelyn N.; Best, William M.J.; Liu, Michael C.; Beichman, Charles A.; Isaacson, Howard; Hansen, Brad M.S.; and Lepine, Sebastien. “Two Transiting Earth-Size Planets Near Resonance Orbiting a Nearby Cool Star.” Astrophysical Journal 811, no. 2 (2015) : 102.

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Friday, February 26th, 2016

Jiajia Dong – Sahoo, Mamata; Dong, Jiajia; and Klumpp, Stefan. “Dynamic Blockage in an Exclusion Process.”Journal of Physics A–Mathematical and Theoretical 48, no. 1 (2015) : 015007.

Jiajia Dong, Associate Professor of Physics & Astronomy

We study an asymmetric exclusion model with one dynamic roadblock particle. The roadblock particle is allowed to move diffusively as well as by longrange jumps mimicking an unbinding/rebinding process. Using Monte Carlo simulations and analytical arguments, the two types of roadblock moves are considered both separately and in combination. Several interesting dynamic phenomena are observed. The long-range jumps of the roadblock lead to a current that depends on the number of particles in the system rather than on the particle density, and thus scales linearly with the system size (up to a critical size). This behavior can be explained by a collective motion of all particles following the roadblock. The diffusive roadblock movements on the other hand lead to a ratcheting motion with the active (driven) particles pushing the roadblock forward.

Sahoo, Mamata; Dong, Jiajia; and Klumpp, Stefan. “Dynamic Blockage in an Exclusion Process.”Journal of Physics A–Mathematical and Theoretical 48, no. 1 (2015) : 015007.

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Friday, February 26th, 2016

Jiajia Dong – Dong, JiaJia. “Spatial Structures in a Simple Model of Population Dynamics for Parasite-Host Interactions.” Europhysics Letters 111, no. 4 (2015) : 48001.

Jiajia Dong, Associate Professor of Physics & Astronomy

Spatial patterning can be crucially important for understanding the behavior of interacting populations. Here we investigate a simple model of parasite and host populations in which parasites are random walkers that must come into contact with a host in order to reproduce. We focus on the spatial arrangement of parasites around a single host, and we derive using analytics and numerical simulations the necessary conditions placed on the parasite fecundity and lifetime for the population’s long-term survival. We also show that the parasite population can be pushed to extinction by a large drift velocity, but, counterintuitively, a small drift velocity generally increases the parasite population.

Dong, JiaJia. “Spatial Structures in a Simple Model of Population Dynamics for Parasite-Host Interactions.” Europhysics Letters 111, no. 4 (2015) : 48001.

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Friday, February 26th, 2016

Jack F. Gallimore – Sales, Dinalva; Robinson, Andy; Axon, David J.; Gallimore, Jack F.; Kharb, Preeti; Curran, Rachel L.; O’Dea, Christopher P.; Baum, Stefi A.; Elitzur, Moshe; and Mittal, Rupal. “An Embedded Active Nucleus in the OH Megamaser Galaxy IRAS16399-0937.” The Astrophysical Journal 799, (2015) : 25.

Jack F. Gallimore, Associate Professor of Physics

We present a multiwavelength study of the OH megamaser galaxy IRAS16399-0937, based on new Hubble Space Telescope (HST)/Advanced Camera for Surveys F814W and Hα+[N II] images and archive data from HST, Two Micron All Sky Survey, Spitzer, Herschel and the Very Large Array. This system has a double nucleus, whose northern (IRAS16399N) and southern (IRAS16399S) components have a projected separation of ~6” (3.4 kpc) and have previously been identified based on optical spectra as a low ionization nuclear emission line region (LINER) and starburst nucleus, respectively. The nuclei are embedded in a tidally distorted common envelope, in which star formation is mostly heavily obscured. The infrared spectrum is dominated by strong polycyclic aromatic hydrocarbon, but deep silicate and molecular absorption features are also present, and are strongest in the IRAS16399N nucleus. The 0.435-500 μm spectral energy distribution was fitted with a model including stellar, interstellar medium and active galactic nucleus (AGN) torus components using our new Markov Chain Monte Carlo code, CLUMPYDREAM. The results indicate that the IRAS16399N contains an AGN (L bol ~ 1044 erg s-1) deeply embedded in a quasi-spherical distribution of optically thick clumps with a covering fraction ≈1. We suggest that these clumps are the source of the OHM emission in IRAS16399-0937. The high torus covering fraction precludes AGN photoionization as the origin of the LINER spectrum, however, the spectrum is consistent with shocks (v ~ 100-200 km s-1). We infer that the ~108 M black hole in IRAS16399N is accreting at a small fraction (~1%) of its Eddington rate. The low accretion rate and modest nuclear star formation rates suggest that while the gas-rich major merger forming the IRAS16399-0937 system has triggered widespread star formation, the massive gas inflows expected from merger simulations have not yet fully developed.

Sales, Dinalva; Robinson, Andy; Axon, David J.; Gallimore, Jack F.; Kharb, Preeti; Curran, Rachel L.; O’Dea, Christopher P.; Baum, Stefi A.; Elitzur, Moshe; and Mittal, Rupal. “An Embedded Active Nucleus in the OH Megamaser Galaxy IRAS16399-0937.” The Astrophysical Journal 799, (2015) : 25.

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Friday, February 26th, 2016

Thomas H. Solomon – Gowen, Savannah and Solomon, Thomas H. “Experimental Studies of Coherent Structures in an Advection-Reaction-Diffusion System.” Chaos (Woodbury, N.Y.) 25, no. 8 (2015) : 087403.

Thomas H. Solomon, Professor of Physics

We present experimental studies of reaction front propagation in a single vortex flow with an imposed external wind. The fronts are produced by the excitable, ferroin-catalyzed Belousov-Zhabotinsky chemical reaction. The flow is generated using an electromagnetic forcing technique: an almost-radial electrical current interacts with a magnetic field from a magnet below the fluid layer to produce the vortex. The magnet is mounted on crossed translation stages allowing for movement of the vortex through the flow. Reaction fronts triggered in or in front of the moving vortex form persistent structures that are seen experimentally for time-independent (constant motion), time-periodic, and time-aperiodic flows. These results are examined with the use of burning invariant manifolds that act as one-way barriers to front motion in the flows. We also explore the usefulness of finite-time Lyapunov exponent fields as an instrument for analyzing front propagation behavior in a fluid flow.

Gowen, Savannah and Solomon, Thomas H. “Experimental Studies of Coherent Structures in an Advection-Reaction-Diffusion System.” Chaos (Woodbury, N.Y.) 25, no. 8 (2015) : 087403.

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Friday, February 26th, 2016

Thomas H. Solomon – Mahoney, John R.; Li, John; Boyer, Carleen; and Solomon, Thomas H. “Frozen Reaction Fronts in Steady Flows: A Burning-Invariant-Manifold Perspective.” Physical Review E 92, no. 6 (2015) : 063005.

Thomas H. Solomon, Professor of Physics

The dynamics of fronts, such as chemical reaction fronts, propagating in two-dimensional fluid flows can be remarkably rich and varied. For time-invariant flows, the front dynamics may simplify, settling in to a steady state in which the reacted domain is static, and the front appears “frozen.” Our central result is that these frozen fronts in the two-dimensional fluid are composed of segments of burning invariant manifolds, invariant manifolds of front-element dynamics in xy theta space, where theta is the front orientation. Burning invariant manifolds (BIMs) have been identified previously as important local barriers to front propagation in fluid flows. The relevance of BIMs for frozen fronts rests in their ability, under appropriate conditions, to form global barriers, separating reacted domains from nonreacted domains for all time. The second main result of this paper is an understanding of bifurcations that lead from a nonfrozen state to a frozen state, as well as bifurcations that change the topological structure of the frozen front. Although the primary results of this study apply to general fluid flows, our analysis focuses on a chain of vortices in a channel flow with an imposed wind. For this system, we present both experimental and numerical studies that support the theoretical analysis developed here.

Mahoney, John R.; Li, John; Boyer, Carleen; and Solomon, Thomas H. “Frozen Reaction Fronts in Steady Flows: A Burning-Invariant-Manifold Perspective.” Physical Review E 92, no. 6 (2015) : 063005.

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Friday, February 26th, 2016

Thomas H. Solomon – Megson, P. W.; Najarian, M. L.; Lilienthal, Katie; and Solomon, Thomas H. “Pinning of Reaction Fronts by Burning Invariant Manifolds in Extended Flows.” Physics of Fluids 27, no. 2 (2015) : 023601.

Thomas H. Solomon, Professor of Physics

We present experiments on the behavior of reaction fronts in extended, vortex-dominated flows in the presence of an imposed wind. We use the ferroin-catalyzed, excitable Belousov-Zhabotinsky chemical reaction, which produces pulse-like reaction fronts. Two time-independent flows are studied: an ordered (square) array of vortices and a spatially disordered flow. The flows are generated with a magneto-hydrodynamic forcing technique, with a pattern of magnets underneath the fluid cell. The magnets are mounted on a translation stage which moves with a constant speed V-d under the fluid, resulting in motion of the vortices within the flow. In a reference frame moving with magnets, the flow is equivalent to one with stationary vortices and a uniform wind with speed W = V-d. For a wide range of wind speeds, reaction fronts pin to the vortices (in a co-moving reference frame), propagating neither forward against the wind nor being blown backward. We analyze this pinning phenomenon and the resulting front shapes using a burning invariant manifold (BIM) formalism. The BIMs are one-way barriers to reaction fronts in the advection-reaction-diffusion process. Pinning occurs when several BIMs overlap to form a complete barrier that spans the width of the system. In that case, the shape of the front is determined by the shape of the BIMs. For the ordered array flow, we predict the locations of the BIMs numerically using a simplified model of the velocity field for the ordered vortex array and compare the BIM shapes to the pinned reaction fronts. We also explore transient behavior of the fronts (before reaching their steady state) to highlight the one-way nature of the BIMs. (C) 2015 Author(s).

Megson, P. W.; Najarian, M. L.; Lilienthal, Katie; and Solomon, Thomas H. “Pinning of Reaction Fronts by Burning Invariant Manifolds in Extended Flows.” Physics of Fluids 27, no. 2 (2015) : 023601.

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Friday, February 26th, 2016

Katharina Vollmayr-Lee – Helfferich, Julian; Vollmayr-Lee, Katharina; Ziebert, Falko; Meyer, Hendrik; and Baschnagel, Joerg. “Glass Formers Display Universal Non-Equilibrium Dynamics on the Level of Single-Particle Jumps.” European Physical Letters 109, (2015) : 36004-p1-36004-p9.

Katharina Vollmayr-Lee, Professor of Physics

Glasses are inherently out-of-equilibrium systems evolving slowly toward their equilibrium state in a process called physical aging. During aging, dynamic observables depend on the history of the system, hampering comparative studies of dynamics in different glass formers. Here, we demonstrate how glass formers can be directly compared on the level of single-particle jumps, i.e. the structural relaxation events underlying the alpha-process. Describing the dynamics in terms of a continuous-time random walk, an analytic prediction for the jump rate is derived. The result is subsequently compared to molecular-dynamics simulations of amorphous silica and a polymer melt as two generic representatives of strong and fragile glass formers, and good agreement is found.

Helfferich, Julian; Vollmayr-Lee, Katharina; Ziebert, Falko; Meyer, Hendrik; and Baschnagel, Joerg. “Glass Formers Display Universal Non-Equilibrium Dynamics on the Level of Single-Particle Jumps.” European Physical Letters 109, (2015) : 36004-p1-36004-p9.

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Friday, February 26th, 2016

Katharina Vollmayr-Lee – Helfferich, J.; Vollmayr-Lee, Katharina; Ziebert, F.; Meyer, H.; and Baschnagel, J. “Glass Formers Display Universal Non-Equilibrium Dynamics on the Level of Single-Particle Jumps.” EPL 109, no. 3 (2015) : 36004.

Katharina Vollmayr-Lee, Professor of Physics

Glasses are inherently out-of-equilibrium systems evolving slowly toward their equilibrium state in a process called physical aging. During aging, dynamic observables depend on the history of the system, hampering comparative studies of dynamics in different glass formers. Here, we demonstrate how glass formers can be directly compared on the level of single-particle jumps, i. e. the structural relaxation events underlying the a-process. Describing the dynamics in terms of a continuous-time random walk, an analytic prediction for the jump rate is derived. The result is subsequently compared to molecular-dynamics simulations of amorphous silica and a polymer melt as two generic representatives of strong and fragile glass formers, and good agreement is found. Copyright (C) EPLA, 2015

Helfferich, J.; Vollmayr-Lee, Katharina; Ziebert, F.; Meyer, H.; and Baschnagel, J. “Glass Formers Display Universal Non-Equilibrium Dynamics on the Level of Single-Particle Jumps.” EPL 109, no. 3 (2015) : 36004.

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