共查询到20条相似文献，搜索用时 31 毫秒
1.
Deborah P. Ruffle Eric Herbst 《Monthly notices of the Royal Astronomical Society》2000,319(3):837-850
In recent years it has become evident that large differences can exist between model results of grain-surface chemistry obtained from a rate equation approach and from a Monte Carlo technique. This dichotomy has led to the development of a modified rate equation method, in which a key element is the artificial slowing down of the diffusion rate of surface hydrogen atoms. Recent laboratory research into the surface diffusion rate of atomic hydrogen suggests that atomic hydrogen moves more slowly on grains than heretofore assumed. This research appears to lessen the need for modifications to the rate equation method. Based on the new laboratory work, we have developed appropriate models of gas-phase and grain-surface chemistry in quiescent dense cloud cores to examine the chemical effects of slowing down the rate at which atomic H can scan over dust surfaces. Furthermore, we have investigated the effect of slowing down the rate at which all species can move over grain surfaces. 相似文献
2.
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
3.
We study, by means of a spherical collapse model, the effect of shear, rotation, and baryons on a generalized Chaplygin gas (gCg) dominated universes. We show that shear, rotation, and the baryon presence slow down the collapse with respect to the simple spherical collapse model. The slowing down in the growth of density perturbation is able to solve the instability of the unified dark matter (UDM) models described in previous papers (e.g., Sandvik et al. 2004) at the linear perturbation level, as also shown by a direct comparison of our model with previous results. 相似文献
4.
We put forward evidence that relates the phenomenon of the chemically peculiar stars of the upper main sequence to Supernovae
explosions in young clusters. In the Upper Scorpius region we find that a supernova shell has interacted with dense clouds
and that the peculiar stars lie close to or along the edges of interaction.
We argue that the stars at or near the cloud faces are capable of acquiring this enriched material which is sufficiently slowed
down. The magnetic accretion process of Havnes and Conti provides the mechanism of acquisition. This process with the associated
magnetic braking accounts for the build-up in abundance anomalies and the slowing down of rotation with age. 相似文献
5.
The collisionless interaction of an expanding plasma cloud with a magnetized background plasma is examined in the framework of a 3D kinetic-hydrodynamic model. The slowing down of a hydrogen cloud is studied for high Alfven-Mach numbers and magneto-laminar interaction parameters. A particle-in-cell method is used to study the dynamics of the magnetic field, plasma cloud, background plasma, and collisionless shock wave generated by the intense particle flux. A numerical simulation is consistent with the nonstationary interactions between the plasma shells formed during nova and supernova explosions and the interstellar plasma medium. 相似文献
6.
Irakli S. Nanobashvili 《Astrophysics and Space Science》2004,294(3-4):125-134
Plasma mechanism for the generation of toroidal magnetic field in the magnetosphere of Crab pulsar is presented. The mechanism
is based on the development of parametric type instability in the relativistic electron-positron plasma of the pulsar magnetosphere.
As a result of plasma corotation with pulsar and its magnetic field, the effect of plasma radial braking takes place and the
time dependence of plasma particle radial velocity is harmonic. This triggers the development of parametric type instability
in the relativistic plasma of the pulsar magnetosphere. The energy for this process is drawn from the slowing down of pulsar
rotation. 相似文献
7.
Hydrodynamically stationary, spherically symmetric accretion onto a neutron star is examined taking the reverse effect of radiation into account. It is assumed that the plasma flow is adiabatic and that radiation is generated in a thin surface layer of the neutron star, where incoming particles are slowed down. It is shown that for stationary accretion, neither a stop, nor a substantial slowing down of the accretion flux is possible for any physically allowed conditions far from the neutron star. 相似文献
8.
L. Mertz 《Astrophysics and Space Science》1974,30(1):43-55
It is proposed that pulsars are mode-locked masers rather than rotating neutron stars. The maser cavity resides at the metallic-molecular interface near the surface of ordinary cold degenerate dwarf configurations. The period distribution, including its position, width, and features, is found in good agreement with that of pulsars. Furthermore, the pulse profile morphology of pulsars and modelocked lasers are found to be strikingly similar. Explanations are given for various other features, such as slowing down and optical pulses, and a possible connection with jovian decametric radiation is suggested. The main shortcoming of the maser theory is that no source of energy is obvious as yet.On leave from the Smithsonian Astrophysical Observatory. 相似文献
9.
The orbit averaged Fokker-Planck equation is used to study the dynamical evolution of globular clusters. Stellar evolutions according to their masses are incorporated in the model. The initial density distribution is chosen by Plummer's model with the initial mass function index =0.65, 1.35, 2.35, and 3.35. The mass-loss rate is given by the model of Fusi-Pecci and Renzini. It is found that the stellar mass loss acts as an energy source, and thereby affects the dynamical evolution of globular clusters by slowing down the evolution rate and extending the core collapse time. Also, the dynamical length scale is extended. 相似文献
10.
The magneto-rotational evolution of a neutron star in the massive binary system 4U 2206+54 is discussed in light of the recent
discovery of its 5555 s rotational period and its average rate of spin-down. We show that this behavior of the neutron star
means that its magnetic field exceeds the quantum mechanical critical limit and it is an accretion magnetar. The system’s
evolution is explained by wind driven mass transfer without formation of an accretion disk. The constant character of the
x-ray source indicates a steady rate of accretion and raises anew the question of the stability of the boundary of the magnetosphere
of a star undergoing spherical accretion. A solution to this problem is also a key to determining the mechanism for the slowing
down of the star’s rotation. 相似文献
11.
E. A. Saad M. A. El Gazzar M. S. Abdel Krim S. A. El Wakil 《Astrophysics and Space Science》1984,107(1):11-18
An asymptotic solution for the equation of radiative transfer in an inhomogeneous medium was obtained on the basis of the corresponding solutions for homogeneous sub-layers in the slowing down region. Function relations between the reflection and transmission coefficients for the whole slab and those of the sublayers are given. The invariant embedding concepts are used to get the reflection and transmission coefficients for the sub-layers. We assumed different models for the slowing-down kernels. Laplace transform was used to transform the Boltzmann equation to one velocity approximation with re-scaled mean-free path and single-scattering albedo. Numerical results are given for energy albedo as a function of the mass number of the host medium. 相似文献
12.
Formed by the freezing of sea water, sea ice defines the character of the marine Arctic. The principal purpose of this review is to synthesize the published efforts that document the potential impact of Arctic sea ice on remote climates. The emphasis is on atmospheric processes and the resulting modifications in surface conditions such as air temperature, precipitation patterns, and storm track behavior at interannual timescales across the middle and low latitudes of the Northern hemisphere during cool months. Addressed also are the theoretical, methodological, and logistical challenges facing the current observational and modeling studies that aim to improve our awareness of the role that Arctic sea ice plays in the definition of global climate. Moving towards an improved understanding of the role that polar sea ice plays in shaping the global climate is a subject of timely importance as the Arctic environment is currently undergoing rapid change with little slowing down forecasted for the future. 相似文献
13.
The planet Jupiter possesses a magnetic field and is surrounded by a magnetosphere. The occurrence of auroral and polar cap phenomena similar to those found on earth is very likely. In this work auroral and polar cap emissions in a model Jovian atmosphere are determined for proton precipitation. The incident protons, which are characterized by representative spectra, are degraded in energy by applying the continuous slowing down approximation. All secondary and higher generation electrons are assumed to be absorbed locally and their contributions to the total emissions are included. Volume emission rates are calculated from the total direct excitation rates with corrections for cascading applied. Results show that most molecular hydrogen and helium emissions for polar cap precipitation are below the ambient dayglow values. Charge capture by precipitating protons is an important source of Lyman α and Balmer α emissions and offers a key to the detection of large fluxes of low energy protons. 相似文献
14.
Douglas C. Hunt 《Planetary and Space Science》1970,18(12):1745-1751
The transfer of energy from an ionizing photon to the atoms and molecules of the neutral gas in the F-region of the atmosphere is investigated. It is found that photoionization heating should be divided into two parts: (1) photoelectron heating associated with the slowing down of the fast photoelectrons formed by photoionization; and (2) reaction heating associated with the chemical reactions undergone by the ions formed in the photoionization process. The photoelectron heating will take place near the time and place of photoionization while the reaction heating will occur at the time and place of the ionic reactions.
Photoelectron and reaction heating rates per unit column are computed for the daytime, and reaction heating rates per unit column are computed for the nighttime. It is concluded that: (1) chemical reactions at night lead to a small but significant amount of F-region heating; and (2) reaction heating during the day is nearly proportional to the cosine of the solar zenith angle except near sunrise and sunset. 相似文献
15.
We discuss the contribution of cosmic-ray protons at all energies above 1 MeV to the absorbed doses of the surface layers of a comet. Since there exists no calculation which takes into account proton energy losses by means of losses to electrons and nuclear collisions (in a cascade process), and losses due to the low energy end-products of the cascade, we have made a rough estimate of all of these contributions. An analytical formula is proposed that allows a rapid estimate of ranges and the dose absorbed at any depth. We give dose-depth curves for two extreme values of the energy at which nuclear collisions begin to dominate the slowing-down process, and for an intermediate value we display the dose-depth curve down to 20 m from the surface. The relevance of these findings to dosimetry in comets and some alterations of cometary material are considered. The need for improving the analytical expression proposed is stressed. 相似文献
16.
Noam Soker 《Monthly notices of the Royal Astronomical Society》2001,328(4):1081-1084
We derive the conditions for a backflow toward the central star(s) of circumstellar material to occur during the post-asymptotic giant branch (post-AGB) phase. The backflowing material may be accreted by the post-AGB star and/or its companion, if such exists. Such a backflow may play a significant role in shaping the descendant planetary nebula, by, among other things, slowing down the post-AGB evolution, and by forming an accretion disc which may blow two jets. We consider three forces acting on a slowly moving mass element: the gravity of the central system, radiation pressure, and fast wind ram pressure. We find that for a significant backflow to occur, a slow dense flow should exist, such that the relation between the total mass in the slow flow, M i , and the solid angle it covers Ω, is given by , where . The requirement for both a high mass-loss rate per unit solid angle and a very slow wind, such that it can be decelerated and flow back, probably requires close binary interaction, hence this process is rare. 相似文献
17.
{W}e consider the gravitational radiation from two time variable mass stars, orbiting around each other under the influence
of gravity. The total rates of the variation of the energy, angular momentum, semimajor axis, eccentricity and orbital period
are obtained. The results could be important for the understanding of general relativistic effects in the case of the variation
of the gravitational mass due to spinning down of the compact stars, which sensitively depends on the equations of state.
The cases of the binary systems PSR 1913+16 and PSR 1534+12 are analyzed in detail, and, for different equations of state
of nuclear matter, the corrections to the orbital decay due to gravitational radiation and to the spinning down of the pulsars
are calculated. The results show that a future significant improvement in the observational techniques could lead to the observation
of the specific general relativistic effect of mass variation of pulsars due to spinning down, via the study of orbital decay,
even in slowly rotating binary systems. 相似文献
18.
As laser–plasma interactions access ever-increasing ranges of plasma temperatures and densities, it is interesting to consider
whether they will some day shed light on questions concerning nuclear synthesis. One such open question is the process of
endothermic nuclear synthesis for elements with A > 60, thought to have taken place at a point in time during the big bang, or currently in supernovae. We present an explanation
based on a Boltzmann equilibrium condition, in combination with the change of the Fermi-statistics from the relativistic branch
for hadrons from higher than nuclear densities to the lower density subrelativistic branch. The Debye length confinement of
nuclei breaks down at the relativistic change, thus leading to the impossibility of nucleation of the quark-gluon state at
higher than nuclear densities. Taking the increment for the proton number Z as Z′ = 10 of the measured standard abundance distribution (SAD) of the elements for a Boltzmann probability for heavy element
synthesis, a sequence 3
n
was found with the exponent n for the sequence of the magic numbers. The jump between the magic numbers 20 and 28 does not need then the usual spin-orbit
explanation. 相似文献
19.
P. Lira A. Lawrence R. A. Johnson 《Monthly notices of the Royal Astronomical Society》2000,319(1):17-42
We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 1038 erg s−1 . The galaxies are selected from a complete, volume-limited sample of 46 galaxies with for which we have extensive multiwavelength data. For an almost complete subsample with (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ∼35 per cent of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity – there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity, and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and H ii region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common superluminous X-ray sources in the off-nuclear population – out of 29 galaxies we find nine sources with a luminosity greater than 1039 erg s−1 . These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery. 相似文献
20.
Large-scale, decelerating, relativistic X-ray jets from microquasar XTE J1550−564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the approaching jet at the late time is consistent with
the well-known Sedov evolutionary phase R∝ t
2/5. A trans-relativistic external shock dynamic model by analogy with the evolution of gamma-ray burst remnants, is shown to
be able to fit the proper-motion data of the approaching jet reasonably well. The inferred interstellar medium density around
the source is well below the canonical value n
ISM∼1 cm−3. The rapidly fading X-ray emission can be interpreted as synchrotron radiation from the non-thermal electrons in the adiabatically
expanding ejecta. These electrons were accelerated by the reverse shock (moving back into the ejecta) which becomes important
when the inertia of the swept external matter leads to an appreciable slowing down of the original ejecta. 相似文献