1,721,010 research outputs found
Planet formation process as a phase transition. I - Elementary model
No full textA model for the condensation of ring-shaped nebular matter into a protoplanet is described. The condensation is considered as a phase transition between a ring phase and a planetary phase. A constant nonzero temperature is assumed. The model is used to study the effect of physical parameters on the critical ring width permitting transition. The parameters considered are mass and temperature of the nebular matter
A tidal hypothesis about the origin of planetary rotation
No full textIt is assumed that the solar tidal action on accreting protoplanets forced them to corotation. If this is true, and assuming a subsequent conservation of spin angular momentum, one obtains a rough explanation of: (1) the actual rotation periods of the planets, (2) the trend toward small inclination angles for the rotation axes, (3) the existence of a relation between rotational angular momenta and planetary masses similar to the empirical ones due to McDonald (1964), Hartmann and Larson (1967), and Fish (1967)
Planet formation process as a phase transition. III - Mass distribution in the outer solar system
No full textOn the basis of the phase transition model for planetary formation, an evaluation is made of the mass which should be present in the part of the solar nebula corresponding to the outer planets. Results confirm the hypothesis of a relevant depletion of the light gas component. This effect increases with the distance from the Sun, lowering the masses of Uranus and Neptune with respect to Saturn and Jupiter: calculations suggest that the original mass in Uranus' and Neptune's zones was almost the same as in that of Saturn
Planet formation process as a phase transition. II - Isoentropic model and comparison with the solar system
No full textWe discuss an adiabatic phase transition between a nebular ring and a protoplanet, taking into account the gravitational corrections to the perfect gas entropy. We compare the results of this model with those found in a previous paper from different assumptions, and we try a qualitative comparison with the structure of the real solar system, using the data for the four giant planets. We discuss the relevance of some primordial phenomena in the formation process of these planets such as a mass depletion within the asteroidal belt, a mass loss from the external regions of the nebula and a decrease of Neptune's orbital radius due to ejection of cometary material. The first and the second process seem to be significantly supported by the results of the phase transition model
Orbital evolution around irregular bodies
Full text linkThe new profiles of the space missions aimed at asteroids and comets, moving from fly-bys to rendezvous and orbiting, call for new spaceflight dynamics tools capable of propagating orbits in an accurate way around these small irregular objects. Moreover, interesting celestial mechanics and planetary science problems, requiring the same sophisticated tools, have been raised by the first images of asteroids (Ida/Dactyl, Gaspra and Mathilde) taken by the Galileo and NEAR probes, and by the discovery that several near-Earth asteroids are probably binary. We have now developed two independent codes which can integrate numerically the orbits of test particles around irregularly shaped primary bodies. One is based on a representation of the central body in terms of "mascons" (discrete spherical masses), while the other one models the central body as a polyhedron with a variable number of triangular faces. To check the reliability and performances of these two codes we have performed a series of tests and compared their results. First we have used the two algorithms to calculate the gravitational potential around non-spherical bodies, and have checked that the results are similar to each other and to those of other, more common, approaches; the polyhedron model appears to be somewhat more accurate in representing the potential very close to the body's surface. Then we have run a series of orbit propagation tests, integrating several different trajectories of a test particle around a sample ellipsoid. Again the two codes give results in fair agreement with each other. By comparing these numerical results to those predicted by classical perturbation formulae, we have noted that when the orbit of the test particle gets close to the surface of the primary, the analytical approximations break down and the corresponding predictions do not match the results of the numerical integrations. This is confirmed by the fact that the agreement gets better and better for orbits farther away from the primary. Finally, we have found that in terms of CPU time requirements, the performances of the two codes are quite similar, and that the optimal choice probably depends on the specific problem under study
Raffaello tifava Roma (antiqua)
No full textAnticipazione sul Domenicale del Sole24Ore di domenica 7 gennaio 2018 di uno stralcio della voce bibliografica "Raffaello Santi o Sanzio" pubblicato nel volume n. 90 del «Dizionario Biografico degli Italiani», Roma, 201
Origin and evolution of the Vesta asteroid family
No full textWe model the formation and subsequent collisional evolution of the Vesta asteroid family. The outcomes of the cratering event(s) which generated the family are predicted from current cratering physics, whereas the subsequent erosion of the family members due to collisions with background asteroids is simulated according to the model of Marzari et al. (1995Icar..113..168M). Comparing the size and orbital distribution of the model Vesta families with the observed family, we estimate the number and size of the projectile(s) which have impacted Vesta. The observed morphology of the family suggests two possible scenarios: (1) The family is the outcome of a major cratering event, resulting from the impact of an asteroid =~40km in diameter on the surface of Vesta about 1Gyr ago, and followed by another more recent lower-energy impact (by a projectile =~20km in diameter), producing the family's subgrouping close to the 3:1 mean motion Jovian resonance. (2) A single impact occurred =~1 Gyr ago and formed the whole family at the same time. In this case we have to assume that the fragments were ejected isotropically over a hemispheric region of Vesta, instead of being concentrated near the surface of a 90deg aperture cone, as suggested by laboratory impact experiments with planar targets. This different ejection geometry yields a more scattered distribution of the orbital elements, resulting into a better agreement with the observed family. In both scenarios the cratering event(s) which formed the family is/are likely to have injected a significant number of km-sized and smaller fragments into the 3:1 resonance, thus generating V-type near-Earth asteroids and HED meteorites. However, it appears likely that the current influx of HED meteorites cannot be directly traced back to the family-forming event(s), but results from more recent, smaller impacts on Vesta (or other family members)
The period distribution of eclipsing and spectroscopic binary systems, II
No full textA statistical analysis of the period distribution for eclipsing and spectroscopic binary systems, based on the spectral types of the components, shows several common features between the two independent samples. The similarity is increased if the geometrical selection effect on the eclipsing binary sample is eliminated by means of the method described in previous papers. The period distribution becomes broader (and probably nonunimodal) for advanced spectral types. Analysis of the mean separation of systems as a function of the spectral type was also performed
Statistics of period for eclipsing binary systems
No full textA statistical analysis of period distribution in eclipsing binary stars based on the spectral types of the components which takes into account the most relevant selection effects, is presented. Results are presented which are concluded to be in line with the fission theory for the origin of these systems. In addition, the results are shown to provide clear indications about evolutionary effects on the average separation. Finally, emphasis is given to the appearance of a marked bimodal distribution for the advanced spectral types
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