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In this paper we briefly discuss the effect of complete homogenization in the intermediate unstable layers of massive stars on the effective temperature of the core He-burning models. To this end, a 20M
star of Population I chemical composition (X=0.700,Z=0.020) has been allowed to evolve from the Main Sequence into the core He-exhaustion stage without taking into account semiconvective mixing. The results show that the models are systematically bluer than those computed with the same physical parameters but with the inclusion of semiconvection. 相似文献
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Summary Some aspects concerning the core-mass increase and the appearance and development of an intermediate semiconvective zone in helium-burning stars are discussed. An iterative method of computation of core increase and semiconvection is also presented. Details of this procedure are given with regard to the horizontal-branch stars of globular clusters.
Riassunto Sono presi in esame alcuni aspetti relativi all'aumento in massa del nucleo ed alla comparsa e sviluppo di una zona semiconvettiva intermedia in modelli di stelle con combustione centrale dell'elio. E'inoltre schematizzato il metodo iterativo di calcolo con particolare riferimento alle stelle di ramo orizzontale degli ammassi globulari.相似文献
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Adnan Aslam Mubasher Jamil Davood Momeni Ratbay Myrzakulov Muneer Ahmad Rashid Muhammad Raza 《Astrophysics and Space Science》2013,348(2):533-540
In literature usual point like symmetries of the Lagrangian have been introduced to study the symmetries and the structure of the fields. This kind of Noether symmetry is a subclass of a more general family of symmetries, called Noether gauge symmetries (NGS). Motivated by this mathematical tool, in this paper, we study the generalized Noether symmetry of quintom model of dark energy, which is a two component fluid model with quintessence and phantom scalar fields. Our model is a generalization of the Noether symmetries of a single and multiple components which have been investigated in detail before. We found the general form of the quintom potential in which the whole dynamical system has a point like symmetry. We investigated different possible solutions of the system for diverse family of gauge function. Specially, we discovered two family of potentials, one corresponds to a free quintessence (phantom) and the second is in the form of quadratic interaction between two components. These two families of potential functions are proposed from the symmetry point of view, but in the quintom models they are used as phenomenological models without clear mathematical justification. From integrability point of view, we found two forms of the scale factor: one is power law and second is de-Sitter. Some cosmological implications of the solutions have been investigated. 相似文献
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The stratified interstellar medium has been known to be subject to the Parker instability, and the instability has been considered as a plausible mechanism for the formation of giant molecular clouds (GMCs). By presenting our recent efforts to understand the instability through linear analyses and numerical simulations, we raise a negative point of view that the Parker instability alone could not form GMCs. 相似文献
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The connection between orbital period modulation and magnetic activity in close binaries is reviewed with an emphasis on the comparison between observational data for RS CVn systems and recently proposed theoretical models. The orbital period changes occurring on timescales of the order of a few decades can be accounted for by means of a standing torsional Alfven wave in the convection zone of the magnetically active components of such systems. Two resonant excitation mechanisms based on the coupling between the wave and an αΩ dynamo are discussed from a qualitative point of view. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Turbulent convection models (TCMs) based on hydrodynamic moment equations are compared with the classical mixing-length theory (MLT) in solar models. The aim is to test the effects of some physical processes on the structure of the solar convection zone, such as the dissipation, diffusion and anisotropy of turbulence that have been ignored in the MLT. Free parameters introduced by the TCMs are also tested in order to find appropriate values for astrophysical applications. It is found that the TCMs usually give larger convective heat fluxes than the MLT does, and the heat transport efficiency is sensitively related to the dissipation parameters used in the TCMs. As a result of calibrating to the present solar values, our solar models usually have rather smaller values of the mixing length to local pressure scaleheight ratio than the standard solar model. The turbulent diffusion is found to have important effects on the structure of the solar convection zone. It leads to significantly lowered and expanded profiles for the Reynolds correlations, and a larger temperature gradient in the central part of the superadiabatic convection region but a smaller one near the boundaries of the convection zone. It is interesting to note that, due to a careful treatment of turbulence developing towards isotropic state, our non-local TCM results in radially dominated motion in the central part and horizontally dominated motion near the boundaries of the convection zone, just as what has been observed in many 3D numerical simulations. Our solar models with the TCMs give small but meaningful differences in the temperature and sound speed profiles compared with the standard solar model using the MLT. 相似文献
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Roberto Bruno Raffaella D&#x;Amicis Bruno Bavassano Vincenzo Carbone Luca Sorriso-Valvo 《Planetary and Space Science》2007,55(15):2233-2238
The interplanetary medium is characterized by a very high Reynolds number and is pervaded by fluctuations providing information on a wide range of scales, from fractions of second up to the solar rotation period. In the past decade or so, turbulence in the solar wind has been used as a large wind tunnel to investigate scaling laws of turbulent fluctuations and multifractal models. Moreover, new interesting insights in the theory of turbulence have been derived from the point of view which considers a turbulent flow as a complex system, a sort of benchmark for the theory of dynamical systems. Important finding like the lack of a strict self-similarity of the fluctuations with the consequent nonapplicability of strict scale invariance, the strong anisotropy of velocity and magnetic field fluctuations, the clear lack of equipartition between magnetic and kinetic fluctuations all contributed to suggest the idea that interplanetary fluctuations could possibly be due to a mixture of propagating waves and static structures convected by the wind. In this paper we further discuss this point and bring new evidence about the fact that the presence of a background magnetic field introduces not only a symmetry breaking in interplanetary space but also organizes fluctuations about its large scale orientation. 相似文献
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Solar Physics - We consider two aspects of solar flares from the point of view of circuit theory. First, we show that the so-called “dynamo models”, which invoke an analogy between the... 相似文献
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An attempt is made to infer the structure of the solar convection zone from observedp-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to find the sound speed in
the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the
convective flux give significantly better agreement with observations than models constructed using the mixing length formalism.
This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in
the CM envelope model is within 0.2% of that in the Sun except in the region withr > 0.99R
⊙. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium
beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection
zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observations. 相似文献
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The problem of collapse of a dust ball has been studied in detail from the point of view of an external observer (O). It is seen that although there is no material pressure from the point of view of the comoving observer, there is a non-vanishing material pressure in the ball from the point of view of O. In the early stages of collapse (w.r.t. O), this pressure is positive everywhere inside the ball, while during the later stages, if one goes outwards from the centre towards the surface of the ball, this pressure is negative up to a certain value of the radial coordinate, and then changes its sign thereafter. On the basis of this pressure, one can understand the physics of the whole scenario of collapse w.r.t. O; in particular, the important feature is that in the ultimate stages an event horizon is formed asymptotically and there is no collapse to a point (as takes place from the point of view of the comoving observer). 相似文献
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It is widely accepted that lunar volcanism started before the emplacement of the mare fills ( b.p.) and lasted for probably more than 3.0 Ga. While the early volcanic activity is relatively easy to understand from a thermal point of view, the late stages of volcanism are harder to explain, because a relatively small body like the Earth's Moon is expected to cool rapidly and any molten layer in the interior should solidify rather quickly. We present several thermal evolution models, in which we varied the boundary conditions at the model surface in order to evaluate the influence on the extent and lifetime of a molten layer in the lunar interior. To investigate the influence of a top insulating layer we used a fully three-dimensional spherical shell convection code for the modelling of the lunar thermal history. In all our models, a partial melt zone formed nearly immediately after the simulation started (early in lunar history), consistent with the identification of lunar cryptomare and early mare basalt volcanism on the Moon. Due to the characteristic thickening of the Moon's lithosphere the melt zone solidified from above. This suggests that the source regions of volcanic rock material proceeded to increasing depth with time. The rapid growth of a massive lithosphere kept the Moon's interior warm and prevented the melt zone from fast freezing. The lifetimes of the melt zones derived from our models are consistent with basalt ages obtained from crater chronology. We conclude that an insulating megaregolith layer is sufficient to prevent the interior from fast cooling, allowing for the thermal regime necessary for the production and eruption of young lava flows in Oceanus Procellarum. 相似文献
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In this paper the sublimation mechanisms of parent molecules from nuclei will be reviewed from the point of view of theoretical models, and the results of models will be compared with the results of the extensive observation campaign of C/1995 O1 Hale-Bopp. The simple model of a mixture of ices in which each gas sublimates independently from the others when the right temperature has been reached is in many cases inadequate to explain the observations. Many minor volatiles can be trapped in the amorphous water ice and released in a complex way when particular ranges of temperature are reached. The presence of sublimating icy grains in the inner coma of comets, suggested many years ago, seems now to be proven, at least for Hale-Bopp. From these grains a significant amount of water and other volatiles could contribute to the total flux measured in the coma. The unprecedented coverage of Hale-Bopp’s gas production curve for such a long time and with so many instrumentshas offered to modellers a wonderful occasion to test and compare observation results with the predictions of sublimation models, demonstrating that current models are able to explain observed results. 相似文献
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Comparison of recent spectra by V. Motta et al. (V. Motta et al.: Astrophys. J. 574, 719–725) of two images of a distant gravitationally lensed quasar (z = 1.38) leads to an extinction curve of the lens galaxy (z = 0.83) showing clear evidence of a 2175A absorption feature. We compare these data with measured extinction curves from
the Galaxy, LMC and SMC and with predictions from models. As it has been found by other authors in the Local Group galaxies
we found that the SBS 0909 + 532 curve is compatible with three component models of classical spherical grains and PAHs. We
note that apart from this standard point of view, the fits also admit an interpretation in terms of products of a biological
origin involving three grain components. 相似文献
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Harold Zirin 《Solar physics》1996,169(2):313-326
We discuss many aspects of the solar chromosphere from an observational point of view, and show that most existing models are in direct contradiction to radio and eclipse measurements. We plead for attention to the actual observed radio temperatures and density gradients, as well as images of the chromosphere. We find that the chromosphere is not in hydrostatic equilibrium and suggest that the support is due to the tangled intranetwork fields.Dedicated to Cornelis de Jager 相似文献
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The stability of the solutions of the mean-field theories of turbulent media is questioned. It is done here for the model equations for the solar convection zone which have been used, in particular, to explain the differential rotation. We present an approximation valid for axisymmetric, short-wave disturbances. A critical local Rayleigh number can be defined - involving eddy diffusivities - above which the stratification becomes unstable. For mixing-length models of the solar convection zone we always find sub-critical Rayleigh numbers. One must be careful, however, with other theoretical models. Those we have considered do not reach sufficiently high surface pressure values so that there the associated Rayleigh numbers exceed their critical limits. In the outermost layers in such models, therefore, the solutions could really be unstable. 相似文献
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The properties of Saturn's electrostatic discharges (SED) as observed by the Voyager Planetary Radio Astronomy experiment during the two Voyager encounters with Saturn are summarized. Several models for the formation of SED are discussed in light of these observations. The most likely source regions appear to be either the equatorial zone of the planet or the dense part of the B ring near 1.80 Rs. The strenghts and weaknesses of each of these possibilities are examined. Neither possibility accounts fully for the observed SED properties in a simple way. A search for an anomaly near 1.80 Rs in the data of other experiments aboard Voyager has been carried out, and at least one and possibly more such experiments do indeed obtain anomalous data at this point in the ring system. There thus appears to be unexplained phenomena at this point, independent of the PRA data, and it is a short step to postulate that a single object may be the cause of all such phenomena. 相似文献
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New solar models are presented, which have been computed with the most recent physical inputs (nuclear reaction rates, equation
of state, opacities, microscopic diffusion). Rotation-induced mixing has been introduced in a way which includes the feed-back
effect of the μ-gradient induced by helium settling. A parametrization of the tachocline region below the convective zone
has also been added in the computations. The sound velocities have been computed in the models and compared to the seismic
Sun. Our best model is described in some detail. Besides the new physical inputs, the most important improvement concerns
the computations of μ-gradients during the solar evolution and their influence in slowing down rotation-induced mixing. This
process can explain why lithium is depleted in the present Sun while beryllium is not, and meanwhile why 3He has not increased at the solar surface for at least 3 Gyrs. 相似文献
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The temporal evolutions of the planetesimals scattered from the Jupiter zone for different masses of the proto-Jupiter [(a) 0.1 and (b) 1.0 of the present mass] are investigated. Due to the combined effects of the orbital evolution of the planetesimals and the elimination of these projectiles either via impact capture or injection into escape velocity by the outer planets, the whole scattering process lasts about 108 yr for case (a) and about 107 yr for case (b). The longer time scale may be a good estimate for the accretion time interval of Jupiter while the shorter one (107) gives the upper time limit of the late heavy-bombardment epoch of the terrestrial planets due to planetesimals scattered from the Jupiter zone. The limiting value of the encounter velocity U at the end of the scattering process is ≈0.6. Consideration of the collisional interaction of these projectiles with the asteroids indicates that the corresponding bombardment effect could be rather appreciable. Also, the asteroids on the inner edge of the main asteroid belt would have been bombarded more severely than those on the outer edge. From this point of view, the structure of the asteroidal belt could be affected significantly not only by Jupiter's gravitational perturbation effect but also by its early scattering process. 相似文献