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Search Results: 1 - 10 of 8724 matches for " Stefano Guido "
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AGN Obscuration and the Unified Model
Stefano Bianchi,Roberto Maiolino,Guido Risaliti
Advances in Astronomy , 2012, DOI: 10.1155/2012/782030
Abstract: Unification Models of Active Galactic Nuclei postulate that all the observed differences between type 1 and type 2 objects are due to orientation effects with respect to the line of sight to the observer. The key ingredient of these models is the obscuring medium, historically envisaged as a toroidal structure on a parsec scale. However, many results obtained in the last few years are clearly showing the need for a more complex geometrical distribution of the absorbing media. In this paper, we review the various pieces of evidence for obscuring media on different scales, from the vicinity of the black hole to the host galaxy, in order to picture an updated unification scenario explaining the complex observed phenomenology. We conclude by mentioning some of the open issues. 1. Introduction: The Standard Unified Model In this paper, we discuss the recent developments on the AGN unified models, specifically for what concern the geometry, location, and physics of the absorbing medium. Before discussing the more recent results, in this section we shortly review the early, classical arguments that historically led to the formulation of the standard Unified Model. An early review of the initial results was also given in Antonucci [1]. The first unification attempts have been focussed on polarization measurements. In particular Antonucci [2] found a perpendicular alignment of optical polarization relative to the radio axis in a sample of radio galaxies, which was interpreted as due to scattering of photons, whose direction before entering the line of sight was primarily in the vertical direction. Shortly after, additional evidence was found in low luminosity, local AGN, and specifically Seyfert galaxies [3]. Seyfert 1 galaxies are characterized by the presence of broad optical permitted lines ( ?km/s), such as H and H , that are not observed in Seyfert 2 galaxies. However, the presence of both strong high ionization and low ionization narrow ( ?km/s) forbidden lines (such as [O?III], [Ne?III], [O?II], [O?I], [N?II], [S?II]), and several very high ionization coronal lines (such as [Fe?X], [Fe?XI], [Si?IX], [Si?X]) is common to both types of Seyfert galaxies and with similar line ratios. The latter finding suggested that all Seyfert galaxies are powered by the same intrinsic engine. A strong observational evidence of a unification between type 2 and type 1 Seyfert nuclei has been the discovery of broad optical lines in the polarized spectrum of the archetypal Seyfert 2, NGC 1068, obtained by Antonucci and Miller [4]. This finding revealed the presence of a Broad
Techno-economic evaluation of 2nd generation bioethanol production from sugar cane bagasse and leaves integrated with the sugar-based ethanol process
Stefano Macrelli, Johan Mogensen, Guido Zacchi
Biotechnology for Biofuels , 2012, DOI: 10.1186/1754-6834-5-22
Abstract: The addition of a steam dryer, doubling of the enzyme dosage in enzymatic hydrolysis, including leaves as raw material in the 2G process, heat integration and the use of more energy-efficient equipment led to a 37% reduction in MESP-2G compared to the Base case. Modelling showed that the MESP for 2G ethanol was 0.97 US$/L, while in the future it could be reduced to 0.78 US$/L. In this case the overall production cost of 1G + 2G ethanol would be about 0.40 US$/L with an output of 102 L/ton dry sugar cane including 50% leaves. Sensitivity analysis of the future scenario showed that a 50% decrease in the cost of enzymes, electricity or leaves would lower the MESP-2G by about 20%, 10% and 4.5%, respectively.According to the simulations, the production of 2G bioethanol from sugar cane bagasse and leaves in Brazil is already competitive (without subsidies) with 1G starch-based bioethanol production in Europe. Moreover 2G bioethanol could be produced at a lower cost if subsidies were used to compensate for the opportunity cost from the sale of excess electricity and if the cost of enzymes continues to fall.Currently in Brazil, the production of sugar cane bioethanol is based entirely on the fermentation of sugar juice from sugar cane and/or molasses in autonomous distilleries (39% of the cases) and in plants associated with sugar mills (61%) [1]. This technology has been in commercial use for the past 30 years and can be considered to be mature as the cost of feedstock accounts for a major part of the production cost [2], around 60-70% [3,4]. Compared to other crops used for bioethanol production from sugar and starch, i.e. first-generation (1G) bioethanol, bioethanol from sugar cane is claimed to have the lowest production cost worldwide [5]. The low cost of Brazilian 1G bioethanol can be explained by a combination of favourable conditions such as the photosynthetic rate of the sugar cane crop per hectare, the meteo-climatic conditions and a renewable energy ratio close t
Similar folds with different stabilization mechanisms: the cases of prion and doppel proteins
Stefano Colacino, Guido Tiana, Giorgio Colombo
BMC Structural Biology , 2006, DOI: 10.1186/1472-6807-6-17
Abstract: In this paper, the dynamical and energetic properties of the two proteins in solution is comparatively analyzed by means of long time scale explicit solvent, all-atom molecular dynamics in different temperature conditions. The trajectories are analyzed by means of a recently introduced energy decomposition approach (Tiana et al, Prot. Sci. 2004) aimed at identifying the key residues for the stabilization and folding of the protein. Our analysis shows that Prion and Doppel have two different cores stabilizing the native state and that the relative contribution of the nucleus to the global stability of the protein for Doppel is sensitively higher than for PrP. Moreover, under misfolding conditions the Doppel core is conserved, while the energy stabilization network of PrP is disrupted.These observations suggest that different sequences can share similar native topology with different stabilizing interactions and that the sequences of the Prion and Doppel proteins may have diverged under different evolutionary constraints resulting in different folding and stabilization mechanisms.The molecular determinants of neurodegenerative diseases have been the subject of very intense research over recent years [1,2]. Particular attention in this field has been devoted to Prion proteins (PrP) due to their fundamental role as infective agents in diseases generally known as Transmissible Spongiform Encephalopathies (TSE) that affect humans and animals, including Creuzfeld-Jakob disease, fatal familial insomnia and Gerstmann-Straussler-Scheinker disease in humans, scrapie in sheep and mad-cow disease in cattle. The distinctive trait of Prion-related diseases is that PrP proteins seem to act as the only infectious agents, with no intervention of genetic material, by causing self-propagating conformational changes [3-5]. Experimental evidences unveiled that in all these cases the normal and benign form of the Prion Protein (PrPC) can undergo a conformational change of the native state
Benthic macroinvertebrate communities as indicators of river environment quality: an experience in Nicaragua
Fenoglio,Stefano; Badino,Guido; Bona,Francesca;
Revista de Biología Tropical , 2002,
Abstract: biotic indexes are one of severas types of measures that are routinely used in biological monitoring in temperate streams and offer interesting possibilities to assess the environmental quality of rivers in the neotropics. macroinvertebrate communities of seven southeastern nicaraguan streams were monitored and seven ecological indexes were applied. the results suggest that information from the indice biótico esteso (i.b.e.) is closely correlated to the results of other methods, but the i.b.e. index is easier to apply as well as avoiding high costs and time-consuming procedures. a calibration of the method is necessary for rapid assessment approaches in the neotropics.
Benthic macroinvertebrate communities as indicators of river environment quality: an experience in Nicaragua
Stefano Fenoglio,Guido Badino,Francesca Bona
Revista de Biología Tropical , 2002,
Abstract: Biotic indexes are one of severas types of measures that are routinely used in biological monitoring in temperate streams and offer interesting possibilities to assess the environmental quality of rivers in the neotropics. Macroinvertebrate communities of seven southeastern Nicaraguan streams were monitored and seven ecological indexes were applied. The results suggest that information from the Indice Biótico Esteso (I.B.E.) is closely correlated to the results of other methods, but the I.B.E. index is easier to apply as well as avoiding high costs and time-consuming procedures. A calibration of the method is necessary for rapid assessment approaches in the neotropics. Los índices bióticos representan una topología de medidas que se emplea rutinariamente en el control biológico de los ambientes lóticos templados: ellos ofrecen también interesantes posibilidades en el área neotropical. Para evaluar la calidad ambiental de algunos ríos de Nicaragua, investigamos las comunidades macrobentónicas y aplicamos siete índices ecológicos. Los resultados sugieren que el índice I.B.E. se relaciona fuertemente a los otros índices, pero permite ahorrar costos y tiempo. Una calibración del método sería necesaria para obtener un método de valoración rápida de la calidad ambiental de los ríos de Centro América.
Are Parton Distributions Positive?
Stefano Forte,Guido Altarelli,Giovanni Ridolfi
Physics , 1998, DOI: 10.1016/S0920-5632(99)00150-4
Abstract: We show that the naive positivity conditions on polarized parton distributions which follow from their probabilistic interpretation in the naive parton model are reproduced in perturbative QCD at the leading log level if the quark and gluon distribution are defined in terms of physical processes. We show how these conditions are modified at the next-to-leading level, and discuss their phenomenological implications, in particular in view of the determination of the polarized gluon distribution
Singlet parton evolution at small x: a theoretical update
Guido Altarelli,Richard D. Ball,Stefano Forte
Physics , 2000,
Abstract: This is an extended and pedagogically oriented version of our recent work, in which we proposed an improvement of the splitting functions at small x which overcomes the apparent problems encountered by the BFKL approach.
Small-x Resummation and HERA Structure Function Data
Guido Altarelli,Richard D. Ball,Stefano Forte
Physics , 2000, DOI: 10.1016/S0550-3213(01)00023-2
Abstract: We apply our systematic NLO small x resummation of singlet splitting functions to the scaling violations of structure functions and compare the results with data. We develop various theoretical tools which are needed in order to relate resummed parton distributions to measurable structure functions, and we present results from a variety of fits to HERA data for the structure functions F_2 and F_L using the resummation. The behaviour of the singlet splitting functions at small x and fixed Q^2 is effectively parametrized as x^{-lambda}. We find that, for lambda small or negative, the resummed description of scaling violations may be phenomenologically as good as or even better than the standard next-to-leading order treatment. However, the best fit gluon density and value of alpha_s can be significantly modified by the resummation.
On positivity of parton distributions
Guido Altarelli,Stefano Forte,Giovanni Ridolfi
Physics , 1998, DOI: 10.1016/S0550-3213(98)00661-0
Abstract: We discuss the bounds on polarized parton distributions which follow from their definition in terms of cross section asymmetries. We spell out how the bounds obtained in the naive parton model can be derived within perturbative QCD at leading order when all quark and gluon distributions are defined in terms of suitable physical processes. We specify a convenient physical definition for the polarized and unpolarized gluon distributions in terms of Higgs production from gluon fusion. We show that these bounds are modified by subleading corrections, and we determine them up to NLO. We examine the ensuing phenomenological implications, in particular in view of the determination of the polarized gluon distribution.
Factorization and Resummation of Small x Scaling Violations with Running Coupling
Guido Altarelli,Richard D. Ball,Stefano Forte
Physics , 2001, DOI: 10.1016/S0550-3213(01)00563-6
Abstract: We discuss the inclusion of running coupling effects in perturbative small x evolution equations. We show that a running coupling BFKL-like x-evolution equation is fully compatible, up to higher twist corrections, with the standard factorized perturbative evolution of parton distributions. We then use this result, combined with the well-known Airy asymptotics, to prove that the oscillations which are present in the running-coupling BFKL solution do not affect the associated splitting functions, which instead remain smooth in the small x limit. This allows us to give a prescription to include running-coupling corrections in the small-x resummation of scaling violations. We show that these corrections are small in the HERA region.
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