How to Mix Molecules with Mathematics

In this paper we develop two methods to calculate thermodynamic properties of mixtures. Starting point are the basic assumptions that also form the basis for the COSMO-RS model. In this approach, the individual molecules are represented by their geometrical shape with an electrical charge density on their surfaces. Next, the surface is split up into surface segments each with its own charge. In COSMO-RS a strong reduction is introduced by treating the segments as if they are completely independent. In the present study we take into account that the coupling between two patches is essentially dependent on the charge distribution on neighboring segments and on the local geometrical structure of the surface. Two approaches are followed. The first one points out how the model equations, which comprise the optimization of the entropy and conservation of internal energy, can efficiently be solved in general, thus also if the dependency between segments and the local geometry is included in the expression for the coupling energy between segments. In the second method the configuration with maximal entropy and prescribed energy is sought via simulation. Successive molecular configurations of the mixture are simulated and updated via a genetic algorithm to optimize the entropy. The second method is more time consuming but very general

Parallax and Kinematics of PSR B0919+06 from VLBA Astrometry and Interstellar Scintillometry

Results are presented from a long-term astrometry program on PSR B0919+06 using the NRAO Very Long Baseline Array. With ten observations (seven epochs) between 1994--2000, we measure a proper motion of 18.35 +/- 0.06 mas/yr in RA, 86.56 +/- 0.12 mas/yr in Dec, and a parallax of 0.83 +/- 0.13 mas (68% confidence intervals). This yields a pulsar distance of 1.21 +/- 0.19 kpc, making PSR B0919+06 the farthest pulsar for which a trigonometric parallax has been obtained, and the implied pulsar transverse speed is 505 +/- 80 km/s. Combining the distance estimate with interstellar scintillation data spanning 20 years, we infer the existence of a patchy or clumpy scattering screen along the line of sight in addition to the distributed electron density predicted by models for the Galaxy, and constrain the location of this scattering region to within about 250 parsecs of the Sun. Comparison with the lines of sight towards other pulsars in the same quadrant of the Galaxy permits refinement of our knowledge of the local interstellar matter in this direction.Comment: 12 pages, includes 4 figures and 3 tables, uses AASTeX 5 (included); ApJ submitte

Nofs ta' kelma

Ġabra ta’ poeżiji u proża li tinkludi: Alla kbir bla qies! ta’ R. M. B. – Tantum ergo – Lil kewkba feġġa – Għajjiena le xebagħna ta’ Ros. Briffa – Sliem ta’ Dun Karm – Kewkba ta’ Dun Karm – Nofs ta’ kelma ta’ A. C.N/

Kinematic Age Estimates for 4 Compact Symmetric Objects from the Pearson-Readhead Survey

Based on multi-epoch observations at 15 and 43 GHz with the Very Long Baseline Array (VLBA) we detect significant angular expansions between the two hot spots of 4 Compact Symmetric Objects (CSOs). From these relative motions we derive kinematic ages of between 300 and 1200 years for the radio emission. These ages lend support to the idea that CSOs are produced in a recent phase of activity. These observations also allow us to study the evolution of the hot spots dynamically in individual sources. In all 4 sources the hot spots are separating along the source axis, but in 1031+567 the tip of one hot spot appears to be moving almost orthogonally to the source axis. Jet components, seen in 3 of the 4 sources observed, are found to be moving relativistically outward from the central engines towards the more slowly moving hot spots.Comment: in press at ApJ for v. 541 Oct. 1, 2000, 23 page LaTeX document includes 6 postscript figure

L'usage des systèmes d'informations électroniques en recherche scientifique : le cas de la neurophysiologie.

Nous présentons les premiers résultats d'une enquête destinée à mieux connaître les pratiques de recherche d'information bibliographique et documentaire chez les chercheurs scientifiques. 64 chercheurs et étudiants doctorants en neurophysiologie ont répondu à un questionnaire portant sur les méthodes, les outils, et les objectifs des recherches d'information typiques dans leur activité. De plus, 11 personnes parmi les répondants ont participé à un entretien individuel semi structuré. Il en ressort que l'usage d'outils informatisés de recherche d'information bibliographique (RIB) est désormais pratique courante, au détriment des index et autres sources imprimées. Les principaux outils utilisés sont la base de données bibliographiques PubMed et le moteur de recherche Google, avec toutefois de nombreux autres outils plus spécifiques utilisés à titre complémentaire. Les répondants mentionnent des objectifs très variés, comme l'acquisition de connaissances nouvelles, mais aussi la recherche de techniques expérimentales, la veille documentaire, l'alimentation du débat scientifique, ou l'aide à l'enseignement. Les difficultés que rencontrent les experts en neurosciences intégratives dans l'exploitation des outils informatiques de RIB spécialisés semblent surtout liées à l'absence de formation des experts à ces outils. Les chercheurs définissent l'outil informatique de RIB « idéal » comme fiable et exhaustif, mais aussi rapide et facile à utiliser et apprendre . De fait, le facteur temps apparaît déterminant dans leur choix d'utilisation ou non d'un outil particulier. Cette étude ouvre la voie à des expériences plus spécifiques, qui porteront sur les stratégies cognitives des experts dans ce type de tâches

Rethinking the divide: Exploring the interdependence between global and nested local markets

The debate on smallholder commodification trajectories tends to be polarised between mainstream approaches that advocate tighter integration of smallholders into global value chains, and alternative approaches that favour localised markets on the grounds that these provide greater autonomy over production and marketing, and allow a greater share of value to be realised for producers and the wider community. This debate obscures the interrelations and possible synergies between them; a critique taken up in this paper. Using a case study on agricultural diversification in the former homeland of Venda, South Africa, we explore the usefulness of the nested markets concept to make sense of smallholders' patterning of markets by combining tree crops for export with seasonal vegetables for local markets. Exploring the drivers of diversification, we show how farmers’ patterning of markets depends on their profiles and corresponding trajectory of accumulation. Local markets are articulated systems that function as hybrid spaces of interaction that enable farmers without any alternative off-farm income to gain and sustain access to global commodity markets. This challenges the framing of nested markets as an act of resistance as well as the dichotomy between local versus global markets as mutually exclusive. Instead, we argue that these markets can be interconnected and mutually supportive and are opportunistically used as such by petty commodity producers to sustain their export-oriented production system. If these relations are better understood, they stand to enable agrarian policy, which currently favours high-value tree crops, to be more inclusive of young and less well-resourced farmers

Magnetic Fields in Quasar Cores II

Multi-frequency polarimetry with the Very Long Baseline Array (VLBA) telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of 1000 rad/m/m in the central regions of 7 out of 8 strong quasars studied (e.g., 3C 273, 3C 279, 3C 395). Beyond a projected distance of ~20 pc, however, the jets are found to have |RM| < 100 rad/m/m. Such sharp RM gradients cannot be produced by cluster or galactic-scale magnetic fields, but rather must be the result of magnetic fields organized over the central 1-100 pc. The RMs of the sources studied to date and the polarization properties of BL Lacs, quasars and galaxies are shown to be consistent so far with the predictions of unified schemes. The direct detection of high RMs in these quasar cores can explain the low fractional core polarizations usually observed in quasars at centimeter wavelengths as the result of irregularities in the Faraday screen on scales smaller than the telescope beam. Variability in the RM of the core is reported for 3C 279 between observations taken 1.5 years apart, indicating that the Faraday screen changes on that timescale, or that the projected superluminal motion of the inner jet components samples a new location in the screen with time. Either way, these changes in the Faraday screen may explain the dramatic variability in core polarization properties displayed by quasars.Comment: Accepted to the ApJ. 27 pages, 9 figures including figure 6 in colo

A Search for Ultra-High Energy Counterparts to Gamma-Ray Bursts

A small air shower array operating over many years has been used to search for ultra-high energy (UHE) gamma radiation ($\geq 50$ TeV) associated with gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each burst are presented for seven GRBs located with zenith angles $\theta < 20^{\circ}$. A $4.3\sigma$ excess over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8\%. If this effect is real then cosmological models are excluded for this burst because of absorption of UHE gamma rays by the intergalactic radiation fields.Comment: 4 pages LaTeX with one postscript figure. This version does not use kluwer.sty and will allow automatic postscript generatio