Showing papers by "International School for Advanced Studies published in 2003"

Neutral theory and relative species abundance in ecology.

Abstract: The theory of island biogeography asserts that an island or a local community approaches an equilibrium species richness as a result of the interplay between the immigration of species from the much larger metacommunity source area and local extinction of species on the island (local community). Hubbell generalized this neutral theory to explore the expected steady-state distribution of relative species abundance (RSA) in the local community under restricted immigration. Here we present a theoretical framework for the unified neutral theory of biodiversity and an analytical solution for the distribution of the RSA both in the metacommunity (Fisher's log series) and in the local community, where there are fewer rare species. Rare species are more extinction-prone, and once they go locally extinct, they take longer to re-immigrate than do common species. Contrary to recent assertions, we show that the analytical solution provides a better fit, with fewer free parameters, to the RSA distribution of tree species on Barro Colorado Island, Panama, than the lognormal distribution.

Sounds and silence: An optical topography study of language recognition at birth

Abstract: Does the neonate's brain have left hemisphere (LH) dominance for speech? Twelve full-term neonates participated in an optical topography study designed to assess whether the neonate brain responds specifically to linguistic stimuli. Participants were tested with normal infant-directed speech, with the same utterances played in reverse and without auditory stimulation. We used a 24-channel optical topography device to assess changes in the concentration of total hemoglobin in response to auditory stimulation in 12 areas of the right hemisphere and 12 areas of the LH. We found that LH temporal areas showed significantly more activation when infants were exposed to normal speech than to backward speech or silence. We conclude that neonates are born with an LH superiority to process specific properties of speech.

Global protein function prediction from protein-protein interaction networks

Abstract: Determining protein function is one of the most challenging problems of the post-genomic era. The availability of entire genome sequences and of high-throughput capabilities to determine gene coexpression patterns has shifted the research focus from the study of single proteins or small complexes to that of the entire proteome 1 . In this context, the search for reliable methods for assigning protein function is of primary importance. There are various approaches available for deducing the function of proteins of unknown function using information derived from sequence similarity or clustering patterns of coregulated genes 2,3 , phylogenetic profiles 4 , protein-protein interactions (refs. 5‐8 and Samanta, M.P. and Liang, S., unpublished data), and protein complexes 9,10 . Here we propose the assignment of proteins to functional classes on the basis of their network of physical interactions as determined by minimizing the number of protein interactions among different functional categories. Function assignment is proteome-wide and is determined by the global connectivity pattern of the protein network. The approach results in multiple functional assignments, a consequence of the existence of multiple equivalent solutions. We apply the method to analyze the yeast Saccharomyces cerevisiae protein-protein interaction network 5 .

What are the facts of semantic category-specific deficits? A critical review of the clinical evidence.

Abstract: In this study we provide a critical review of the clinical evidence available to date in the field of semantic category-specific deficits. The motivation for undertaking this review is that not all the data reported in the literature are useful for adjudicating among extant theories. This project is an attempt to answer two basic questions: (1) what are the categories of category-specific deficits, and (2) is there an interaction between impairment for a type of knowledge (e.g., visual, functional, etc.) and impairment for a given category of objects (e.g., biological, artefacts, etc.). Of the 79 case studies in which the reported data are sufficiently informative with respect to the aims of our study, 61 presented a disproportionate impairment for biological categories and 18 presented a disproportionate impairment for artefacts. Less than half of the reported cases provide statistically and theoretically interpretable data. Each case is commented upon individually. The facts that emerge from our critical review are that (1) the categories of category-specific semantic deficits are animate objects, inanimate biological objects, and artefacts (the domain of biological objects fractionates into two independent semantic categories: animals, and fruit/ vegetables); (2) the types of category-specific deficits are not associated with specific types of conceptual knowledge deficits. Other conclusions that emerge from our review are that the evidence in favour of the existence of cases of reliable category-specific agnosia or anomia is not very strong, and that the visual structural description system functions relatively autonomously from conceptual knowledge about object form.

Photometry and Spectroscopy of the Type IIP SN 1999em from Outburst to Dust Formation

Abstract: We present photometry and spectra of the Type IIP supernova 1999em in NGC 1637 from several days after the outburst till day 642. The radioactive tail of the recovered bolometric light curve of SN 1999em indicates that the amount of the ejected 5 6 Ni is 0.02 M O .. The Ha and HeI 10 830-A lines at the nebular epoch show that the distribution of the bulk of 5 6 Ni can be represented approximately by a sphere of 5 6 Ni with a velocity of 1500 km s - 1 , which is shifted towards the far hemisphere by about 400 km s - 1 . The fine structure of the Ha at the photospheric epoch reminiscent of the 'Bochum event' in SN 1987A is analysed in terms of two plausible models: bipolar 5 6 Ni jets and non-monotonic behaviour of the Ha optical depth combined with the one-sided 5 6 Ni ejection. The late-time spectra show a dramatic transformation of the [O I] 6300-A line profile between days 465 and 510, which we interpret as an effect of dust condensation during this period. Late-time photometry supports the dust formation scenario after day 465. The [O I] line profile suggests that the dust occupies a sphere with velocity 800 km s - 1 and optical depth >> 10. The latter exceeds the optical depth of the dusty zone in SN 1987A by more than 10 times. Use is made of the Expanding Photosphere Method to estimate the distance and the explosion time, D 7.83 Mpc and t 0 ≃ 1999 October 24.5 UT, in accord with observational constraints on the explosion time and with other results of detailed studies of the method. The plateau brightness and duration combined with the expansion velocity suggest a pre-supernova radius of 120-150 R O ., ejecta mass of 10-11 M O . and explosion energy of (0.5-1) x 10 5 1 erg. The ejecta mass combined with the neutron star and a conservative assumption about mass loss implies the main sequence progenitor of M m s 12-14 M O .. The derived mass range is in agreement with the upper limit to the mass found using pre-supernova field images by Smartt et al. From the [OI] 6300, 6364 A doublet luminosity we infer the oxygen mass to be a factor four lower than in SN 1987A which is consistent with the estimated SN 1999em progenitor mass according to nucleosynthesis and stellar evolution theory. We note a 'second-plateau' behaviour of the light curve after the main plateau at the beginning of the radioactive tail. This feature seems to be common to SNe IIP with low 5 6 Ni mass.

Oxygen, carbon and nitrogen evolution in galaxies

Abstract: We discuss the evolution of oxygen, carbon and nitrogen in galaxies of different morphological type by adopting detailed chemical evolution models with different star formation histories (continuous star formation or starbursts). In all the models detailed nucleosynthesis prescriptions from supernovae of all types and low- and intermediate-mass stars are taken into account. We start by computing chemical evolution models for the Milky Way with different stellar nucleosynthesis prescriptions. Then, a comparison between model results and ‘key’ observational constraints allows us to choose the best set of stellar yields. Once the best set of yields is identified for the Milky Way, we apply the same nucleosynthesis prescriptions to other spirals (in particular M101) and dwarf irregular galaxies. We compare our model predictions with the [C,N,O/Fe] versus [Fe/H], log(C/O) versus 12 + log(O/H), log(N/O) versus 12+ log(O/H) and [C/O] versus [Fe/H] relations observed in the solar vicinity and draw the following conclusions. (i) There is no need to invoke strong stellar winds in massive stars in order to explain the evolution of the C/O ratio, as often claimed in the literature. (ii) The predicted [O/Fe] ratio as a function of metallicity is in very good agreement with the most recent data available for the solar vicinity, especially for halo stars. This fact again suggests that the oxygen stellar yields in massive stars computed by either Woosley & Weaver or Thielemann, Nomoto & Hashimoto without taking into account mass loss, reproduce the observations well. (iii) We predict that the gap observed in the [Fe/O] versus [O/H] at [O/H] ∼− 0.3 dex should be observed also in C/O versus O/H. The existence of such a gap is predicted by our model for the Milky Way and is caused by a halt in the star formation between the end of the thick disc and the beginning of the thin disc phase. Such a halt is produced by the adopted threshold gas density for the star formation rate. (iv) This threshold is also responsible for the prediction of a very slow chemical enrichment between the time of formation of the solar system (4.5 Gyr ago) and the present time, in agreement with new abundance measurements. (v) The chemical evolution models for dwarf irregulars and spirals, adopting the same nucleosynthesis prescriptions of the best model for the solar neighbourhood, well reproduce the available constraints for these objects. (vi) By taking into account the results obtained for all the studied galaxies (Milky Way, M101, dwarf galaxies and DLAs) we conclude that there is no need for claiming a strong primary component of N produced in massive stars (M > 10 M� ). (vii) Moreover, there is a strong indication that C and N are mainly produced in low- and intermediate-mass stars, at variance with recent suggestions that most of the C should come from massive stars. In particular, intermediate-mass stars with masses between 4 and 8 M� contribute mostly to N (both primary and secondary) whereas those with masses between 1 and

Asymptotic safety of gravity coupled to matter

Abstract: Nonperturbative treatments of the UV limit of pure gravity suggest that it admits a stable fixed point with a positive Newton constant and a cosmological constant. We prove that this result is stable under the addition of a scalar field with a generic potential and nonminimal coupling to the scalar curvature. There is a fixed point where the mass and all nonminimal scalar interactions vanish, while the gravitational couplings have values which are almost identical to those in the pure gravity case. We discuss the linearized flow around this fixed point and find that the critical surface is four dimensional. In the presence of other, arbitrary, massless minimally coupled matter fields, the existence of the fixed point, the sign of the cosmological constant, and the dimension of the critical surface depend on the type and number of fields. In particular, for some matter content, there exist polynomial asymptotically free scalar potentials, suggesting a gravitational solution to the well-known problem of triviality.

Low-mass relics of early star formation

Abstract: The earliest stars to form in the Universe were the first sources of light, heat and metals after the Big Bang The products of their evolution will have had a profound impact on subsequent generations of stars Recent studies of primordial star formation have shown that, in the absence of metals (elements heavier than helium), the formation of stars with masses 100 times that of the Sun would have been strongly favoured, and that low-mass stars could not have formed before a minimum level of metal enrichment had been reached The value of this minimum level is very uncertain, but is likely to be between 10(-6) and 10(-4) that of the Sun Here we show that the recent discovery of the most iron-poor star known indicates the presence of dust in extremely low-metallicity gas, and that this dust is crucial for the formation of lower-mass second-generation stars that could survive until today The dust provides a pathway for cooling the gas that leads to fragmentation of the precursor molecular cloud into smaller clumps, which become the lower-mass stars

Singularly perturbed elliptic equations with symmetry: existence of solutions concentrating on spheres, Part I

Abstract: We deal with the existence of positive radial solutions concentrating on spheres to a class of singularly perturbed elliptic problems like −ɛ2Δu+V(|x|)u=u p ,uH 1 (ℝ n ). Under suitable assumptions on the auxiliary potential M(r)=r n−1 V θ (r), θ(p+1)/(p−1)−1/2, we provide necessary and sufficient conditions for concentration as well as the bifurcation of non-radial solutions.

Early reionization by the first galaxies

Abstract: Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, τ e ∼ 0.17. The reionization of the Universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results (Ω m = 0.3, Ω Λ = 0.7, h = 0.7, Ω b = 0.04, n = 1 and σ 8 = 0.9). Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with ionizing photon production as expected for zero-metallicity stars distributed according to a standard stellar initial mass function (IMF) (10 6 1 photons per unit solar mass of formed stars) and with a moderate photon escape fraction from galaxies (5 per cent), produces τ e = 0.104, which is within 1.0 to 1.5σ of the 'best' WMAP value. Values of up to 0.16 can be produced by taking larger escape fractions or a top-heavy IMF. The data do not require a separate populations of 'miniquasars' or of stars forming in objects with total masses below 10 9 M O .. Reconciling such early reionization with the observed Gunn-Peterson troughs in z > 6 quasars may be challenging. Possible resolutions of this problem are discussed.

Replica bounds for optimization problems and diluted spin systems

Abstract: In this paper we generalize to the case of diluted spin models and random combinatorial optimization problems a technique recently introduced by Guerra (cond-mat/0205123) to prove that the replica method generates variational bounds for disordered systems. We analyze a family of models that includes the Viana–Bray model, the diluted p-spin model or random XOR-SAT problem, and the random K-SAT problem, showing that the replica/cavity method, at the various levels of approximation, provides systematic schemes to obtain lower bounds of the free-energy at all temperatures and of the ground state energy. In the case of K-SAT and XOR-SAT it thus gives upper bounds of the satisfiability threshold. Our analysis underlines deep connections with the cavity method which are not evident in the long range case.

Constraints on matter from asymptotic safety

Abstract: Recent studies of the ultraviolet behavior of pure gravity suggest that it admits a non-Gaussian attractive fixed point, and therefore that the theory is asymptotically safe. We consider the effect on this fixed point of massless minimally coupled matter fields. The existence of a UV attractive fixed point puts bounds on the type and number of such fields.

A new simple model for high-frequency quasi-periodic oscillations in black hole candidates

Abstract: Observations of X-ray emissions from binary systems have long since been considered important tools to test general relativity in strong-field regimes. The high-frequency quasi-periodic oscillations (HFQPOs) observed in binaries containing a black hole candidate, in particular, have been proposed as a means to measure more directly the properties of the black hole, such as its mass and spin. Numerous models have been suggested to explain the HFQPOs and the rich phenomenology accompanying them. Many of these models rest on a number of assumptions and are at times in conflict with the most recent observations. We here propose a new, simple model in which the HFQPOs result from basic p-mode oscillations of a small accretion torus orbiting close to the black hole. We show that within this model the key properties of the HFQPOs can be explained simply, given a single reasonable assumption. We also discuss observational tests that can refute the model.

Nuclear matter and neutron-star properties calculated with the Skyrme interaction

Abstract: The effective Skyrme interaction has been used extensively in mean-field models for several decades and many different parametrizations of the interaction have been proposed. All of these give similar agreement with the experimental observables of nuclear ground states as well as with the properties of infinite symmetric nuclear matter at the saturation density n0. However, when applied over a wider range of densities (up to ∼3n0) they predict widely varying behavior for the observables of both symmetric and asymmetric nuclear matter. A particularly relevant example of naturally occurring asymmetric nuclear matter is the material of which neutron stars are composed. At around nuclear matter density, this can be well represented as a mixture of neutrons, protons, electrons, and muons (n+p+e+μ matter) in β-equilibrium, and these densities turn out to be the key ones for determining the properties of neutron-star models with masses near to the widely used “canonical” value of 1.4M⊙. By constructing equations of state for neutron-star matter using the different Skyrme parametrizations, calculating corresponding neutron-star models and then comparing these with observational data, an additional constraint can be obtained for the values of the Skyrme parameters. Such a constraint is particularly relevant because the parametrizations are initially determined by fitting to the properties of doubly closed-shell nuclei and it is an open question how suitable they then are for nuclei with high values of isospin, such as those at the neutron drip-line and beyond. The neutron-star environment provides an invaluable testing ground for this. We have carried out an investigation of 87 different Skyrme parametrizations in order to examine how successful they are in predicting the expected properties of infinite nuclear matter and generating plausible neutron-star models. This is the first systematic study of the predictions of the various Skyrme parametrizations for the density dependence of the characteristic observables of nuclear matter; the density dependence of the symmetry energy for β-equilibrium matter turns out to be a crucial property for indicating which Skyrme parameter sets will apply equally well for finite nuclei and for neutron-star matter. Only 27 of the 87 parametrizations investigated pass the test of giving satisfactory neutron-star models and we present a list of these.

The detectability of the first stars and their cluster enrichment signatures

Abstract: We conduct a comprehensive investigation of the detectability of the first stars and their enrichment signatures in galaxy clusters. As the initial mass function (IMF) of these Population III stars is unknown and likely to be biased to high masses, we base our study on analytical models that parameterize these uncertainties and allow us to make general statements. We show that the mean metallicity of outflows from Population III objects containing these stars is well above the critical transition metallicity (Zcr ~ 10-4 Z☉) that marks the formation of normal stars. Thus, the fraction of Population III objects formed as a function of redshift is heavily dependent on the distribution of metals and fairly independent of the mean metallicity of the universe, or the precise value of Zcr. Using an analytic model of inhomogeneous structure formation, we study the evolution of Population III objects as a function of the star formation efficiency, IMF, and efficiency of outflow generation. For all models, Population III objects tend to be in the 106.5-107.0 M☉ mass range, just large enough to cool within a Hubble time, but small enough that they are not clustered near areas of previous star formation. Although the mean metallicity exceeds Zcr at z ~ 15 in all models, the peak of Population III star formation occurs at z ~ 10, and such stars continue to form well into the observable range. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyα emitters. Finally, we combine our Population III distributions with the yield models of Heger & Woosley to study their impact on the intracluster medium (ICM) in galaxy clusters. We find that Population III stars can contribute no more than 20% of the iron observed in the ICM, but if they form with characteristic masses ~200-260 M☉, their peculiar elemental yields help to reconcile theoretical models with the observed Fe and Si/Fe abundances. However, these stars tend to overproduce S/Fe and can account only for the O/Fe ratio in the inner regions of poor clusters. Additionally, the associated supernova heating falls far short of the observed level of ~1 keV per ICM gas particle. Thus, the properties of the first objects may be best determined by direct observation.

Resilience to damage of graphs with degree correlations.

Abstract: The existence or nonexistence of a percolation threshold on power law correlated graphs is a fundamental question for which a general criterion is lacking. In this work we investigate the problems of site and bond percolation on graphs with degree correlations and their connection with spreading phenomena. We obtain some general expressions that allow the computation of the transition thresholds or their bounds. Using these results we study the effects of assortative and disassortative correlations on the resilience to damage of networks.

On the life and death of satellite haloes

Abstract: We study the evolution of dark matter satellites orbiting inside more massive haloes using semi-analytical tools coupled with high-resolution N-body simulations. We select initial satellite sizes, masses, orbital energies, and eccentricities as predicted by hierarchical models of structure formation. Both the satellite (of initial mass M s , 0 ) and the main halo (of mass M h ) are described by a Navarro, Frenk & White density profile with various concentrations. We explore the interplay between dynamic friction and tidal mass loss/evaporation in determining the final fate of the satellite. We provide a user-friendly expression for the dynamic friction time-scale τ d f . l i v e and for the disruption time for a live (i.e. mass-losing) satellite. This can be easily implemented into existing semi-analytical models of galaxy formation improving considerably the way they describe the evolution of satellites. Massive satellites (M s , 0 > 0.1M h ) starting from typical cosmological orbits sink rapidly (irrespective of the initial circularity) toward the centre of the main halo where they merge after a time τ d f . r i g , as if they were rigid. Satellites of intermediate mass (0.01M h < M s , 0 < 0.1 M h ) suffer severe tidal mass losses as dynamic friction reduces their pericentre distance. In this case, mass loss increases substantially their decay time with respect to a rigid satellite. The final fate depends on the concentration of the satellite, c s , relative to that of the main halo, c h . Only in the unlikely case where c s /c h ≤ 1 are satellites disrupted. In this mass range, τ d f , l i v e gives a measure of the merging time. Among the satellites whose orbits decay significantly, those that survive must have been moving preferentially on more circular orbits since the beginning as dynamical friction does not induce circularization. Lighter satellites (M s , 0 < 0.01M h ) do not suffer significant orbital decay and tidal mass loss stabilizes the orbit even further. Their orbits should map those at the time of entrance into the main halo. After more than a Hubble time satellites have masses M s ∼ 1-10 per cent M s , 0 , typically, implying M s < 0.001 M h for the remnants. In a Milky-Way-like halo, light satellites should be present even after several orbital times with their baryonic components experimenting morphological changes due to tidal stirring. They coexist with the remnants of more massive satellites depleted in their dark matter content by the tidal field, which should move preferentially on tightly bound orbits.

Extremely hard GRB spectra prune down the forest of emission models

Abstract: We consider the evidence for very hard low energy spectra during the prompt phase of Gamma-Ray Bursts (GRB). In particular we examine the spectral evolution of GRB 980306 together with the detailed analysis of some other bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927 and GRB 970111), and check for the significance of their hardness (i.e. extremely steep spectral slopes below the EFE peak) by applying dierent tests. These bursts, detected by the Burst And Transient Source Experiment (BATSE) in the30 keV-2 MeV energy range, are suciently bright to allow time resolved spectral studies on time intervals of the order of tenths of a second. We discuss the hard spectra of these bursts and their evolution in the context of several non-thermal emission models, which all appear inadequate to account for these cases. The extremely hard spectra, which are detected in the early part of the BATSE light curve, are also compared with a black body spectral model: the resulting fits are remarkably good, except for an excess at high energies (in several cases) which could be simply accounted for by the presence of a supra-thermal component. The findings on the possible thermal character of the evolving spectrum and the implications on the GRB physical scenario are considered in the frameworks of photospheric models for a fireball which is becoming optically thin, and of Compton drag models, in which the fireball boosts "ambient" seed photons by its own bulk motion. Both models, according to simple estimates, appear to be qualitatively and quantitatively consistent with the found spectral characteristics, although their possible caveats are discussed.

Evidence for supernova signatures in the spectrum of the late-time bump of the optical afterglow of GRB 021211

Abstract: We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ∼25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained ∼10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ∼14 400 km s −1 , thus indicating that a supernova (SN) component is indeed powering the "bump" in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.

Accurate relic densities with neutralino, chargino and sfermion coannihilations in mSUGRA

Abstract: Neutralinos arise as natural dark matter candidates in many supersymmetric extensions of the Standard Model. We present a novel calculation of the neutralino relic abundance in which we include all so-called coannihilation processes between neutralinos, charginos and sfermions, and, at the same time, we apply the state of the art technique to trace the freeze-out of a species in the early Universe. As a first application, we discuss here results valid in the mSUGRA framework; we describe general trends as well as performing a detailed study of the neutralino relic densities in the mSUGRA parameter space. The emerging picture is fair agreement with previous analyses in the same framework, however we have the power to discuss it in much more detail than previously done. For example, we find that the cosmological bound on the neutralino mass is pushed up to ~565 GeV in the stau coannihilation region and to ~1500 GeV in the chargino coannihilation region.

The imprint of the cosmic dark ages on the near-infrared background

Abstract: The redshifted light of the first (Population III) stars might contribute substantially to the near-infrared background (NIRB). By fitting recent data with models including up-to-date Population III stellar spectra, we find that such stars can indeed account for the whole NIRB residual (i.e. after 'normal' galaxy contribution subtraction) if the high-redshift star formation efficiency is f* = 10-50 per cent, depending on the initial mass function (the top-heaviest requiring lowest efficiency) and on the unknown galaxy contribution in the L band (our models, however, suggest it to be negligible). Such an epoch of Population III star formation ends in all models by z e n d 8.8, with a hard limit z e n d 260 M O ., locking their nucleosynthetic products in the compact remnant or by postulating an extremely inhomogeneous metal enrichment of the Lya forest. We discuss these possibilities in detail along with the uncertainties related to the adopted zodiacal light model.

Non-Linear Resonance in Nearly Geodesic Motion in Low-Mass X-Ray Binaries

Abstract: We have explored the ideas that parametric resonance affects nearly geodesic motion around a black hole or a neutron star, and that it may be relevant to the high-frequency (twin) quasi-periodic oscillations that occur in some low-mass X-ray binaries. We have assumed the particles or uid elements of an accretion disc to be subject to an isotropic perturbation having a hypothetical but rather general form. We nd that the parametric resonance is indeed excited close to the radius where epicyclic frequencies of the radial and meridional oscillations are in a 2 : 3 ratio. The location and frequencies of the highest amplitude excitation vary with the strength of the perturbation. These results agree with actual frequency ratios of twin kHz QPOs that have been reported in some black hole candidates, and they may be consistent also with correlation of the twin peaks in Sco X-1.

Encoding of Whisker Vibration by Rat Barrel Cortex Neurons: Implications for Texture Discrimination

Abstract: Rats, using their whiskers, have excellent capabilities in texture discrimination. What is the representation of texture in rat somatosensory cortex? We hypothesize that as rats “whisk” over a surface, the spatial frequency of a grooved or pebbled texture is converted to a temporal frequency of whisker vibration. Surface features such as groove depth or grain size modulate the amplitude of this vibration. Validation of the hypothesis depends on showing that vibration parameters have distinct neuronal representations in cortex. To test this, we delivered sinusoidal vibrations to the whisker shaft and analyzed cortical neuronal activity. Seven amplitudes and seven frequencies were combined to construct 49 stimuli while recording activity through a 10 × 10 microelectrode array inserted into the middle layers of barrel cortex. We find that cortical neurons do not explicitly encode vibration frequency ( f ) or amplitude ( A ) by any coding measure (average spike counts over different time windows, spike timing patterns in the peristimulus time histograms or in autocorrelograms). Instead, neurons explicitly encode the product of frequency and amplitude, which is proportional to the mean speed of the vibration. The quantity Af is an invariant because neuronal response encodes this feature independently of the values of the individual terms A and f . This was true across a wide time scale of firing rate measurements, from 5 to 500 msec. We conclude that vibration kinetics are rapidly and reliably encoded in the firing rate of cortical ensembles. Therefore, the cortical representation of vibration speed could underlie texture discrimination.

A radio catalog of Galactic HII regions for applications from decimeter to millimeter wavelengths

Abstract: LBNL, SSL, Physics Department, University of California, Berkeley, CA 94720, USAReceived July 27, 2002; accepted October 10, 2002Abstract. By collecting the information from 24 previously published lists and catalogs, we produce a compre-hensive catalog (Master Catalog) of 1442 Galactic HII regions. For each object, we quote the original fluxes anddiameters as well as the available information on radio line velocities, line widths and line temperatures andthe errors on these quantitities. References to the original works are also reported. By exploiting all these datawe produce a Synthetic Catalog of fluxes and diameters (with corresponding errors) at 2.7 GHz. This choiceis motivated by the extensive, although not complete, information available at this frequency, widely spreadamong many different catalogs, and by its relevance for both detailed studies on Galactic HII regions and theextrapolation up to millimetric wavelengths. The catalog can be used for detailed studies of Galactic HII regionsand, by extrapolation, for investigations of HII regions up to millimetric wavelengths. In particular, we discussthe study of the effects of microwave emission from HII regions on the new generation of Cosmic MicrowaveBackground (CMB) experiments. We present simulations of the detection of HII regions in the Planck highresolution CMB survey, and briefly analize some of the typical applications of our catalog to the evaluation ofCMB anisotropy experiments such as calibration, beam reconstruction and straylight effects. The Master Catalogand the Synthetic Catalog are available via ftp at: cdsarc.u-strasbg.fr. This work is related to Planck-LFIactivities.Key words. HII Regions – Catalogs – cosmic microwave background

First-principles study of lattice instabilities in ferromagnetic Ni 2 MnGa

Abstract: The phonon-dispersion relations and elastic constants for ferromagnetic ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ in the cubic and tetragonally distorted Heusler structures are computed using density-functional and density-functional-perturbation theory within the spin-polarized generalized-gradient approximation. For $0.9lc/al1.06,$ the ${\mathrm{TA}}_{2}$ transverse- acoustic branch along $[110]$ and the symmetry-related directions exhibit a dynamical instability at a wave vector that depends on $c/a.$ Through examination of the Fermi-surface nesting and electron-phonon coupling, this is identified as a Kohn anomaly. In the parent cubic phase the computed tetragonal shear elastic constant, ${C}^{\ensuremath{'}}{=(C}_{11}\ensuremath{-}{C}_{12})/2,$ is close to zero, indicating a marginal elastic instability towards a uniform tetragonal distortion. We conclude that the cubic Heusler structure is unstable against a family of energy-lowering distortions produced by the coupling between a uniform tetragonal distortion and the corresponding $[110]$ modulation. The computed relation between the $c/a$ ratio and the modulation wave vector is in excellent agreement with structural data on the premartensitic $(c/a=1)$ and martensitic $(c/a=0.94)$ phases of ${\mathrm{Ni}}_{2}\mathrm{MnGa}.$

Merger of binary neutron stars of unequal mass in full general relativity

Abstract: We present results of three dimensional numerical simulations of the merger of unequal-mass binary neutron stars in full general relativity. A $\Gamma$-law equation of state $P=(\Gamma-1)\rho\epsilon$ is adopted, where $P$, $\rho$, $\varep$, and $\Gamma$ are the pressure, rest mass density, specific internal energy, and the adiabatic constant, respectively. We take $\Gamma=2$ and the baryon rest-mass ratio $Q_M$ to be in the range 0.85--1. The typical grid size is $(633,633,317)$ for $(x,y,z)$ . We improve several implementations since the latest work. In the present code, the radiation reaction of gravitational waves is taken into account with a good accuracy. This fact enables us to follow the coalescence all the way from the late inspiral phase through the merger phase for which the transition is triggered by the radiation reaction. It is found that if the total rest-mass of the system is more than $\sim 1.7$ times of the maximum allowed rest-mass of spherical neutron stars, a black hole is formed after the merger irrespective of the mass ratios. The gravitational waveforms and outcomes in the merger of unequal-mass binaries are compared with those in equal-mass binaries. It is found that the disk mass around the so formed black holes increases with decreasing rest-mass ratios and decreases with increasing compactness of neutron stars. The merger process and the gravitational waveforms also depend strongly on the rest-mass ratios even for the range $Q_M= 0.85$--1.

Probing the seesaw mechanism with neutrino data and leptogenesis

Abstract: We use the current low-energy neutrino data to understand the structure of the neutrino mass matrix. Considering this information and assuming hierarchical neutrino Yukawa couplings, we use the seesaw formula to study the properties of the heavy right-handed neutrinos Ni. We find that successful baryogenesis via leptogenesis requires mass degeneracy and maximal mixing of N1 and N2.

Oscillations of vertically integrated relativistic tori – I. Axisymmetric modes in a Schwarzschild space–time

Abstract: This is the first of a series of papers investigating the oscillation properties of relativistic, non-self-gravitating tori orbiting around a black hole. In this initial paper we consider the axisymmetric oscillation modes of a torus constructed in a Schwarzschild space-time. To simplify the treatment and make it as analytical as possible, we build our tori with vertically integrated and vertically averaged quantities, thus transforming the eigenvalue problem into a set of coupled ordinary differential equations. The tori are also modelled with a number of different non-Keplerian distributions of specific angular momentum, and we discuss how the oscillation properties change when different distributions of angular momentum are considered. Our investigation progresses by steps. We first consider a local analysis in Newtonian gravity and determine the properties of acoustic wave propagation within these objects, as well as the relations between acoustic and epicyclic oscillations. Next, we extend the local analysis to a general relativistic framework. Finally, we perform a global analysis and determine both the eigenfunctions and the eigenfrequencies of the axisymmetric oscillations corresponding to the p modes of relativistic tori. These behave as sound waves globally trapped in the torus and possess eigenfrequencies appearing in the simple sequence 2:3:4:..., independently of the distribution of angular momentum considered. The properties of the modes investigated here are in good agreement with those observed in recent numerical simulations, and could have a number of different applications. In X-ray binary systems containing a black hole candidate, for instance, p-mode oscillations could be used to explain the harmonic relations in the high-frequency quasi-periodic oscillations observed. In systems comprising a massive torus orbiting a black hole, on the other hand, p-mode oscillations could be used to explain the development or the suppression of the runaway instability.

Separation of Variables for Bi-Hamiltonian Systems

Abstract: We address the problem of the separation of variables for the Hamilton–Jacobi equation within the theoretical scheme of bi-Hamiltonian geometry. We use the properties of a special class of bi-Hamiltonian manifolds, called ωN manifolds, to give intrisic tests of separability (and Stackel separability) for Hamiltonian systems. The separation variables are naturally associated with the geometrical structures of the ωN manifold itself. We apply these results to bi-Hamiltonian systems of the Gel'fand–Zakharevich type and we give explicit procedures to find the separated coordinates and the separation relations.