Showing papers by "National University of La Plata published in 2008"

Observation of the suppression of the flux of cosmic rays above 4x10(19) eV

Abstract: The energy spectrum of cosmic rays above 2.5 x 10;{18} eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the particle flux, J proportional, variantE;{-gamma}, at energies between 4 x 10;{18} eV and 4 x 10;{19} eV is 2.69+/-0.02(stat)+/-0.06(syst), steepening to 4.2+/-0.4(stat)+/-0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min.

The emerging functions and mechanisms of mammalian fatty acid-binding proteins

Abstract: Fatty acid-binding proteins (FABPs) are abundant intracellular proteins that bind long-chain fatty acids with high affinity. Nine separate mammalian FABPs have been identified, and their tertiary structures are highly conserved. The FABPs have unique tissue-specific distributions that have long suggested functional differences among them. In the last decade, considerable progress has been made in understanding the specific functions of the FABPs and, in some cases, their mechanisms of action at the molecular level. The FABPs appear to be involved in the extranuclear compartments of the cell by trafficking their ligands within the cytosol via interactions with organelle membranes and specific proteins. Several members of the FABP family have been shown to function directly in the regulation of cognate nuclear transcription factor activity via ligand-dependent translocation to the nucleus. This review will focus on these emerging functions and mechanisms of the FABPs, highlighting the unique functional properties of each as well as the similarities among them.

Thermodynamic signature of growing amorphous order in glass-forming liquids

Abstract: That the dynamical properties of a glass-forming liquid at high temperature are different from behaviour in the supercooled state has already been established. Numerical simulations now suggest that the static length scale over which spatial correlations exist also changes on approaching the glass transition. Supercooled liquids exhibit a pronounced slowdown of their dynamics on cooling1 without showing any obvious structural or thermodynamic changes2. Several theories relate this slowdown to increasing spatial correlations3,4,5,6. However, no sign of this is seen in standard static correlation functions, despite indirect evidence from considering specific heat7 and linear dielectric susceptibility8. Whereas the dynamic correlation function progressively becomes more non-exponential as the temperature is reduced, so far no similar signature has been found in static correlations that can distinguish qualitatively between a high-temperature and a deeply supercooled glass-forming liquid in equilibrium. Here, we show evidence of a qualitative thermodynamic signature that differentiates between the two. We show by numerical simulations with fixed boundary conditions that the influence of the boundary propagates into the bulk over increasing length scales on cooling. With the increase of this static correlation length, the influence of the boundary decays non-exponentially. Such long-range susceptibility to boundary conditions is expected within the random first-order theory4,9,10 (RFOT) of the glass transition. However, a quantitative account of our numerical results requires a generalization of RFOT, taking into account surface tension fluctuations between states.

Biosensing and supramolecular bioconjugation in single conical polymer nanochannels. Facile incorporation of biorecognition elements into nanoconfined geometries.

Abstract: There is a growing quest for tailorable nanochannels or nanopores having dimensions comparable to the size of biological molecules and mimicking the function of biological ion channels. This interest is based on the use of nanochannels as extremely sensitive single molecule biosensors. The biosensing capabilities of these nanochannels depend sensitively on the surface characteristics of their inner walls to achieve the desired functionality of the biomimetic system. Nanoscale control over the surface properties of the nanochannel plays a crucial role in the biosensing performance due to the chemical groups incorporated on the inner channel walls that act as binding sites for different analytes and interact with molecules passing through the channel. Here we report a new approach to incorporate biosensing elements into polymer nanochannels by using electrostatic self-assembly. We describe a facile strategy based on the use of bifunctional macromolecular ligands to electrostatically assemble biorecongnition sites into the nanochannel wall, which can then be used as recognition elements for constructing a nanobiosensor. The experimental results demonstrate that the ligand-functionalized nanochannels are very stable and the biorecognition event (protein conjugation) does not promote the removal of the ligands from the channel surface. In addition, control experiments indicated that the electrostatically assembled nanochannel surface displays good biospecificity and nonfouling properties. Then, we demonstrate that this approach also enables the creation of supramolecular multilayered structures inside the nanopore that are stabilized by strong ligand-receptor interactions. We envision that the formation of multilayered supramolecular assemblies inside solid-state nanochannels will play a key role in the further expansion of the toolbox called "soft nanotechnology", as well as in the construction of new multifunctional biomimetic systems.

A multifractal approach for stock market inefficiency

Abstract: In this paper, the multifractality degree in a collection of developed and emerging stock market indices is evaluated. Empirical results suggest that the multifractality degree can be used as a quantifier to characterize the stage of market development of world stock indices. We develop a model to test the relationship between the stage of market development and the multifractality degree and find robust evidence that the relationship is negative, i.e., higher multifractality is associated with a less developed market. Thus, an inefficiency ranking can be derived from multifractal analysis. Finally, a link with previous volatility time series results is established.

Update on vegetable lecithin and phospholipid technologies

Abstract: This paper reviews the production technologies for sourcing lecithins from the oil-bearing seeds soybean, rapeseed and sunflower kernel. The phospholipid composition is measured by newly developed HPLC-LSD and 31P-NMR methods. The phospholipid compositions of the three types of lecithin show small differences, while the fatty acid composition is largely equivalent to the oil source. Regulatory specifications (FAO/WHO, EU, FCC) and DGF and AOCS analytical methods for product quality are compiled. Phospholipid modifications by enzymatic hydrolysis, solvent fractionation, acetylating and hydroxylation processes result in lecithins with specific enhanced hydrophilicity and oil-in-water emulsifying properties. New available phospholipase and lipase enzymes represent opportunities for the esterification of phospholipids with special omega fatty acids and serine groups. Application characteristics are given for use in yellow fat spreads, baked goods, chocolate, agglomerated instant powders, liposome encapsulation, animal feed, food supplements and pharmaceutics.

Upper limit on the diffuse flux of ultrahigh energy tau neutrinos from the Pierre Auger Observatory

Abstract: The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from ντ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ντ at EeV energies. Assuming an Eν-2 differential energy spectrum the limit set at 90% C.L. is Eν2dNντ/dEν<1.3×10-7GeVcm-2s-1sr-1 in the energy range 2×1017eV

The Social Production of Toxic Uncertainty

Abstract: Based on both archival research and two and a half years of ethnographic fieldwork in an Argentine shantytown with high levels of air, water, and ground contamination, this article examines the social production of environmental uncertainty First, we dissect residents' perceptions of contamination, finding widespread doubts and mistakes about the polluted habitat Second, we provide a sociologically informed account of uncertainty and the erroneous perceptions that underlie it Along with inherent ambiguity surrounding toxic contamination, the generalized confusion about sources and effects of pollution is the result of two factors: (1) the “relational anchoring” of risk perceptions and (2) the “labor of confusion” generated by powerful outside actors We derive two implications from this ethnographic case study: (1) Cognitive psychology and organizational sociology can travel beyond the boundaries of self-bounded communities and laboratory settings to understand and explain the collective production and

A preliminary study on various implementation approaches of domain-specific language

Abstract: Various implementation approaches for developing a domain-specific language are available in literature. There are certain common beliefs about the advantages/disadvantages of these approaches. However, it is hard to be objective and speak in favor of a particular one, since these implementation approaches are normally compared over diverse application domains. The purpose of this paper is to provide empirical results from ten diverse implementation approaches for domain-specific languages, but conducted using the same representative language. Comparison shows that these discussed approaches differ in terms of the effort need to implement them, however, the effort needed by a programmer to implement a domain-specific language should not be the only factor taken into consideration. Another important factor is the effort needed by an end-user to rapidly write correct programs using the produced domain-specific language. Therefore, this paper also provides empirical results on end-user productivity, which is measured as the lines of code needed to express a domain-specific program, similarity to the original notation, and how error-reporting and debugging are supported in a given implementation.

Modeling He-rich subdwarfs through the hot-flasher scenario

Abstract: We present 1D numerical simulations aimed at studying the hot-flasher scenario for the formation of He-rich subdwarf stars. Sequences were calculated for a wide range of metallicities and physical assumptions, such as the stellar mass at the moment of the helium core flash. This allows us to study the two previously proposed flavors of the hot-flasher scenario (“deep” and “shallow” mixing cases) and to identify a third transition type. Our sequences are calculated by solving simultaneously the mixing and burning equations within a diffusive convection picture, and in the context of standard mixing length theory. We are able to follow chemical evolution during deep-mixing events in which hydrogen is burned violently, and therefore able to present a homogeneous set of abundances for different metallicities and varieties of hot-flashers. We extend the scope of our work by analyzing the effects of non-standard assumptions, such as the effect of chemical gradients, extra-mixing at convective boundaries, possible reduction in convective velocities, or the interplay between difussion and mass loss. Particular emphasis is placed on the predicted surface properties of the models. We find that the hot-flasher scenario is a viable explanation for the formation and surface properties of He-sdO stars. Our results also show that, during the early He-core burning stage, element diffusion may produce the transformation of (post hot-flasher) He-rich atmospheres into He-deficient ones. If this is so, then we find that He-sdO stars should be the progenitors of some of the hottest sdB stars.

'Senescence-associated vacuoles' are involved in the degradation of chloroplast proteins in tobacco leaves.

Abstract: Massive degradation of photosynthetic proteins is the hallmark of leaf senescence; however the mechanism involved in chloroplast protein breakdown is not completely understood. As small 'senescence-associated vacuoles' (SAVs) with intense proteolytic activity accumulate in senescing leaves of soybean and Arabidopsis, the main goal of this work was to determine whether SAVs are involved in the degradation of chloroplastic components. SAVs with protease activity were readily detected through confocal microscopy of naturally senescing leaves of tobacco (Nicotiana tabacum L.). In detached leaves incubated in darkness, acceleration of the chloroplast degradation rate by ethylene treatment correlated with a twofold increase in the number of SAVs per cell, compared to untreated leaves. In a tobacco line expressing GFP targeted to plastids, GFP was re-located to SAVs in senescing leaves. SAVs were isolated by sucrose density gradient centrifugation. Isolated SAVs contained chloroplast-targeted GFP and the chloroplast stromal proteins Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) and glutamine synthetase, but lacked the thylakoid proteins D1 and light-harvesting complex II of the photosystem II reaction center and photosystem II antenna, respectively. In SAVs incubated at 30 degrees C, there was a steady decrease in Rubisco levels, which was completely abolished by addition of protease inhibitors. These results indicate that SAVs are involved in degradation of the soluble photosynthetic proteins of the chloroplast stroma during senescence of leaves.

Effects of AGN feedback on ΛCDM galaxies

Abstract: We study the effects of Active Galactic Nuclei (AGN) feedback on the formation andevolution of galaxies by using a combination of a cosmological N-body simulation ofthe concordance Λ cold dark matter (ΛCDM) paradigm and a semi-analytic modelof galaxy formation. This model is an improved version of the one described by Cora(2006), which now considers the growth of black holes (BHs) as driven by (i) gasaccretion during merger-driven starbursts and mergers with other BHs, (ii) accretionduring starbursts triggered by disc instabilities, and (iii) accretion of gas cooled fromquasi-hydrostatic hot gas haloes. It is assumed that feedback from AGN operatesin the later case. The model has been calibrated in order to reproduce observationalcorrelations between BH mass and mass, velocity dispersion, and absolute magnitudesofthe galaxybulge. AGN feedback hasa strongimpact on reducing oreven suppressinggas cooling, an effect that becomes important at lower redshifts. This phenomenonhelpsto reproducethe observedgalaxyluminosityfunction(LF) inthe opticalandnearIR bands at z = 0, and the cosmic star formation rate and stellar mass functions over awide redshift range (0 . z . 5). It also allows to have a population of massive galaxiesalready in place at z & 1, which are mostly early-type and have older and redderstellar populations than lower mass galaxies, reproducing the observed bimodalityin the galaxy colour distribution, and the morphological fractions. The evolution ofthe optical QSO LF is also reproduced, provided that the presence of a significantfraction of obscured QSOs is assumed. We explore the effects of AGN feedback duringstarbursts finding that, in order to obtain a good agreement with observations, theseneed to be strong enough to expell the reheated gas away from the galaxy halo.We also test new, recent prescriptions for dynamical friction time-scales, and findthat they produce an earlier formation of elliptical galaxies, and a larger amount ofdisc instabilites, which compensate the change in the merger frequency such that theproperties of z = 0 galaxies remain almost unaffected.Key words: galaxies: evolution - galaxies: formation - galaxies: statistics - quasars:general

Comparative structural and chemical studies of ferritin cores with gradual removal of their iron contents.

Abstract: Transmission Electron Microscopy (TEM), X-ray Absorption Near Edge Spectroscopy (XANES), Electron Energy-Loss Spectroscopy (EELS), Small-Angle X-ray Scattering (SAXS), and SQUID magnetic studies were performed in a batch of horse spleen ferritins from which iron had been gradually removed, yielding samples containing 2200, 1200, 500, and 200 iron atoms. Taken together, findings obtained demonstrate that the ferritin iron core consists of a polyphasic structure (ferrihydrite, magnetite, hematite) and that the proportion of phases is modified by iron removal. Thus, the relative amount of magnetite in ferritin containing 2200 to 200 iron atoms rose steadily from approximately 20% to approximately 70% whereas the percentage of ferrihydrite fell from approximately 60% to approximately 20%. These results indicate a ferrihydrite-magnetite core-shell structure. It was also found that the magnetite in the ferritin iron core is not a source of free toxic ferrous iron, as previously believed. Therefore, the presence of magnetite in the ferritin cores of patients with Alzheimer's disease is not a cause of their increased brain iron(II) concentration.

Unveiling the nature of INTEGRAL objects through optical spectroscopy. VI. A multi-observatory identification campaign

Abstract: Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of still unidentified hard X-ray sources.

The periodicity of the η Carinae events

Abstract: Extensive spectral observations of η Carinae over the last cycle, and particularly around the 2003.5 low-excitation event, have been obtained. The variability of both narrow and broad lines, when combined with data taken from two earlier cycles, reveal a common and well-defined period. We have combined the cycle lengths derived from the many lines in the optical spectrum with those from broad-band X-rays, optical and near-infrared observations, and obtained a period length of P pres = 2022.7 ± 1.3 d. Spectroscopic data collected during the last 60 yr yield an average period of P avg = 2020 ± 4 d, consistent with the present-day period. The period cannot have changed by more than AP/P = 0.0007 since 1948. This confirms the previous claims of a true, stable periodicity, and gives strong support to the binary scenario. We have used the disappearance of the narrow component of He I 6678 to define the epoch of the Cycle 11 minimum, To = JD 245 2819.8. The next event is predicted to occur on 2009 January 11 (±2 d). The dates for the start of the minimum in other spectral features and broad-bands are very close to this date, and have well-determined time-delays from the He I epoch.

Improved tolerance of wheat plants (Triticum aestivum L.) to drought stress and rewatering by the arbuscular mycorrhizal fungus Glomus claroideum: Effect on growth and cell membrane stability

Abstract: The aim of this paper was to investigate the contribution of the arbuscular mycorrhizal fungus Glomus claroideum to drought stress tolerance in wheat plants grown under controlled conditions in a growth chamber, and subjected to moderate or severe water stress and rewatering. Water stress tolerance was determined through total dry weight, leaf relative water content, leakage of solutes and leaf chlorophyll and protein concentrations in mycorrhizal and non-mycorrhizal wheat plants. Total dry weight and leaf chlorophyll concentrations were significantly higher in mycorrhizal plants after moderate or severe water stress treatments compared with non-mycorrhizal ones. Electrolyte leakage was significantly lower in water-stressed inoculated plants. Compared to non-inoculated plants, leaf relative water content and total protein concentration of inoculated individuals increased only under severe water stress. When irrigation was re-established, mycorrhizal plants increased their total dry weight and leaf chlorophyll concentration, and recovered cell membrane permeability in leaves compared with non-mycorrhizal plants. In conclusion, root colonization by G. claroideum could be an adequate strategy to alleviate the deleterious effects of drought stress and retard the senescence syndrome in wheat.

Metric character of the quantum Jensen-Shannon divergence

Abstract: In a recent paper, the generalization of the Jensen-Shannon divergence in the context of quantum theory has been studied [Majtey et al., Phys. Rev. A 72, 052310 (2005)]. This distance between quantum states has shown to verify several of the properties required for a good distinguishability measure. Here we investigate the metric character of this distance. More precisely we show, formally for pure states and by means of a numerical procedure for mixed states, that its square root verifies the triangle inequality.