1,783 research outputs found
Instabilities in the Gamma Ray Burst central engine. What makes the jet variable?
Both types of long and short gamma ray bursts involve a stage of a
hyper-Eddington accretion of hot and dense plasma torus onto a newly born black
hole. The prompt gamma ray emission originates in jets at some distance from
this 'central engine' and in most events is rapidly variable, having a form of
spikes and subpulses. This indicates at the variable nature of the engine
itself, for which a plausible mechanism is an internal instability in the
accreting flow. We solve numerically the structure and evolution of the
neutrino-cooled torus. We take into account the detailed treatment of the
microphysics in the nuclear equation of state that includes the neutrino
trapping effect. The models are calculated for both Schwarzschild and Kerr
black holes. We find that for sufficiently large accretion rates (> 10 Msun/s
for non-rotating black hole, and >1 Msun/s for rotating black hole, depending
on its spin), the inner regions of the disk become opaque, while the helium
nuclei are being photodissociated. The sudden change of pressure in this region
leads to the development of a viscous and thermal instability, and the neutrino
pressure acts similarly to the radiation pressure in sub-Eddington disks. In
the case of rapidly rotating black holes, the instability is enhanced and
appears for much lower accretion rates. We also find the important and possibly
further destabilizing role of the energy transfer from the rotating black hole
to the torus via the magnetic coupling.Comment: 4 pages, 4 figures; Proceedings of the 275 IAU Symposium "Jets at all
scales", Buenos Aires, 13-17.09.2010; eds. G. Romero, R. Sunyaev, T. Bellon
Performance Assessment of Different Roof Integrated Photovoltaic Modules under Mediterranean Climate
AbstractMany countries, for aesthetic purposes, offer economic advantages (tax deductions, incentives, etc..) for the installation of building integrated photovoltaic modules (BIPV), with water-tightness capability and adequate mechanical resistance in order to substitute tile covering or part of it. Nevertheless, poor or absent ventilation under BIPV panels could cause them to overheat and reduce their efficiency.It is well established that the presence of an air gap between a photovoltaic (PV) module and roof covering facilitates ventilation cooling under the device and consequently reduces cell temperature and improves its performance.In this study, we investigated the thermal performance of PV modules installed in a real scale experimental building over a traditional clay tile pitched roof in Italy for almost one year (from August 2009 to June 2010). One PV module was rack-mounted over the roof covering with a 0.2 m air gap; the others were fully integrated and installed at the same level of the roof covering (one with an air gap of 0.04 m, the other mounted directly in contact with the insulation). Temperature and heat flux measurements for each panel, and environmental parameters were recorded.Experimental results demonstrate that even though the rack-mounted PV module constantly maintains cell temperature below that of the other full-building integrated modules, due to the presence of a higher air gap, the difference in the energy produced by the PV modules estimated for the entire monitoring period is less than 4%
Characterizing the time variability in magnetized neutrino--cooled accretion disks: signatures of the gamma-ray burst central engine
The central engine of Gamma Ray Bursts is hidden from direct probing with
photons mainly due to the high densities involved. Inferences on their
properties are thus made from their cosmological setting, energetics,
low-energy counterparts and variability. If GRBs are powered by hypercritical
accretion onto compact objects, on small spatial scales the flow will exhibit
fluctuations, which could in principle be reflected in the power output of the
central engine and ultimately in the high energy prompt emission. Here we
address this issue by characterizing the variability in neutrino cooled
accretion flows through local shearing box simulations with magnetic fields,
and then convolving them on a global scale with large scale dynamical
simulations of accretion disks. The resulting signature is characteristic, and
sensitive to the details of the cooling mechanism, providing in principle a
discriminant for GRB central engine properties.Comment: Accepted for publication in ApJ Letter
PLEISTOCENE BATHYAL MOLLUSCAN ASSEMBLAGES FROM SOUTHERN ITALY
Four Pleistocene bathyal molluscan assemblages from southern Italy (Calabria and Messina area) were studied. One hundred and thirty-six species were recorded. Twenty-four were classified and described in detail and thirty-five were illustrated. The following new combinations are pro posed: Solariella marginulata (Philippi, 1844), Iphitus tenuisculptus (Seguenza, 1876), Benthomangelia tenuicostata (Seguenza, 1879), Chrysallida microscalaria (Seguenza, 1876), Ennucula corbuloides (Seguenza, 1877), Ennucula rotundata (Seguenza, 1877), Thestyleda cuspidata (Philippi, 1844), Katadesmia confusa (Seguenza, 1877), Austrotindaria pusio (Philippi, 1844), Austrotindaria salicensis (Seguenza, 1877). Comments concerning the taxonomy of Fissurisepta Seguenza, 1862, Solariella Wood, 1842, Ennucula Iredale, 1931, Thestyleda Iredale, 1929, Ledella Verrill & Bush, 1897, Yoldiella Verrill & Bush, 1897, Bathyspinula Filatova, 1958, Katadesmia Dall, 1908, Austrotindaria Fleming, 1948 and Cadulus Philippi, 1844 are included. The assemblages are dominated by nuculoids and fit the general compositional pattern of the deep-sea molluscan communities. A paleodepth of 500-600 m is inferred for two assemblages, whereas a greater depth, pro bably not exceeding 1,000 m, is suggested for the other two. Taxonomic affinities with northeast Atlantic and more generally with World Ocean deep-sea molluscan faunas are remarkable. The Plio-Quaternary evolution of the deep Mediterranean benthos is discussed.  
Traditional Houses with Stone Walls in Temperate Climates: The Impact of Various Insulation Strategies
The present chapter focuses on the yearly behaviour of a traditional farmhouse in a
temperate Italian climate and analyses the impact of alternative energy saving strategies on
summer comfort and winter consumptions.
The aim of the current study was to:
- identify combinations of thermal insulation interventions which: optimise winter
energy saving and summer internal comfort without modifying the close relationship
between architecture and specific climate typical of traditional buildings; respect the
building material consistency and the façades aesthetic appearance.
- compare the performance of this traditional architecture (after the retrofit intervention)
with that of a modern building, of the type encouraged by new energy saving
legislation (lightweight and super-insulated).
To that aim a series of monitoring activities in summer and in winter were carried out to
investigate the internal environmental conditions and to calibrate a simulation model with
the software Energyplus. This model was used to assess the impact of various energy-saving
strategies on winter energy consumptions and summer comfort with the method of
Percentage outside the comfort range (EN 15251:2007-08)
Therapeutic Strategies in Pulmonary Hypertension
Pulmonary hypertension (PH) is a life-threatening condition characterized by elevated pulmonary arterial pressure. It is clinically classified into five groups: patients in the first group are considered to have pulmonary arterial hypertension (PAH) whereas patients of the other groups have PH that is due to cardiopulmonary or other systemic diseases. The management of patients with PH has advanced rapidly over the last decade and the introduction of specific treatments especially for PAH has lead to an improved outcome. However, despite the progress in the treatment, the functional limitation and the survival of these patients remain unsatisfactory and there is no cure for PAH. Therefore the search for an “ideal” therapy still goes on. At present, two levels of treatment can be identified: primary and specific therapy. Primary therapy is directed at the underlying cause of the PH. It also includes a supportive therapy consisting in oxygen supplementation, diuretics, and anticoagulation which should be considered in all patients with PH. Specific therapy is directed at the PH itself and includes treatment with vasodilatators such as calcium channel blockers and with vasodilatator and pathogenetic drugs such as prostanoids, endothelin receptor antagonists and phosphodiesterase type-5 inhibitors. These drugs act in several pathogenetic mechanisms of the PH and are specific for PAH although they might be used also in the other groups of PH. Finally, atrial septostomy and lung transplantation are reserved for patients refractory to medical therapy. Different therapeutic approaches can be considered in the management of patients with PH. Therapy can be established on the basis of both the clinical classification and the functional class. It is also possible to adopt a goal-oriented therapy in which the timing of treatment escalation is determined by inadequate response to known prognostic indicators
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