Topological closed-string interpretation of Chern-Simons theory

The exact free energy of SU($N$) Chern-Simons theory at level $k$ is expanded in powers of $(N+k)^{-2}.$ This expansion keeps rank-level duality manifest, and simplifies as $k$ becomes large, keeping $N$ fixed (or vice versa)---this is the weak-coupling (strong-coupling) limit. With the standard normalization, the free energy on the three-sphere in this limit is shown to be the generating function of the Euler characteristics of the moduli spaces of surfaces of genus $g,$ providing a string interpretation for the perturbative expansion. A similar expansion is found for the three-torus, with differences that shed light on contributions from different spacetime topologies in string theory.Comment: 6 pages, iassns-hep-93-30 (title change, omitted refs. added, two sign errors corrected, no significant change

Develando con T-ReCS las regiones oscurecidas del núcleo starburst mas cercano

Presentamos las imágenes de la región nuclear de NGC 253 obtenidas con el instrumento T-ReCS (Thermal-Region Camera Spectrograph) del telescopio Gemini Sur. Las mismas fueron tomadas en las bandas Qa, Si-2, NeII, y continuo de NeII (18.3, 8.7, 12.8, 13.1 µm respectivamente), con un muestreo espacial medio de 0.2” (3.4 pc). Se identifican y analizan las subestructuras más importantes alrededor de los posibles núcleos y se discuten sus roles en el escenario dinámico de la galaxia.We present the images of the nuclear region of NGC 253 obtained with the T-ReCS instrument (Thermal-Region Camera Spectrograph) of the Gemini South Telescope. Such images were taken in the Qa, Si-2, NeII and NeII continuum bands (18.3, 8.7, 12.8 and 13.1 µm respectively) with an average spatial sampling of 0.2” (3.4 pc). We also identify and analyze the most important sub-structures around the possible nuclei and we discuss their roles in the dynamic scenario of the galaxy.Fil: Camperi, Javier Antonio. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Gunthardt, Guillermo Ivan. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Agüero, Maria Paz. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Diaz, Ruben Joaquin. Gemini Observatory; Chile. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; ArgentinaFil: Schirmer, M.. Gemini Observatory; Chile. Universitaet Bonn; Alemani

Micro-opto-mechanical devices and systems using epitaxial lift off

The integration of high quality, single crystal thin film gallium arsenide (GaAs) and indium phosphide (InP) based photonic and electronic materials and devices with host microstructures fabricated from materials such as silicon (Si), glass, and polymers will enable the fabrication of the next generation of micro-opto-mechanical systems (MOMS) and optoelectronic integrated circuits. Thin film semiconductor devices deposited onto arbitrary host substrates and structures create hybrid (more than one material) near-monolithic integrated systems which can be interconnected electrically using standard inexpensive microfabrication techniques such as vacuum metallization and photolithography. These integrated systems take advantage of the optical and electronic properties of compound semiconductor devices while still using host substrate materials such as silicon, polysilicon, glass and polymers in the microstructures. This type of materials optimization for specific tasks creates higher performance systems than those systems which must use trade-offs in device performance to integrate all of the function in a single material system. The low weight of these thin film devices also makes them attractive for integration with micromechanical devices which may have difficulty supporting and translating the full weight of a standard device. These thin film devices and integrated systems will be attractive for applications, however, only when the development of low cost, high yield fabrication and integration techniques makes their use economically feasible. In this paper, we discuss methods for alignment, selective deposition, and interconnection of thin film epitaxial GaAs and InP based devices onto host substrates and host microstructures

Uncovering the nucleus candidate for NGC 253

NGC253 is the nearest spiral galaxy with a nuclear starburst which becomes the best candidate to study the relationship between starburst and AGN activity. However, this central region is veiled by large amounts of dust, and it has been so far unclear which is the true dynamical nucleus. The near infrared spectroscopy could be advantageous in order to shed light on the true nucleus identity. Using Flamingos-2 at Gemini South we have taken deep K-band spectra along the major axis and through the brightest infrared source. We present evidence showing that the brightest near infrared and mid infrared source in the central region, already known as radio source TH7 and so far considered just a stellar supercluster, in fact, presents various symptoms of a genuine galactic nucleus. Therefore, it should be considered a valid nucleus candidate. It is the most massive compact infrared object in the central region, located at 2.0" of the symmetry center of the galactic bar. Moreover, our data indicate that this object is surrounded by a large circumnuclear stellar disk and it is also located at the rotation center of the large molecular gas disk of NGC 253. Furthermore, a kinematic residual appears in the H2 rotation curve with a sinusoidal shape consistent with an outflow centered in the candidate nucleus position. The maximum outflow velocity is located about 14 pc from TH7, which is consistent with the radius of a shell detected around the nucleus candidate observed at 18.3 {\mu}m (Qa) and 12.8 {\mu}m ([NeII]) with T-ReCS. Also, the Br_gamma emission line profile is blue-shifted and this emission line has also the highest equivalent width at this position. All these evidences point out TH7 as the best candidate to be the galactic nucleus of NGC 253.Comment: Accepted for publication in Astronomical Journa

Diet of the black-chinned siskin (<i>Carduelis barbata</i>) during autumn

Se analizó el contenido estomacal de 28 individuos de Cabecitanegra Austral (<i>Carduelis barbata</i>) colectados durante el otoño en el norte de la Patagonia. Todos los estómagos contuvieron alimento, cuyo peso promedio fue de 0.209 g. Se identificaron 11 ítems (7 de origen vegetal y 4 de origen animal). Las plantas predominaron en la dieta (85.8% en peso) y pertenecieron a las familias Asteraceae, Brassicaceae, Chenopodiaceae y Solanaceae. Las semillas más abundantes fueron de Camelina microcarpa y Sisymbrium sp. La fracción animal estuvo representada solo por insectos (14.2%) de los órdenes Diptera (larvas) y Homoptera (familias Psyllidae y Aphididae). Los Aphididae estuvieron representados en gran cantidad de muestras por <i>Pterocomma populeum</i>. Los gastrolitos aparecieron en todas las muestras, con un peso promedio de 0.093 g. Es destacable el alto porcentaje de insectos que el Cabecitanegra Austral incluye en su dieta, teniendo en cuenta que pertenece a una familia típicamente granívora.The contents of 28 stomachs of the Black-chinned Siskin (<i>Carduelis barbata</i>) were analyzed from an autumn collection from the north of Patagonia. All stomachs contained food. The mean mass was 0.209 g. Eleven items (seven vegetal and four animal) were identified in the samples. The plant fraction was predominant (85.8% in weight) and corresponded to the families Asteraceae, Brassicaceae, Chenopodiaceae and Solanaceae. <i>Camelina microcarpa</i> and <i>Sisymbrium sp</i>. were the more abundant seeds. The animal fraction was represented only by insects (14.2%) in the orders Diptera (larvae) and Homoptera (families Psyllidae and Aphididae). Aphididae was represented by <i>Pterocomma populeum</i> in many samples. Gastroliths were present in all samples, with a mean mass of 0.093 g. The high proportion of insects in the diet of the Black-chinned Siskin is striking considering that it belongs to a family of seed-eater

Combining spatial and parametric working memory in a dynamic neural field model

We present a novel dynamic neural field model consisting of two coupled fields of Amari-type which supports the existence of localized activity patterns or “bumps” with a continuum of amplitudes. Bump solutions have been used in the past to model spatial working memory. We apply the model to explain input-specific persistent activity that increases monotonically with the time integral of the input (parametric working memory). In numerical simulations of a multi-item memory task, we show that the model robustly memorizes the strength and/or duration of inputs. Moreover, and important for adaptive behavior in dynamic environments, the memory strength can be changed at any time by new behaviorally relevant information. A direct comparison of model behaviors shows that the 2-field model does not suffer the problems of the classical Amari model when the inputs are presented sequentially as opposed to simultaneously

Winter diet of Great Pampa-finches Embernagra platensis in Guaminí Lagoon, Argentina

The diet of the Great Pampa-finch includes seeds rather than insects. Our objective was to study its diet during the winter season and, whenever possible, to record differences between the sexes in diet patterns. This species was studied at Guamini Lagoon, Buenos Aires Province, Argentina. The stomach content of 38 adult birds was investigated. The analysis shows that the food eaten by E. platensis consisted of seeds (60%) (Chenopodiaceae, Asteraceae, Ciperaceae and Poaceae) and insects (39%) (Bellostomatidae, Dytiscidae, Curculionidae, Coccinelidae, Formicidae and Apidae). Males ate more insects and, overall, more items than females. These, in turn, consumed a greater amount of plants. Niche breadth values were 2.9 for males and 3.1 for females. The seeds encountered were those of plant species typically associated with the pampas.Facultad de Ciencias Naturales y MuseoFacultad de Ciencias Agrarias y Forestale

Correlation Functions of Large N Chern-Simons-Matter Theories and Bosonization in Three Dimensions

We consider the conformal field theory of N complex massless scalars in 2+1 dimensions, coupled to a U(N) Chern-Simons theory at level k. This theory has a 't Hooft large N limit, keeping fixed \lambda = N/k. We compute some correlation functions in this theory exactly as a function of \lambda, in the large N (planar) limit. We show that the results match with the general predictions of Maldacena and Zhiboedov for the correlators of theories that have high-spin symmetries in the large N limit. It has been suggested in the past that this theory is dual (in the large N limit) to the Legendre transform of the theory of fermions coupled to a Chern-Simons gauge field, and our results allow us to find the precise mapping between the two theories. We find that in the large N limit the theory of N scalars coupled to a U(N)_k Chern-Simons theory is equivalent to the Legendre transform of the theory of k fermions coupled to a U(k)_N Chern-Simons theory, thus providing a bosonization of the latter theory. We conjecture that perhaps this duality is valid also for finite values of N and k, where on the fermionic side we should now have (for N_f flavors) a U(k)_{N-N_f/2} theory. Similar results hold for real scalars (fermions) coupled to the O(N)_k Chern-Simons theory.Comment: 49 pages, 16 figures. v2: added reference

Working Memory Cells' Behavior May Be Explained by Cross-Regional Networks with Synaptic Facilitation

Neurons in the cortex exhibit a number of patterns that correlate with working memory. Specifically, averaged across trials of working memory tasks, neurons exhibit different firing rate patterns during the delay of those tasks. These patterns include: 1) persistent fixed-frequency elevated rates above baseline, 2) elevated rates that decay throughout the tasks memory period, 3) rates that accelerate throughout the delay, and 4) patterns of inhibited firing (below baseline) analogous to each of the preceding excitatory patterns. Persistent elevated rate patterns are believed to be the neural correlate of working memory retention and preparation for execution of behavioral/motor responses as required in working memory tasks. Models have proposed that such activity corresponds to stable attractors in cortical neural networks with fixed synaptic weights. However, the variability in patterned behavior and the firing statistics of real neurons across the entire range of those behaviors across and within trials of working memory tasks are typical not reproduced. Here we examine the effect of dynamic synapses and network architectures with multiple cortical areas on the states and dynamics of working memory networks. The analysis indicates that the multiple pattern types exhibited by cells in working memory networks are inherent in networks with dynamic synapses, and that the variability and firing statistics in such networks with distributed architectures agree with that observed in the cortex