30,149 research outputs found
The long-wavelength view of GG Tau A: rocks in the ring world
No full textWe present the first detection of GG Tau A at centimetre wavelengths, made with the Arcminute Microkelvin Imager Large Array at a frequency of 16 GHz (λ = 1.8 cm). The source is detected at >6 σrms with an integrated flux density of S16GHz = 249 ± 45 µJy. We use these new centimetre-wave data, in conjunction with additional measurements compiled from the literature, to investigate the long-wavelength tail of the dust emission from this unusual protoplanetary system. We use an MCMC-based method to determine maximum likelihood parameters for a simple parametric spectral model and consider the opacity and mass of the dust contributing to the microwave emission. We derive a dust mass of Md ~ 0.1 Msun, constrain the dimensions of the emitting region and find that the opacity index at λ > 7 mm is less than unity, implying a contribution to the dust population from grains exceeding ~4 cm in size. We suggest that this indicates coagulation within the GG Tau A system has proceeded to the point where dust grains have grown to the size of small rocks with dimensions of a few centimetres. Considering the relatively young age of the GG Tau association in combination with the low derived disc mass, we suggest that this system may provide a useful test case for rapid core accretion planet formation models
E-journals in a networked environment : its impact on academic libraries in the digital millennium
No full textThis paper tries to describe the Advantages of E-journals in terms of Accessibility, Speed distribution and production, Subscription Cost, Multimedia Capabilities, Internal and External Links etc and also tries to find out the Technological, Socio-cultural and Economic Barriers. The issues like Refereeing, Copyright and Licensing, Longevity and Storage and recent trends in E-journal publication and its implication on Academic Libraries in selection and acquisition, Cataloguing, archiving, user's access, training and support to staff and users are discussed in this paper
A new look at the pathogenesis of asthma
No full textAsthma is an inflammatory disorder of the conducting airways that has strong association with allergic sensitization. The disease is characterized by a polarized Th-2 (T-helper-2)-type T-cell response, but in general targeting this component of the disease with selective therapies has been disappointing and most therapy still relies on bronchodilators and corticosteroids rather than treating underlying disease mechanisms. With the disappointing outcomes of targeting individual Th-2 cytokines or manipulating T-cells, the time has come to re-evaluate the direction of research in this disease. A case is made that asthma has its origins in the airways themselves involving defective structural and functional behaviour of the epithelium in relation to environmental insults. Specifically, a defect in barrier function and an impaired innate immune response to viral infection may provide the substrate upon which allergic sensitization takes place. Once sensitized, the repeated allergen exposure will lead to disease persistence. These mechanisms could also be used to explain airway wall remodelling and the susceptibility of the asthmatic lung to exacerbations provoked by respiratory viruses, air pollution episodes and exposure to biologically active allergens. Variable activation of this epithelial-mesenchymal trophic unit could also lead to the emergence of different asthma phenotypes and a more targeted approach to the treatment of these. It also raises the possibility of developing treatments that increase the lung's resistance to the inhaled environment rather than concentrating all efforts on trying to suppress inflammation once it has become established.<br/
Self-archiving practice and the influence of publisher policies in the social sciences
No full textAuthors in different disciplines exhibit very different behaviours on the so-called ‘green’ road to open access, i.e. self-archiving. This study looks at the self-archiving behaviour of authors publishing in leading journals in six social science disciplines. It tests the hypothesis that authors are self-archiving according to the norms of their respective disciplines rather than following self-archiving policies of publishers, and that, as a result, they are self-archiving significant numbers of publisher PDF versions. It finds significant levels of
self-archiving, as well as significant self-archiving of
the publisher PDF version, in all the disciplines
investigated. Publishers’ self-archiving policies have
no influence on author self-archiving practice
Testing protoplanetary disc dispersal with radio emission
No full textWe consider continuum free–free radio emission from the upper atmosphere of protoplanetary discs as a probe of the ionized luminosity impinging upon the disc. Making use of previously computed hydrodynamic models of disc photoevaporation within the framework of extreme-ultraviolet (EUV) and X-ray irradiation, we use radiative transfer post-processing techniques to predict the expected free–free emission from protoplanetary discs. In general, the free–free luminosity scales roughly linearly with ionizing luminosity in both EUV- and X-ray-driven scenarios, where the emission dominates over the dust tail of the disc and is partial optically thin at cm wavelengths. We perform a test observation of GM Aur at 14–18?GHz and detect an excess of radio emission above the dust tail to a very high level of confidence. The observed flux density and spectral index are consistent with free–free emission from the ionized disc in either the EUV- or the X-ray-driven scenario. Finally, we suggest a possible route to testing the EUV- and X-ray-driven dispersal model of protoplanetary discs, by combining observed free–free flux densities with measurements of mass-accretion rates. On the point of disc dispersal one would expect to find an M?2? scaling with free–free flux in the case of EUV-driven disc dispersal or an ?* scaling in the case of X-ray-driven disc dispersa
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Nematic-isotropic phase transition in diblock fused-sphere chain fluids
No full textA density-functional theory for the isotropic-nematic phase transition in fluids of rigid or semiflexible fused hard-sphere chains, developed previously by the authors, is extended to diblock chains each consisting of both a rigid and a flexible part. The theory is compared with recent Monte Carlo simulation results of McBride The theoretical results for the variation of pressure and nematic order parameter with density agree well with the simulation data over density ranges where the simulations find isotropic and nematic phases.PT: J; CR: BOUBLIK T, 1974, MOL PHYS, V27, P1415 BOUBLIK T, 1975, J CHEM PHYS, V63, P4084 BOUBLIK T, 1981, MOL PHYS, V44, P1369 BOUBLIK T, 1989, MOL PHYS, V68, P191 COTTER MA, 1977, J CHEM PHYS, V66, P1098 COTTER MA, 1978, PHYS REV A, V18, P2669 FORSMAN J, 2003, J CHEM PHYS, V119, P1889 FYNEWEVER H, 1998, J CHEM PHYS, V108, P1636 HONNELL KG, 1989, J CHEM PHYS, V90, P1841 JAFFER KM, 1999, J CHEM PHYS, V110, P11630 JAFFER KM, 2001, J CHEM PHYS, V114, P3314 KHOKHLOV AR, 1981, PHYSICA A, V108, P546 KHOKHLOV AR, 1982, PHYSICA A, V112, P605 MCBRIDE C, 2001, PHYS REV E 1, V64 MCBRIDE C, 2002, J CHEM PHYS, V117, P10370 MEHTA SD, 1996, J PHYS CHEM-US, V100, P10408 MULLER M, 2003, J CHEM PHYS, V118, P2929 SATO T, 1996, ADV POLYM SCI, V126, P85 TIAN P, 2001, J CHEM PHYS, V115, P9055 VARGA S, 2000, MOL PHYS, V98, P693 VEGA C, 1994, J CHEM PHYS, V100, P6727 WERTHEIM MS, 1987, J CHEM PHYS, V87, P7323 WHITTLE M, 1991, MOL PHYS, V72, P247 WILLIAMSON DC, 1995, MOL PHYS, V86, P819 WILLIAMSON DC, 1998, J CHEM PHYS, V108, P10294 YETHIRAJ A, 1998, MOL PHYS, V93, P693 ZHOU YQ, 1995, J CHEM PHYS, V103, P2688; NR: 27; TC: 3; J9: PHYS REV E; PN: Part 1; PG: 4; GA: 835RHSource type: Electronic(1
A tetrahedron to model e-learning Mathematics
No full textIn this paper we introduce a model to 'read' the impact of e-learning technologies on the teaching/learning process, usually interpreted through the “didactical triangle” - knowledge, student, teacher (Chevallard, 1985). The idea we present here starts increasing the number of actors -that are author (A), student/s (S), tutor (T), mathematical knowledge (M) - and makes the model to become three-dimensional, passing from a triangle to a tetrahedron. The mediation of technology is represented by a sphere inscribed in the tetrahedron. The tangent point of the sphere with each face of the tetrahedron allows us to look at the relations among the three actors (the vertices of the face) in the e-learning environment and then how they can be transmitted /fostered/created by the new technological tools
Measurement of Collins asymmetries in inclusive production of charged pion pairs in e+e− annihilation at BABAR
No full textWe present measurements of Collins asymmetries in the inclusive process e+e−→ππX, where π stands for charged pions, at a center-of-mass energy of 10.6 GeV. We use a data sample of 468 fb−1 collected by the BABAR experiment at the PEP-II B factory at SLAC, and consider pairs of charged pions produced in opposite hemispheres of hadronic events. We observe clear asymmetries in the distributions of the azimuthal angles in two distinct reference frames. We study the dependence of the asymmetry on several kinematic variables, finding that it increases with increasing pion momentum and momentum transverse to the analysis axis, and with increasing angle between the thrust and beam axis
Fracture toughness determination by repetitive nano-impact testing in Cu/W nanomultilayers with length-scale-dependent films properties
No full textAbstractNanoscale metallic multilayers based on Cu/W have been considered as a potential material for structural applications in nuclear reactors and for the cladding of storage tanks for advanced fuels kept at high temperatures. The understanding of how mechanical properties change in relation to periodicity, λ, is required in order to use Cu/W nano-multilayers as a protective coating against radiation damage. The aim of this work is to demonstrate the feasibility of using the repetitive-nano-impact technique to obtain quantitative fracture toughness, KC, values in nano-multilayers and assess its variation as a function of λ
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