A new model to simulate impact breakup     

Alessandro Cordelli  Paolo Farinella 《Planetary and Space Science》1997,45(12):1639-1647

We have developed a preliminary version of a new type of code to simulate the outcomes of impacts between solid bodies, which we plan to further refine for application to both asteroid science and space debris studies. In the current code, colliding objects are modeled as two-dimensional arrays of finite elements, which can interact with each other in both an elastic and a shock-wave regime. The finite elements are hard spheres with a given value for mass and radius. When two of them come into contact the laws of inelastic scattering are applied, thus giving rise to the propagation of shock waves. Moreover each spherical element interacts elastically with its nearest neighbours. The interaction force corresponds to that of a spring having an equilibrium length equal to the lattice spacing, and results into the propagation of elastic waves in the lattice. Dissipation effects are modeled by means of a dissipative force term proportional to the relative velocity, with a given characteristic time of decay. The possible occurrence of fractures in the material is modeled by assuming that when the distance of two neighbouring elements exceeds a threshold value, the binding force between them disappears for ever. This model requires finding a plausible correspondence between the input parameters appearing in the equations of motion, and the physical properties of real solid materials. Some of the required links are quite obvious (e.g., the relationship between mass of the elements and elastic constant on one side, and material density and sound velocity on the other side), some others a priori are unclear, and additional hypotheses on them must be made (e.g., on the restitution coefficient of inelastic scattering). Despite the preliminary character of the model, we have obtained some interesting results, which appear to mimic in a realistic way the outcomes of actual impacts. For instance, we have observed the formation of craters and fractures, and (for high impact energies) the occurrence of catastrophic breakup. The masses and velocities of the fragments resemble those found in laboratory impact experiments.  相似文献   

Propagation of vortices within a rotating,fluid shell     

Paolo Lanzano 《Earth, Moon, and Planets》1992,58(1):39-55

We consider a spherical, solid planet surrounded by a thin layer of an incompressible, inviscid fluid. The planet rotates with constant angular velocityWe study the vortex motion within this rotating ocean. For this purpose, we obtain a linearized version of the Navier-Stokes equation and adopt it as our ocean model; next, we prove analytically that a certain function of vorticity is an invariant of motion.Using this ocean model and this invariant property of vorticity, we are able to establish a general equation governing the motion of vortices within a fluid shell: it is a nonlinear partial differential equation of the third order for the stream function of motion.We finally examine some particular solutions of this vorticity equation that represent solitary waves of permanent form and decay within a finite distance. These solutions have been represented in terms of quadratic, exponential, and hyperbolic functions.The question whether these vortices that propagate as solitary waves could be solitons depends on their behavior when they collide with each other; this has not yet been resolved.Retired, U.S. Naval Research Laboratory, Washington, D.C., U.S.A.  相似文献   

Pollen Viability for Air Pollution Bio-Monitoring     

Elena Gottardini  Fabiana Cristofolini  Elena Paoletti  Paolo Lazzeri  Giancarlo Pepponi 《Journal of Atmospheric Chemistry》2004,49(1-3):149-159

Biological characterization of Corylus avellana L. and Pinus nigra L. pollen samples was carried out to determine the actual value of pollen as a bio-indicator of the effects of atmospheric pollution, using samples from plants naturally developed in sites controlled for air pollution. In Trentino (North Italy), we selected six stations at three different levels of air pollution, which are constantly monitored with automatic gauges by the Environmental Protection Agency of Trento.First results showed that pollen viability of both species, germinability and pollen tube length of P. nigra, were higher in areas with no road traffic compared to heavy traffic ones. Pollen viability of P. nigra was positively correlated to ozone (O₃) concentrations and altitude but negatively to sulphur dioxide (SO₂), particulate matter with a diameter less than 10 m (PM₁₀), nitrogen oxides (NO_x) and nitrogen dioxide (NO₂) concentrations.  相似文献   

Defining a model of 3D seismogenic sources for Seismic Hazard Assessment applications: The case of central Apennines (Italy)   总被引：6，自引：0，他引：6  

Paolo Boncio  Giusy Lavecchia  Bruno Pace 《Journal of Seismology》2004,8(3):407-425

Geology-based methods for Probabilistic Seismic Hazard Assessment (PSHA) have been developing in Italy. These methods require information on the geometric, kinematic and energetic parameters of the major seismogenic faults. In this paper, we define a model of 3D seismogenic sources in the central Apennines of Italy. Our approach is mainly structural-seismotectonic: we integrate surface geology data (trace of active faults, i.e. 2D features) with seismicity and subsurface geological–geophysical data (3D approach). A fundamental step is to fix constraints on the thickness of the seismogenic layer and deep geometry of faults: we use constraints from the depth distribution of aftershock zones and background seismicity; we also use information on the structural style of the extensional deformation at crustal scale (mainly from seismic reflection data), as well as on the strength and behaviour (brittle versus plastic) of the crust by rheological profiling. Geological observations allow us to define a segmentation model consisting of major fault structures separated by first-order (kilometric scale) structural-geometric complexities considered as likely barriers to the propagation of major earthquake ruptures. Once defined the 3D fault features and the segmentation model, the step onward is the computation of the maximum magnitude of the expected earthquake (M _max). We compare three different estimates of M _max: (1) from association of past earthquakes to faults; (2) from 3D fault geometry and (3) from geometrical estimate corrected by earthquake scaling laws. By integrating all the data, we define a model of seismogenic sources (seismogenic boxes), which can be directly used for regional-scale PSHA. Preliminary applications of PSHA indicate that the 3D approach may allow to hazard scenarios more realistic than those previously proposed.  相似文献   

Solitary waves of vortices     

Paolo Lanzano 《Earth, Moon, and Planets》1995,69(3):271-283

We study the propagation of solitary waves of vortices within a spherical shell which constitutes the uppermost layer of a solid planet. This solid-liquid configuration rotates with constant angular velocity about an axis which is fixed with respect to the solid surface. The fluid within the shell is inviscid, incompressible, and of constant density. The motion imparted by the planetary rotation upon this fluid mass is governed by the Laplace tidal equation from which the potential of the extraplanetary forces has been deleted. Consistent with this ocean model, we establish that the stream function of a solitary wave of vortices must satisfy a third-order partial differential equation. We obtain solutions to this wave equation by imposing the condition that the vertical component of vorticity be functionally related to the stream function. We find that this dependence must necessarily be of the exponential type and that the solution to the wave equation then reduces to a quadrature depending on some arbitrary parameters. We prove that we can always choose the values of these parameters in order to approximate the integral in question by means of an analytic function: we reach a representation of the stream function of a solitary wave of vortices in terms of hyperbolic functions of time and position.This paper is dedicated to the memory of Professor Zdenek Kopal.  相似文献   

The dangerous border of the 5:2 mean motion resonance     

Zoran Kne?evi?  Andrea Milani  Paolo Farinella 《Planetary and Space Science》1997,45(12):1581-1585

After discovering that asteroid (2953) Vysheslavia, a Koronis family member, has a typical dynamical lifetime of the order of 10 Myr only against “falling” into the 5:2 Kirkwood gap and ending up in a hyperbolic orbit (Milani and Farinella, 1995. Icarus 114, 209–212), we decided to repeat the same study for two other asteroids, 1991 UA₂ and 1993 FR₅₈, lying also very close to the outer edge of the same resonance. The orbital elements of these bodies were not yet accurate enough and we appealed to observers, both professional and amateur, to obtain more astrometric data. Eight groups in three different countries carried out observations, and as a result the Minor Planet Center could derive updated, accurate orbits for both objects. Here we report on the preliminary results of our study. Whereas the long-term behavior of 1991 UA₂ looks fairly regular and stable, 1993 FR₅₈ exhibits a typical “stable chaos” behavior; nevertheless, it does not end up in the resonance within a time span of 50 Myr. On the other hand, its fictitious clones, placed just 3–4 × 10⁻³ AU closer to the resonance, undergo large irregular semimajor axis changes, fall into the resonance and escape on a timescale <1 Myr.  相似文献   

Intrinsic and extrinsic galaxy alignment     

Paolo Catelan  Marc Kamionkowski  Roger D. Blandford 《Monthly notices of the Royal Astronomical Society》2001,320(1):L7-L13

We show with analytic models that the assumption of uncorrelated intrinsic ellipticities of target sources that is usually made in searches for weak gravitational lensing arising from large-scale mass inhomogeneities ('field lensing') is unwarranted. If the orientation of the galaxy image is determined either by the angular momentum or by the shape of the halo in which it forms, then the image should be aligned preferentially with the component of the tidal gravitational field perpendicular to the line of sight. Long-range correlations in the tidal field will thus lead to long-range ellipticity–ellipticity correlations that mimic the shear correlations arising from weak gravitational lensing. We calculate the ellipticity–ellipticity correlation expected if halo shapes determine the observed galaxy shape, and we discuss uncertainties (which are still considerable) in the predicted amplitude of this correlation. The ellipticity–ellipticity correlation induced by angular momenta should be smaller. We consider several methods for discriminating between the weak-lensing (extrinsic) and intrinsic correlations, including the use of redshift information. An ellipticity–tidal-field correlation also implies the existence of an alignment of images of galaxies near clusters. Although the intrinsic alignment may complicate the interpretation of field-lensing results, it is inherently interesting as it may shed light on galaxy formation as well as on structure formation.  相似文献   

On the presence of silicon and carbon in the pre-maximum spectrum of the Type Ia SN 1990N     

Paolo A. Mazzali 《Monthly notices of the Royal Astronomical Society》2001,321(2):341-346

The spectrum of the normal Type Ia SN 1990N observed very early on (14 days before B maximum) was analysed by Fisher et al., who showed that the large width and the unusual profile of the strong line near 6000 Å can be reproduced if the line is assumed to be due to C  ii 6578, 6583 Å and if carbon is located in a high-velocity shell. This line is one of the characterizing features of SNe Ia, and is usually thought to be due to Si  ii . A Monte Carlo spectrum synthesis code is used to investigate this suggestion further. The result is that if a standard explosion model is used, the mass enclosed in the shell at the required high velocity (25 000–35 000 km s⁻¹) is too small to give rise to a strong C  ii line. At the same time, removing silicon has a negative effect on the synthetic spectrum at other wavelengths, and removing carbon from the lower velocity regions near the photosphere makes it difficult to reproduce two weak lines which are naturally explained as C  ii , one of them being the line which Fisher et al. suggested is responsible for the strong 6000-Å feature. However, synthetic spectra confirm that although Si  ii can reproduce most of the observed 6000-Å line, the red wing of the line extends too far to be compatible with a Si  ii origin, and that the flat bottom of the line is also not easy to reproduce. The best fit is obtained for a normal SN Ia abundance mix at velocities near the photosphere (15 500–19 000 km s⁻¹) and an outer carbon–silicon shell beyond 20 000 km s⁻¹. This suggests that mixing is not complete in the outer ejecta of an SN Ia. Observations at even earlier epochs might reveal to what extent a carbon shell is unmixed.  相似文献   

Conservation laws and mass distribution in the planet formation process     

Paolo Farinella  Paolo Paolicchi 《Earth, Moon, and Planets》1977,17(4):401-408

Within the framework of the nebular theory of the origin of the solar system, conservation laws are applied to the condensation of a ring shaped cloud of orbiting particles. The final configuration is assumed to be a point-like planet in a circular orbit around the Sun. On this ground, it is possible to relate the masses of the planets with the interplanetary distances. This relation is confirmed satisfactorily by the observed masses and orbital radii of several planets and satellites of the solar system.  相似文献   

A search for solar neutrons near solar maximum,II     

Paolo Cortellessa  Paolo Di Benedetto  Constantinos Paizis 《Solar physics》1971,20(2):474-490

A plastic scintillator counter with anticoincidence screen has flown in four balloon flights at a floating altitude variable between 4 and 5 mb, during night to day, day-time and night-time flights. The analysis and comparison of the day-time and night-time parts of the flights has given for the continuous emission flux an upper limit of 5.5 × 10^–3 n/cm²s over the energy range from 10 MeV to 200 MeV. This upper limit is converted into upper limit differential solar neutron spectrum to be compared with the results obtained from other experiments.An irregular excess in counting rate observed in one of the flights, resulting in a day-night difference, is discussed.Finally the problems encountered in the operation of this type of detector at a high level of sensitivity are also discussed.This research has been sponsored in part by the Air Force Cambridge Research Laboratories through the European Office of Aerospace Research, OAR, USAF under contract F61052-68-C-0050.  相似文献