The topology of U-duality (sub-)groups

We discuss the topology of the symmetry groups appearing in compactified (super-)gravity, and discuss two applications. First, we demonstrate that for 3 dimensional sigma models on a symmetric space G/H with G non-compact and H the maximal compact subgroup of G, the possibility of oxidation to a higher dimensional theory can immediately be deduced from the topology of H. Second, by comparing the actual symmetry groups appearing in maximal supergravities with the subgroups of SL(32,R) and Spin(32), we argue that these groups cannot serve as a local symmetry group for M-theory in a formulation of de Wit-Nicolai type.Comment: 18 pages, LaTeX, 1 figure, 2 table

Dynamic Stark Effect in Strongly Coupled Microcavity Exciton-Polaritons

We present experimental observations of a non-resonant dynamic Stark shift in strongly coupled microcavity quantum well exciton-polaritons - a system which provides a rich variety of solid-state collective phenomena. The Stark effect is demonstrated in a GaAs/AlGaAs system at 10K by femtosecond pump-probe measurements, with the blue shift approaching the meV scale for a pump fluence of 2 mJcm^-2 and 50 meV red detuning, in good agreement with theory. The energy level structure of the strongly coupled polariton Rabi-doublet remains unaffected by the blue shift. The demonstrated effect should allow generation of ultrafast density-independent potentials and imprinting well-defined phase profiles on polariton condensates, providing a powerful tool for manipulation of these condensates, similar to dipole potentials in cold atom systems

Prospecting in ultracool dwarfs : Measuring the metallicities of mid- and late-m dwarfs

© 2014. The American Astronomical Society. All rights reserved.Metallicity is a fundamental parameter that contributes to the physical characteristics of a star. The low temperatures and complex molecules present in M dwarf atmospheres make it difficult to measure their metallicities using techniques that have been commonly used for Sun-like stars. Although there has been significant progress in developing empirical methods to measure M dwarf metallicities over the last few years, these techniques have been developed primarily for early- to mid-M dwarfs. We present a method to measure the metallicity of mid- to late-M dwarfs from moderate resolution (R ∼ 2000) K-band (≃ 2.2 μm) spectra. We calibrate our formula using 44 wide binaries containing an F, G, K, or early-M primary of known metallicity and a mid- to late-M dwarf companion. We show that similar features and techniques used for early-M dwarfs are still effective for late-M dwarfs. Our revised calibration is accurate to ∼0.07 dex for M4.5-M9.5 dwarfs with -0.58 <[Fe/H] <+0.56 and shows no systematic trends with spectral type, metallicity, or the method used to determine the primary star metallicity. We show that our method gives consistent metallicities for the components of M+M wide binaries. We verify that our new formula works for unresolved binaries by combining spectra of single stars. Lastly, we show that our calibration gives consistent metallicities with the Mann et al. study for overlapping (M4-M5) stars, establishing that the two calibrations can be used in combination to determine metallicities across the entire M dwarf sequence.Peer reviewe

Trumpeting M Dwarfs with CONCH-SHELL: a Catalog of Nearby Cool Host-Stars for Habitable ExopLanets and Life

We present an all-sky catalog of 2970 nearby ($d \lesssim 50$ pc), bright ($J< 9$) M- or late K-type dwarf stars, 86% of which have been confirmed by spectroscopy. This catalog will be useful for searches for Earth-size and possibly Earth-like planets by future space-based transit missions and ground-based infrared Doppler radial velocity surveys. Stars were selected from the SUPERBLINK proper motion catalog according to absolute magnitudes, spectra, or a combination of reduced proper motions and photometric colors. From our spectra we determined gravity-sensitive indices, and identified and removed 0.2% of these as interloping hotter or evolved stars. Thirteen percent of the stars exhibit H-alpha emission, an indication of stellar magnetic activity and possible youth. The mean metallicity is [Fe/H] = -0.07 with a standard deviation of 0.22 dex, similar to nearby solar-type stars. We determined stellar effective temperatures by least-squares fitting of spectra to model predictions calibrated by fits to stars with established bolometric temperatures, and estimated radii, luminosities, and masses using empirical relations. Six percent of stars with images from integral field spectra are resolved doubles. We inferred the planet population around M dwarfs using $Kepler$ data and applied this to our catalog to predict detections by future exoplanet surveys.Comment: Accepted to MNRAS 22 figures, 3 tables, 2 electronic tables. Electronic tables are available as links on this pag

Deweyan tools for inquiry and the epistemological context of critical pedagogy

This article develops the notion of resistance as articulated in the literature of critical pedagogy as being both culturally sponsored and cognitively manifested. To do so, the authors draw upon John Dewey\u27s conception of tools for inquiry. Dewey provides a way to conceptualize student resistance not as a form of willful disputation, but instead as a function of socialization into cultural models of thought that actively truncate inquiry. In other words, resistance can be construed as the cognitive and emotive dimensions of the ongoing failure of institutions to provide ideas that help individuals both recognize social problems and imagine possible solutions. Focusing on Dewey\u27s epistemological framework, specifically tools for inquiry, provides a way to grasp this problem. It also affords some innovative solutions; for instance, it helps conceive of possible links between the regular curriculum and the study of specific social justice issues, a relationship that is often under-examined. The aims of critical pedagogy depend upon students developing dexterity with the conceptual tools they use to make meaning of the evidence they confront; these are background skills that the regular curriculum can be made to serve even outside social justice-focused curricula. Furthermore, the article concludes that because such inquiry involves the exploration and potential revision of students\u27 world-ordering beliefs, developing flexibility in how one thinks may be better achieved within academic subjects and topics that are not so intimately connected to students\u27 current social lives, especially where students may be directly implicated

Internal dynamics of the galaxy cluster Abell 545

Diffuse radio emission in galaxy clusters, and their connection with cluster mergers, are still debated. We seek to explore the internal dynamics of the radio halo cluster Abell 545. This cluster is also peculiar for hosting in its center a very bright, red, diffuse intracluster light due to an old, stellar population, so bright to be named as "star pile". Our analysis is based on redshift data for 110 galaxies. We identify 95 cluster members and analyze the cluster internal dynamics by combining galaxy velocities and positions. We also use both photometric and X-ray data. We estimate the cluster redshift, z=0.1580, a velocity dispersion of 1200 km/s, and ICM temperature kT_X~8 keV. Our optical and X-ray analyses detect substructures. Optical data reveal three main galaxy clumps (center, NNW, and NE), and possibly a fourth clump at South. There is not a dominant galaxy and the four brightest galaxies avoid the cluster core (>~0.4h distant from the cluster center) and are >~1500 km/s far from the mean cluster velocity. The analysis of the X-ray surface brightness distribution provides us evidence of a disturbed dynamical phase. Located in the star pile region there is the brightest galaxies of the cluster core (CBCG) and a very compact elliptical galaxy. We show that the star pile has a similar redshift to that of the CBCG. Both the star pile and the CBCG are at rest in the cluster rest frame. The emerging picture of Abell 545 is that of a massive, M(R<1.6 h_70^-1 Mpc)=1.1-1.8x10^15 h_70^-1 Msun, very complex cluster with merging occurring along two directions. A545 gives another proof in the favor of the connection between cluster merger and extended, diffuse radio emission. The star pile, likely due to the process of a brightest galaxy forming in the cluster core. A545 represents a textbook cluster where to study the simultaneous formation of a galaxy system and its brightest galaxy.Comment: 16 pages, 11 figures and 2 tables. Accepted in A&

Spacelike Singularities and Hidden Symmetries of Gravity

We review the intimate connection between (super-)gravity close to a spacelike singularity (the "BKL-limit") and the theory of Lorentzian Kac-Moody algebras. We show that in this limit the gravitational theory can be reformulated in terms of billiard motion in a region of hyperbolic space, revealing that the dynamics is completely determined by a (possibly infinite) sequence of reflections, which are elements of a Lorentzian Coxeter group. Such Coxeter groups are the Weyl groups of infinite-dimensional Kac-Moody algebras, suggesting that these algebras yield symmetries of gravitational theories. Our presentation is aimed to be a self-contained and comprehensive treatment of the subject, with all the relevant mathematical background material introduced and explained in detail. We also review attempts at making the infinite-dimensional symmetries manifest, through the construction of a geodesic sigma model based on a Lorentzian Kac-Moody algebra. An explicit example is provided for the case of the hyperbolic algebra E10, which is conjectured to be an underlying symmetry of M-theory. Illustrations of this conjecture are also discussed in the context of cosmological solutions to eleven-dimensional supergravity.Comment: 228 pages. Typos corrected. References added. Subject index added. Published versio