Showing papers by "National University of La Plata published in 2001"

Correlated X-Ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550–564: A New Interpretation of Black Hole States

Abstract: We present an analysis of data of the black hole candidate and X-ray transient XTE J1550-564, taken with the Rossi X-Ray Timing Explorer between 1998 November 22 and 1999 May 20. During this period the source went through several different states, which could be divided into soft and hard states based on the relative strength of the high-energy spectral component. These states showed up as distinct branches in the color-color and hardness-intensity diagrams, connecting to form a structure with a comblike topology, the branch corresponding to the soft state forming the spine and the branches corresponding to the various hard states forming the teeth of the comb. The power spectral properties of the source were strongly correlated with its position on the branches. The broadband noise became stronger and changed from power law-like to band-limited, as the spectrum became harder. Three types of quasi-periodic oscillations (QPOs) were found: 1-18 Hz and 102-284 Hz QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The 1-18 Hz QPOs on the hard branches could be divided into three subtypes. The frequencies of the high- and low-frequency QPOs on the hard branches were correlated with each other and were anticorrelated with spectral hardness. The changes in QPO frequency suggest that the inner disk radius only increases by a factor of 3-4 as the source changes from a soft to a hard state. Our results on XTE J1550-564 strongly favor a two-dimensional description of black hole behavior, where the regions near the spine of the comb in the color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which includes the very high state) to the low state, and back. The two physical parameters underlying this two-dimensional behavior vary to a large extent independently and could for example be the accretion rate through the disk and the size of the Comptonizing region causing the hard tail. The difference between the various teeth is then associated with the mass accretion rate through the disk, suggesting that high state ↔ low state transitions can occur at any disk mass accretion rate and that these transitions are primarily caused by another, independent parameter. We discuss how this picture could tie in with the canonical, one-dimensional behavior of black hole candidates that has usually been observed.

Chemical and microbiological characterisation of kefir grains.

Abstract: Chemical and microbiological composition of four Argentinean kefir grains from different sources as well as characteristics of the corresponding fermented milk were studied. Kefir grains CIDCA AGK1, AGK2 and AGK4 did not show significant differences in their chemical and microbiological composition. In contrast, protein and yeast content of AGK3 was higher than in the other grains. Although grain microflora comprised lactobacilli, lactococcus, acetic acid bacteria and yeast, we found an important difference regarding species. Lactococcus lactis subsp. lactis, Lactobacillus kefir, Lactobacillus plantarum, Acetobacter and Saccharomyces were present in all types of kefir grain. While Leuconostoc mesenteroides was only isolated from grains CIDCA AGK1 and Lactococcus lactis subsp. lactis biovar diacetylactis, Lactobacillus parakefir and Kluyveromyces marxianus were only isolated from CIDCA AGK2 grains. All grains produced acid products with pH between 3.5 and 4.0. The apparent viscosity of AGK1 fermented milk was greater than the product obtained with AGK4. All fermented milks had inhibitory power towards Escherichia coli but AGK1 and AGK2 supernatants were able to halt the bacterial growth for at least 25 h. Grain weight increment in AGK1, AGK2 and AGK3 during growth in milk did not show significant differences. Despite their fermenting activity, AGK4 grains did not increase their weight.

Designing personalized web applications

Abstract: The goal of this paper is to argue the need to approach the personalization issues in Web applications from the very beginning in the application’s development cycle. Since personalization is a critical aspect in many popular domains such as e-commerce, it important enough that it should be dealt with through a design view, rather than only an implementation view (which discusses mechanisms, rather than design options). We present different scenarios of personalization covering most existing applications. Since our design approach is based on the Object-Oriented Hypermedia Design Method, we briefly introduce i the way in which we build Web application models as object -oriented views of conceptual models. We show how we specify personalized Web applications by refining views according to users’ profiles or preferences; we show that an object -oriented approach allows maximizing reuse in these specifications. We discuss some implementation aspects and compare our work with related approaches, and present some concluding remarks.

Individual heterogeneity in the returns to schooling: instrumental variables quantile regression using twins data

Abstract: Considerable effort has been exercised in estimating mean returns to education while carefully considering biases arising from unmeasured ability and measurement error. Recent work has investigated whether there are variations from the “mean” return to education across the population with mixed results. We use an instrumental variables estimator for quantile regression on a sample of twins to estimate an entire family of returns to education at different quantiles of the conditional distribution of wages while addressing simultaneity and measurement error biases. We test whether there is individual heterogeneity in returns to education and find that: more able individuals obtain more schooling perhaps due to lower marginal costs and/or higher marginal benefits of schooling and that higher ability individuals (those further to the right in the conditional distribution of wages) have higher returns to schooling consistent with a non-trivial interaction between schooling and unobserved abilities in the generation of earnings. The estimated returns are never lower than 9 percent and can be as high as 13 percent at the top of the conditional distribution of wages but they vary significantly only along the lower to middle quantiles. Our findings may have meaningful implications for the design of educational policies.

Specifying Quality Characteristics and Attributes for Websites

Abstract: In this work, we outline more than a hundred characteristics and attributes for the domain of academic sites in order to analyze the quality requirement tree and a way to specify them These elements are used in a quantitative methodology for assessment, comparison, and ranking processes The proposed Web-site Quality Evaluation Methodology (QEM) can be a useful approach to assess the quality in different phases of a Web product life cycle In the academic study, we have observed three different evaluation audiences regarding visitor users: current and prospective students, academic personnel, and research sponsors Besides, the aim of this work is to show a hierarchical and descriptive specification framework for characteristics, subcharacteristics and attributes This framework is a key underlying piece in the construction of a hyperdocumented evaluation tool Finally, some results are presented and concluding remarks are discussed

First Isolation of Neospora caninum From the Feces of a Naturally Infected Dog

Abstract: Neospora caninum is a major cause of abortion in cattle worldwide. Cattle become infected with N. caninum by ingesting oocysts from the environment or transplacentally from dam to fetus. Experimentally, dogs can act as definitive hosts, but dogs excrete few oocysts after ingesting tissue cysts. A natural definitive host was unknown until now. In the present study, N. caninum was isolated from the feces of a dog. Gerbils (Meriones unguiculatus) fed feces from the dog developed antibodies to N. caninum in the Neospora caninum agglutination test, and tissue cysts were found in their brains. Neospora caninum was isolated in cell culture and in gamma-interferon gene knockout mice inoculated with brain homogenates of infected gerbils. The DNA obtained from fecal oocysts of the dog, from the brains of gerbils fed dog feces, and from organisms isolated in cell cultures inoculated with gerbil brains was confirmed as N. caninum. The identification of N. caninum oocyst by bioassay and polymerase chain reaction demonstrates that the dog is a natural definitive host for N. caninum.

Nonextensive Thermostatistics and the H Theorem

Abstract: The kinetic foundations of Tsallis' nonextensive thermostatistics are investigated through Boltzmann's transport equation approach. Our analysis follows from a nonextensive generalization of the "molecular chaos hypothesis." For q>0, the q-transport equation satisfies an H theorem based on Tsallis entropy. It is also proved that the collisional equilibrium is given by Tsallis' q-nonextensive velocity distribution.

Quantification of Active Sites for the Determination of Methanol Oxidation Turn-over Frequencies Using Methanol Chemisorption and in Situ Infrared Techniques. 1. Supported Metal Oxide Catalysts

Abstract: Methanol oxidation over metal oxide catalysts is industrially important for the production of formaldehyde, but knowledge about the intrinsic catalysis taking place is often obscured by a lack of knowledge as to the number of active sites present on the catalyst surface. In the present study, the number of surface sites active in methanol oxidation has been determined over a wide range of supported metal oxide catalysts using quantitative methanol chemisorption and in-situ infrared titration techniques performed at an experimentally optimized temperature of 110 °C. It was found that a steric limitation of about 0.3 methoxylated surface species (e.g., strongly Lewis-bound CH3OHads and dissociatively adsorbed −OCH3,ads, which are the reactive surface intermediates in methanol oxidation) exists per active deposited metal oxide metal atom across all supported metal oxides. Hence, the use of methanol chemisorption for counting active surface sites is more realistic than other site-counting methods for the kine...

Reverse Mode of the Na+-Ca2+ Exchange After Myocardial Stretch : Underlying Mechanism of the Slow Force Response

Abstract: —This study was designed to gain additional insight into the mechanism of the slow force response (SFR) to stretch of cardiac muscle. SFR and changes in intracellular Na+ concentration ([Na+]i) were assessed in cat papillary muscles stretched from 92% to ≈98% of Lmax. The SFR was 120±0.6% (n=5) of the rapid initial phase and coincided with an increase in [Na+]i. The SFR was markedly depressed by Na+-H+ exchanger inhibition, AT1 receptor blockade, nonselective endothelin-receptor blockade and selective ETA-receptor blockade, extracellular Na+ removal, and inhibition of the reverse mode of the Na+-Ca2+ exchange by KB-R7943. KB-R7943 prevented the SFR but not the increase in [Na+]i. Inhibition of endothelin-converting enzyme activity by phosphoramidon suppressed both the SFR and the increase in [Na+]i. The SFR and the increase in [Na+]i after stretch were both present in muscles with their endothelium (vascular and endocardial) made functionally inactive by Triton X-100. In these muscles, phosphoramidon also suppressed the SFR and the increase in [Na+]i. The data provide evidence that the last step of the autocrine-paracrine mechanism leading to the SFR to stretch is Ca2+ entry through the reverse mode of Na+-Ca2+ exchange.

Diffusion and the occurrence of hydrogen‐shell flashes in helium white dwarf stars

Abstract: In this paper we investigate the effects of element diffusion on the structure and evolution of low-mass helium white dwarfs. Attention is focused mainly on the occurrence of hydrogen-shell flashes induced by diffusion processes during cooling phases. Physically sound initial models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196, 0.169 and 0.161 M⊙ are constructed by applying mass-loss rates at different stages of the red giant branch evolution of a solar model up to the moment the model begins to evolve to the blue part of the HR diagram. The multicomponent flow equations describing gravitational settling, and chemical and thermal diffusion are solved and the diffusion calculations are coupled to an evolutionary code. In addition, the same sequences are computed but neglecting diffusion. Results without diffusion are similar to recent results of Driebe, Schonberner, Blocker and Herwig. We find that element diffusion strongly affects the structure and cooling history of helium white dwarfs. In particular, diffusion induces the occurrence of hydrogen-shell flashes in models with masses ranging from 0.18 to 0.41 M⊙, which is in sharp contrast with the situation when diffusion is neglected. In connection with further evolution, these diffusion-induced flashes lead to much thinner hydrogen envelopes, preventing stable nuclear burning from being a sizeable energy source at advanced stages of evolution. This implies much shorter cooling ages than in the case when diffusion is neglected. These new evolutionary models are discussed in light of recent observational data on some millisecond pulsar systems with white dwarf companions. In this context, we find that discrepancies between spin-down ages and the predictions of standard evolutionary models appear to be the result of ignoring element diffusion in such evolutionary models. Indeed, such discrepancies vanish when account is taken of diffusion.

Kinetic Study of the Reactions of Chlorine Atoms and Cl2•- Radical Anions in Aqueous Solutions. II. Toluene, Benzoic Acid, and Chlorobenzene†

Abstract: Laser and conventional flash photolysis of Na2S2O8 aqueous solutions containing Cl- ions were employed to investigate the reactions of chlorine atoms and Cl2•- radical ions with toluene, benzoic acid, and chlorobenzene. A mechanism is proposed which accounts for the faster decay of Cl2•- in aqueous solutions containing increasing concentrations of the organic substrates. Interpretation of the experimental data is supported by kinetic computer simulations. Chlorine atoms react with the three substituted aromatics studied here almost with diffusion-controlled rate constants, k = (1.8 ± 0.3) × 1010 M-1 s-1. The high reactivity observed for Cl atoms contrasts with that of the Cl2•- radical ions, for which the rate constant for its reactions with the substituted benzenes is ≤ 1 × 106 M-1 s-1. The organic radicals produced from these reactions, as well as the nature of the reaction products are discussed. The observed results seem to support an addition mechanism yielding chlorocyclohexadienyl radicals (Cl−CHD)...

Involvement of the Argentine Precordillera terrane in the Famatinian mobile belt: U-Pb SHRIMP and metamorphic evidence from the Sierra de Pie de Palo

Abstract: New data suggest that the eastern margin of the Argentine Precordillera terrane comprises Grenvillian basement and a sedimentary cover derived from it that were together affected by Middle Ordovician deformation and metamorphism during accretion to the Gondwana margin. The basement first underwent low pressure/temperature (P/T) type metamorphism, reaching high-grade migmatitic conditions in places (686 6 40 MPa, 790 6 17 8C), comparable to the Grenvillian M2 metamorphism of the supposed Laurentian counterpart of the terrane. The second metamorphism, recognized in the cover sequence, is of Famatinian age and took place under higher P/T conditions, following a clockwise P-T path (baric peak: 1300 6 100 Mpa, 600 6 50 8C). Low-U zircon overgrew detrital Grenvillian cores as pressure fell from its peak, and yields U-Pb SHRIMP ages of ca. 460 Ma. This is interpreted as the age of ductile thrusting coincident with early uplift; initial accretion to Gondwana must have occurred before this. The absence of late Neoproterozoic detrital zircons is consistent with a Laurentian origin of the Argentine Precordillera terrane.

Inhibition of Giardia intestinalis by Extracellular Factors from Lactobacilli: an In Vitro Study

Abstract: The aim of the present work was to evaluate the effect of spent culture supernatants of different strains of lactobacilli on giardia trophozoites. The growth of Giardia intestinalis strain WB, as well as the attachment to the human intestinal epithelial cell line Caco-2, was evaluated by using proliferation and adhesion assays with radiolabeled parasites. In addition, scanning electron microscopy and flow cytometric analysis were performed. The effect of spent culture supernatants from lactobacilli was strain dependent. Lactobacillus johnsonii La1 significantly inhibited the proliferation of G. intestinalis trophozoites. Although the effect was strongly pH dependent, it was not simply due to lactic acid. According to flow cytometric analysis, trophozoites were arrested in G1 phase but neither significant necrosis nor apoptosis could be detected. Bacterial cells or their spent culture supernatants were unable to modify trophozoite attachment to Caco-2 cells. However, trophozoites treated with spent culture supernatants had little, if any, proliferative capacity. These results suggest that La1 produces some substance(s) able to inhibit proliferation of Giardia trophozoites. Partial characterization of the factors involved in the antigiardiasic action showed that they have a low molecular mass and are inactivated by heating. On this basis, it seems worthwhile to explore how colonization of the proximal small bowel with these lactic acid bacteria could interfere with giardiasis in vivo.

High and intermediate-resolution spectroscopy of Be stars - An atlas of H$\gamma$, He I 4471 and Mg II 4481 lines

Abstract: We present an atlas of H ,H ei 4471 and Mg ii 4481 line proles obtained in a 10 year observation period of 116 Be stars, which enabled many of them to be observed at quite dierent emission epochs. From the best t of the observed He i 4471 line proles with non-LTE, uniform (Te; logg) and full limb-darkened model line proles, we determined the V sini of the program stars. To account, to some degree, for the line formation peculiarities related to the rapid rotation-induced non-uniform distributions of temperature and gravity on the stellar surface, the t was achieved by considering (Te; logg) as free parameters. This method produced V sini estimations that correlate with the rotational velocities determined by Slettebak (1982) within a dispersion 30 km s 1 and without any systematic deviation. They can be considered as given in the new Slettebak's et al. (1975) system. Only 13 program stars have discrepant V sini values. In some objects, this discrepancy could be attributed to binary eects. Using the newly determined V sini parameters, we found that the ratio of true rotational velocities V=Vc of the program Be stars has a very low dispersion around the mean value. Assuming then that all the stars are rigid rotators with the same ratio V (!)=Vc, we looked for the value of ! that better represents the distribution of V sini=Vc for randomly oriented rotational axes. We obtained ! =0 :795. This value enabled us to determine the probable inclination angle of the stellar rotation axis of the program stars. In the observed line proles of H ,H ei 4471, Mg ii 4481 and Fe ii 4351 we measured several parameters related to the absorption and/or emission components, such as: equivalent width, residual emission and/or absorption intensity, FWHM, emission peak separations, etc. The parameters related to the H line emission proles were used to investigate the structure of the nearby environment of the central star. From the characteristics of the correlations between these quantities and the inferred inclination angle, we concluded that in most of cases the H line emission forming regions may not be strongly flattened. Using a simple representation of the radiation flux emitted by the star+envelope system, we derived rst order estimates of physical parameters characterizing the H line emission formation region. Thus, we obtained that the total extent of the H region is Rf' 2:5 1:0 R and that the density distribution in these layers can be mimicked with a power law R ,w here =2 :5 +2:2 0:6 . The same approach enabled us to estimate the optical depth of the H line emission formation region. From its dependence with the aspect angle, we concluded that these regions are caracterized by a modest flattening and that the (equator)=(pole) density contrast of the circumstellar envelope near the star should be two orders of magnitude lower than predicted by models based on a priori disc-shaped circumstellar envelopes. We found that the separation between the emission peaks, p, and the full width at half maximum, 1=2 ,o f the H line emission are not only sensitive to kinematic eects, but to line optical depth as well. This nding agrees with previous theoretical predictions and conrms that Huang's (1972) relation overestimates the extent of the H line emission formation region.

Non-enzymatic glycosylation of a type I collagen matrix: effects on osteoblastic development and oxidative stress.

Abstract: The tissue accumulation of protein-bound advanced glycation endproducts (AGE) may be involved in the etiology of diabetic chronic complications, including osteopenia. The aim of this study was to investigate the effect of an AGE-modified type I collagen substratum on the adhesion, spreading, proliferation and differentiation of rat osteosarcoma UMR106 and mouse non-transformed MC3T3E1 osteoblastic cells. We also studied the role of reactive oxygen species (ROS) and nitric oxide synthase (NOS) expression on these AGE-collagen mediated effects. AGE-collagen decreased the adhesion of UMR106 cells, but had no effect on the attachment of MC3T3E1 cells. In the UMR106 cell line, AGE-collagen also inhibited cellular proliferation, spreading and alkaline phosphatase (ALP) activity. In preosteoblastic MC3T3E1 cells (24-hour culture), proliferation and spreading were significantly increased by AGE-collagen. After one week of culture (differentiated MC3T3E1 osteoblasts) AGE-collagen inhibited ALP activity, but had no effect on cell number. In mineralizing MC3T3E1 cells (3-week culture) AGE-collagen induced a decrease in the number of surviving cells and of extracellular nodules of mineralization, without modifying their ALP activity. Intracellular ROS production, measured after a 48-hour culture, was decreased by AGE-collagen in MC3T3E1 cells, but was increased by AGE-collagen in UMR106 cells. After a 24-hour culture, AGE-collagen increased the expression of endothelial and inducible NOS, in both osteoblastic cell lines. These results suggest that the accumulation of AGE on bone extracellular matrix could regulate the proliferation and differentiation of osteoblastic cells. These effects appear to depend on the stage of osteoblastic development, and possibly involve the modulation of NOS expression and intracellular ROS pathways.

Genotoxic ability of cadmium, chromium and nickel salts studied by kinetochore staining in the cytokinesis-blocked micronucleus assay.

Abstract: The aneugenic and clastogenic ability of cadmium chloride(II), cadmium sulfate(II), nickel chloride(II), nickel sulfate(II), chromium chloride(III) and potassium dichromate(IV) have been evaluated through kinetochore-stained micronucleus test. Traditional genotoxicity assays evaluate DNA damage, gene mutations and chromosome breakage. However, these tests are not adequate to detect aneugenic agents that do not act directly on DNA. Staining kinetochores in the cytokinesis-blocked micronucleus assay is a useful way to discriminate between clastogens and aneuploidogens and may allow a rapid identification of aneuploidy-inducing environmental compounds. Human diploid fibroblasts (MRC-5) were employed. All compounds increased micronuclei frequency in a statistically significant way. However, increases in kinetochore-positive micronuclei frequencies were higher than in kinetochore-negative ones. The present work demonstrates the genotoxic ability of the cadmium and chromium salts studied. Aneugenic as well as clastogenic ability could be observed with this assay. Nickel salts, as it was expected because of their known weak mutagenicity, showed lower genotoxic effects than the other metal salts studied. As the test employed only allows the detection of malsegregation, it is proposed that this mechanism is at least one of those by which the tested metal salts induced aneuploidy. On the other hand, visualization of kinetochores in all experiments suggests that the compounds studied did not act by damaging these structures.

Pharmacology of chiral compounds: 2-arylpropionic acid derivatives.

Abstract: Molecules exist as three dimensional structures. Therefore they can exist in symmetrical and asymmetrical forms. Molecules with an asymmetric centre are chiral. If the molecule and its mirror image are non-superimposable, the relationship between the two molecules is enantiomeric and the two stereoisomers are enantiomers. Since enantiomers have very similar or identical physicochemical properties, it is very difficult to distinguish between them in an achiral environment. However, once in a chiral environment, as in the body, they exhibit clear differences. In fact, most of the physiological processes in nature are stereospecific. Stereospecificity can occur in pharmacokinetic processes, in particular that utilise a carrier protein, receptor or enzyme. In addition, stereoselectivity occurs in pharmacodynamic processes and the differences between enantiomers can be either qualitative and quantitative. 2-arylpropionic acid derivatives (2APAs - profens) are an important subgroup within the class of NSAIDs. These are chiral compounds marketed as racemic mixtures. Some members of the group in an species-dependent manner undergo a special type of metabolic transformation leading to partial inversion to the optical antipode through a specific conjugation with CoA (coenzyme A) and subsequent epimerization. This metabolic inversion has not only pharmacological consequences (related to clinical effect) but also toxicological consequences such as, formation of hybrid triglycerides and even inhibition of fatty acid beta-oxidation. Differences on inversion rate between compounds and species will be discussed as well as its modification by different patho-physiologic processes such as, inflammation.

The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aquila X-1, as a Function of X-ray Intensity

Abstract: We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a set of nearly parallel tracks in a frequency versus X-ray count rate diagram. We find that between two of these tracks, for similar QPO frequency, the 2-60 keV source count rate can differ by up to a factor of ~4, whereas at the same time the rms amplitude of the kHz QPOs is only a factor of ~1.1 different. We also find that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not depend upon which track the source occupies in the frequency versus X-ray count rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with simple "extra source of X-rays" scenarios for the parallel tracks, such as those in which the properties of the kHz QPOs are determined only by the mass accretion rate through the disk, whereas X-ray count rate also depends upon other sources of energy that do not affect the QPOs.

The connection between globular cluster systems and the host galaxies

Abstract: ABSTRA C T A large number of early-type galaxies are now known to possess blue and red subpopulations of globular clusters. We have compiled a data base of 28 such galaxies exhibiting bimodal globular cluster colour distributions. After converting to a common V‐I colour system, we investigate correlations between the mean colour of the blue and red subpopulations with galaxy velocity dispersion. We support previous claims that the mean colours of the blue globular clusters are unrelated to their host galaxy. They must have formed rather independently of the galaxy potential they now inhabit. The mean blue colour is similar to that for halo globular clusters in our Galaxy and M31. The red globular clusters, on the other hand, reveal a strong correlation with galaxy velocity dispersion. Furthermore, in well-studied galaxies the red subpopulation has similar, and possibly identical, colours to the galaxy halo stars. Our results indicate an intimate link between the red globular clusters and the host galaxy; they share a common formation history. A natural explanation for these trends would be the formation of the red globular clusters during galaxy collapse.