The role of the equation of state in models of very low-mass stars     

Marcella Di Criscienzo  Paolo Ventura  Francesca D’Antona 《Astrophysics and Space Science》2010,328(1-2):167-169

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Restrictions on the short time constant astrophysics     

G. Cavallo  A. Ventura 《Astrophysics and Space Science》1972,19(2):431-439

The comparatively new field of short time-constant astrophysics is investigated with the aim of checking the statement that there is a maximum amount of electromagnetic energy which can be radiated at a frequencev _m by an object whose size isL and during a time Δτ. Such limits are found under the assumptions that the emission is isotropic, thatL?cΔτ, and for the following four radiation mechanisms: incoherent synchrotron radiation, synchrotron maser, antenna radiation and for one simple case of radiation by a turbulent plasma. These limits are compared and found to be consistent with experimental data referring to the Sun, the pulsar NP 0532, the Quasar 3C 273 and the Seyfert galaxy NGC 4151. The main conclusion is that extragalactic radio astronomy at the present sensitivity level of observations is not likely to allow for the detection of pulses lasting less than 0.1 s.  相似文献   

Quantum theory of the dielectric constant of a magnetized plasma and astrophysical applications     

V. Canuto  J. Ventura 《Astrophysics and Space Science》1972,18(1):104-120

A quantum mechanical treatment of an electron plasma in a constant and homogeneous magnetic field is considered, with the aim of (a) defining the range of validity of the magnetoionic theory (b) studying the deviations from this theory, in applications involving high densities, and intense magnetic field. While treating the magnetic field exactly, a perturbation approach in the photon field is used to derive general expressions for the dielectric tensor . The properties of are explored in the various limits. Numerical estimates on the range of applicability of the magnetoionic theory are given for the case of the one-dimensional electron gas, where only the lowest Landau level is occupied.  相似文献   

Diffuse CO<Subscript>2</Subscript> degassing at Vesuvio,Italy   总被引：1，自引：0，他引：1  

Francesco?FrondiniEmail author  Giovanni?Chiodini  Stefano?Caliro  Carlo?Cardellini  Domenico?Granieri  Guido?Ventura 《Bulletin of Volcanology》2004,66(7):642-651

At Vesuvio, a significant fraction of the rising hydrothermal–volcanic fluids is subjected to a condensation and separation process producing a CO₂–rich gas phase, mainly expulsed through soil diffuse degassing from well defined areas called diffuse degassing structures (DDS), and a liquid phase that flows towards the outer part of the volcanic cone. A large amount of thermal energy is associated with the steam condensation process and subsequent cooling of the liquid phase. The total amount of volcanic–hydrothermal CO₂ discharged through diffuse degassing has been computed through a sequential Gaussian simulation (sGs) approach based on several hundred accumulation chamber measurements and, at the time of the survey, amounted to 151 t d^–1. The steam associated with the CO₂ output, computed assuming that the original H₂O/CO₂ ratio of hydrothermal fluids is preserved in fumarolic effluents, is 553 t d^–1, and the energy produced by the steam condensation and cooling of the liquid phase is 1.47×10¹² J d^–1 (17 MW). The location of the CO₂ and temperature anomalies show that most of the gas is discharged from the inner part of the crater and suggests that crater morphology and local stratigraphy exert strong control on CO₂ degassing and subsurface steam condensation. The amounts of gas and energy released by Vesuvio are comparable to those released by other volcanic degassing areas of the world and their estimates, through periodic surveys of soil CO₂ flux, can constitute a useful tool to monitor volcanic activity.Editorial responsibility: H. Shinohara  相似文献   

Synthesis, crystal structure and crystal chemistry of ferri-clinoholmquistite, ?Li ₂Mg ₃Fe ³⁺ ₂Si ₈O ₂₂(OH) ₂     

G. Iezzi  F. Cámara  R. Oberti  G. Della Ventura  G. Pedrazzi  J-L Robert 《Physics and Chemistry of Minerals》2004,31(6):375-385

This work reports the synthesis of ferri-clinoholmquistite, nominally _Li2(Mg₃Fe³⁺₂)Si₈O₂₂(OH)₂, at varying f_O2 conditions. Amphibole compositions were characterized by X-ray (powder and single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Mössbauer and Raman) techniques. Under reducing conditions ( NNO+1, where NNO = Nickel–Nickel oxide buffer), the amphibole yield is very high (>90%), but its composition, and in particular the FeO/Fe₂O₃ ratio, departs significantly from the nominal one. Under oxidizing conditions ( NNO+1.5), the amphibole yield is much lower (<60%, with Li-pyroxene abundant), but its composition is close to the ideal stoichiometry. The exchange vector of relevance for the studied system is ^M2(Mg,Fe²⁺) ^M4(Mg,Fe²⁺) ^M2Fe³⁺_–1 ^M4Li_–1, which is still rather unexplored in natural systems. Amphibole crystals of suitable size for structure refinement were obtained only at 800 °C, 0.4 GPa and NNO conditions (sample 152), and have C2/m symmetry. The X-ray powder patterns for all other samples were indexed in the same symmetry; the amphibole closest to ideal composition has a = 9.428(1) Å, b = 17.878(3) Å, c = 5.282(1) Å, = 102.06(2)°, V = 870.8(3) ų. Mössbauer spectra show that Fe³⁺ is strongly ordered at M2 in all samples, whereas Fe²⁺ is disordered over the B and C sites. FTIR analysis shows that the amount of ^CFe²⁺ increases for increasingly reducing conditions. FTIR data also provide strong evidence for slight but significant amounts of Li at the A sites.  相似文献   

Analysis of climatic trends in data from the agrometeorological station of Bologna-Cadriano, Italy (1952–2007)     

Philipp Matzneller  Francesca Ventura  Nicola Gaspari  Paola Rossi Pisa 《Climatic change》2010,100(3-4):717-731

Agriculture is highly exposed to climate change, as farming activities directly depend on climatic conditions. Knowledge of the extent of such change and of related phenomena will help to answer the questions posed by society about adaptation strategies. The global situation is well described by the Fourth IPCC assessment report (IPCC 2007), but local studies are important to understand the impact and the priorities to adopt in adaptation strategies. In this study a historical set of meteorological data, collected during the period 1952–2007 at the University of Bologna (Italy) agrometeorological station, was analysed. Several indexes, such as Frost Severity Index, number of hot days, number of rainy days, etc., were calculated, and their trends in time were analysed. The results show a scenario of increasing temperatures and evapotranspiration, a decrease in rainy days and a deepening of the watertable. The effect of these changes on agriculture will be a decrease in water availability, an increase in heat stress in plants and an increase in drought risk.  相似文献   

Star formation in the starburst cluster in NGC 3603     

Matteo Correnti  Francesco Paresce  Rossella Aversa  Giacomo Beccari  Guido De Marchi  Marcella Di Criscienzo  Xiaoying Pang  Loredana Spezzi  Elena Valenti  Paolo Ventura 《Astrophysics and Space Science》2012,340(2):263-279

We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (∼1–3 M_⊙) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ∼2 to ∼30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20–30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East–North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction.  相似文献   

First results from a dark matter search with liquid argon at 87 K in the Gran Sasso underground laboratory     

P. Benetti  R. Acciarri  F. Adamo  B. Baibussinov  M. Baldo-Ceolin  M. Belluco  F. Calaprice  E. Calligarich  M. Cambiaghi  F. Carbonara  F. Cavanna  S. Centro  A.G. Cocco  F. Di Pompeo  N. Ferrari  G. Fiorillo  C. Galbiati  V. Gallo  L. Grandi  A. Ianni  G. Mangano  G. Meng  C. Montanari  O. Palamara  L. Pandola  F. Pietropaolo  G.L. Raselli  M. Rossella  C. Rubbia  A.M. Szelc  S. Ventura  C. Vignoli  WARP Collaboration 《Astroparticle Physics》2008,28(6):495-507

A new method of searching for dark matter in the form of weakly interacting massive particles (WIMP) has been developed with the direct detection of the low energy nuclear recoils observed in a massive target (ultimately many tons) of ultra pure liquid argon at 87 K. A high selectivity for argon recoils is achieved by the simultaneous observation of both the VUV scintillation luminescence and of the electron signal surviving columnar recombination, extracted through the liquid–gas boundary by an electric field.

First physics results from this method are reported, based on a small 2.3 l test chamber filled with natural argon and an accumulated fiducial exposure of about 100 kg day, supporting the future validity of this method with isotopically purified ⁴⁰Ar and for a much larger unit presently under construction with correspondingly increased sensitivities.  相似文献   

Changes in the rainfall–streamflow regimes related to climate change in a small catchment in Northern Italy     

Pieri  Linda  Rondini  Davide  Ventura  Francesca 《Theoretical and Applied Climatology》2017,129(3-4):1075-1087

Theoretical and Applied Climatology - Climate change has become an important issue for scientific community, for its numerous impacts, especially on agriculture and environment. To shed light on...  相似文献   

A note on maar eruption energetics: current models and their application   总被引：1，自引：1，他引：0  

Jacopo Taddeucci  Gianluca Sottili  Danilo M. Palladino  Guido Ventura  Piergiorgio Scarlato 《Bulletin of Volcanology》2010,72(1):75-83

Hydromagmatic eruptions convert thermal into mechanical energy via the expansion of ground- and/or surface-water. Several models address the energetics of these eruptions based on different physical-volcanological approaches. Here we test different models with two case studies in the Colli Albani Volcanic District (central Italy): the monogenetic Prata Porci and the polygenetic Albano maars. Test results are mutually consistent, and show cumulative mechanical energy releases on the order of 10¹⁵–10¹⁷ J for single maars. The fraction of thermal energy turned into mechanical ranges from 0.45 (as calculated from the theoretical maximum mechanical energy), through 0.1 (calculated from country rock fragmentation, crater formation and ballistic ejection), to 0.03 (derived from magma fragmentation by thermohydraulic explosions). It appears that the energy released during the most intense hydromagmatic events may account for a dominant fraction of the total mechanical energy released during the whole maar eruptive histories. Finally, we consider the role of magmatic explosive activity intervening during maar eruptions in causing departures from predictions of the models evaluated.  相似文献