共查询到20条相似文献，搜索用时 31 毫秒
1.
We consider the consistency of positrons and electrons with a propagation model in which the cosmic rays are stopped by nuclear collisions or energy losses before they can escape from the Galaxy (the closed-galaxy model). The fact that we find no inconsistency between the predictions and the data implies that the protons which produce the positrons by nuclear reactions could have their origin in a large number of distant sources, as opposed to the heavier nuclei which in this model come from a more limited set of sources. The closed-galaxy model predicts steep electron and positron spectra at high energies. None of these are inconsistent with present measurements; but future measurements of the spectrum of high-energy positrons could provide a definite test for the model. The closed-galaxy model also predicts that the interstellar electron intensity below a few GeV is larger than that implied by other models. The consequence of this result is that electron brems-strahlung is responsible for about 50% of the galactic gamma-ray emission at photon energies greater than 100 MeV. 相似文献
2.
A. A. Korchak 《Solar physics》1980,66(1):149-158
The influence of Coulomb collisions in two-component plasma on the nuclear composition and the charge-state of accelerated particles is investigated. The main characteristics are the location and value of the two loss maxima. It is shown that the maximum of energy losses on the electron component of plasma for flares is a high energy threshold which prevents the penetration of a large particle flux into the range 10 MeV nucl–1. The low-energy range up to the maximum is considered in detail. At preliminary or initial stage of acceleration the nuclear composition of accelerated particles is strongly dependent on their energy losses on the proton component of plasma which prevails at low energies. The conditions under which the equilibrium charge is reached are investigated. 相似文献
3.
Collisions between large, similar-sized bodies are believed to shape the final characteristics and composition of terrestrial planets. Their inventories of volatiles such as water are either delivered or at least significantly modified by such events. Besides the transition from accretion to erosion with increasing impact velocity, similar-sized collisions can also result in hit-and-run outcomes for sufficiently oblique impact angles and large enough projectile-to-target mass ratios. We study volatile transfer and loss focusing on hit-and-run encounters by means of smooth particle hydrodynamics simulations, including all main parameters: impact velocity, impact angle, mass ratio and also the total colliding mass. We find a broad range of overall water losses, up to 75% in the most energetic hit-and-run events, and confirm the much more severe consequences for the smaller body also for stripping of volatile layers. Transfer of water between projectile and target inventories is found to be mostly rather inefficient, and final water contents are dominated by pre-collision inventories reduced by impact losses, for similar pre-collision water mass fractions. Comparison with our numerical results shows that current collision outcome models are not accurate enough to reliably predict these composition changes in hit-and-run events. To also account for non-mechanical losses, we estimate the amount of collisionally vaporized water over a broad range of masses and find that these contributions are particularly important in collisions of \(\sim \) Mars-sized bodies, with sufficiently high impact energies, but still relatively low gravity. Our results clearly indicate that the cumulative effect of several (hit-and-run) collisions can efficiently strip protoplanets of their volatile layers, especially the smaller body, as it might be common, e.g., for Earth-mass planets in systems with Super-Earths. An accurate model for stripping of volatiles that can be included in future planet formation simulations has to account for the peculiarities of hit-and-run events and track compositional changes in both large post-collision fragments. 相似文献
4.
S. Yabushita 《Earth, Moon, and Planets》1996,74(3):183-189
Heat transport at very high temperatures is governed by a nonlinear diffusion equation, and in order to estimate the efficiency of nuclear explosions near the surface of a near earth object (NEO) in the orbit deflecting, it is required to solve the non-linear diffusion equation. Here, the solution is obtained by similarity considerations. It is shown that the earlier (approximate) value of the efficiency obtained by Simonenko et al. of the fraction of absorbed energy to the total incident energy is an overestimate by 60%. Other differences are in the propagation velocity of the thermal front and in the time required for hydrodynamical motion to start.A brief comparison is also made with the explosion where neutrons are the major products. 相似文献
5.
Based on the magnetar model, we have studied in detail the processes of neutrino cooling of an electron-positron plasma generating
an SGR giant flare and the influence of the magnetar magnetic field on these processes. Electron-positron pair annihilation
and synchrotron neutrino emission are shown to make a dominant contribution to the neutrino emissivity of such a plasma. We
have calculated the neutrino energy losses from a plasma-filled region at the long tail stage of the SGR 0526-66, SGR 1806–20,
and SGR 1900+14 giant flares. This plasma can emit the energy observed in an SGR giant flare only in the presence of a strongmagnetic
field suppressing its neutrino energy losses. We have obtained a lower bound on the magnetic field strength and showed this
value to be higher than the upper limit following from an estimate of the magnetic dipole losses for the magnetars being analyzed
in a wide range of magnetar model parameters. Thus, it is problematic to explain the observed energy release at the long tail
stage of an SGR giant flare in terms of the magnetarmodel. 相似文献
6.
W
odzimierz Bednarek 《New Astronomy》2002,7(8):471-482
We investigate the cascading effects of extremely high energy (EHE) photons in the Earth’s magnetosphere assuming that these photons arrive with the parameters of the highest energy AGASA events (energies, arrival directions). For the location of the AGASA Observatory, we determine the directions in the sky from which photons can cascade with a high (low) probability. In the case of the primary photons with the parameters of the events with the energies >1020 eV, we compute the average cascade spectra of secondary photons entering the Earth’s atmosphere, and estimate their fluctuations around these average values by selecting the events with the largest and smallest number of secondary cascade photons. It is shown that most photons with the parameters of the highest energy AGASA events should initiate cascades in the Earth’s magnetosphere with a high probability even though they tend to arrive from directions in the sky for which the perpendicular component of the magnetic field is weaker. On the other hand, if these events are caused by the photons with lower energies, then the fluctuations in their shower development in the magnetosphere and the atmosphere should be higher than in the case of photons with the energies estimated by the AGASA experiment. 相似文献
7.
P. Bordas J. M. Paredes V. Bosch-Ramon M. Orellana 《Astrophysics and Space Science》2007,309(1-4):339-343
We present a model to estimate the synchrotron radio emission generated in microquasar (MQ) jets due to secondary pairs created
via decay of charged pions produced in proton-proton collisions between stellar wind ions and jet relativistic protons. The
synchrotron radiation produced by secondary electrons/positrons is computed using consistently derived particle energy distributions.
Energy losses due to synchrotron and inverse Compton (IC) processes, and adiabatic expansion, are taken into account. The
space parameter for the model is explored and the corresponding spectral energy distributions (SEDs) are presented. We conclude
that secondary leptonic emission represents a significant though hardly dominant contribution to the total radio emission
in MQs, with observational consequences that can be used to test some still unknown processes occurring in these objects as
well as the nature of the matter outflowing in their jets.
相似文献
8.
D. B. Melrose R. Fok † D. P. Menezes ‡ 《Monthly notices of the Royal Astronomical Society》2006,371(1):204-210
The dominant emission from bare strange stars is thought to be electron–positron pairs, produced through spontaneous pair creation (SPC) in a surface layer of electrons tied to the star by a superstrong electric field. The positrons escape freely, but the electrons are directed towards the star and quickly fill all available states, such that their degeneracy suppresses further SPC. An electron must be reflected and gain energy in order to escape, along with the positron. Each escaping electron leaves a hole that is immediately filled by another electron through SPC. We discuss the collisional processes that produce escaping electrons. When the Landau quantization of the motion perpendicular to the magnetic field is taken into account, electron–electron collisions can lead to an escaping electron only through a multistage process involving higher Landau levels. Although the available estimates of the collision rate are deficient in several ways, it appears that the rate is too low for electron–electron collisions to be effective. A simple kinetic model for electron–quark collisions leads to an estimate of the rate of pair production that is analogous to thermionic emission, but the work function is poorly determined. 相似文献
9.
The space weathering, i.e. the evolution of surface properties over time, due to the exposure to external factors, has been shown to affect the optical properties of the asteroids, usually causing reddening (an effect which is measured in terms of the spectral slope in the visible and near infrared range) and darkening over time. However, some problems remain open. In particular, the timescale for reddening, which we estimate from laboratory experiments, is shorter—maybe, by two or even more orders of magnitude—than the typical asteroidal ages. Thus we should expect a complete saturation of the reddening effects for most of the objects, which does not happen, instead of a general significant dependence of the slope on the age, as indeed we find.In this paper we discuss, with the aid of a simplified model, how the collisions may affect the timing of the reddening process. We show that the collisions might halt the reddening, unless a significant reaccumulation of the fragments created in the cratering collisions takes place. In this case the timing for the complete reddening is driven by the collisional events, thus providing a rationale for the observed slope-age and slope-exposure relations. 相似文献
10.
A hypothesis is proposed for the mechanism of superrotation of the atmosphere of Venus involving the following processes. Winds near the planet's surface, making up part of the Hadley cell, flow past the surface relief and excite internal atmospheric gravity waves. While moving upward, these waves become unstable and collapse generating turbulent eddies. Some portion of the energy of these eddies is transferred to a two-dimensional flow, which is maintained also at the expense of instability of large-scale motions. In this flow, the inverse energy cascade (transition from smaller eddies to larger eddies), or negative viscosity, appears as a key element of this mechanism. Large-scale quasi-two-dimensional turbulent eddies transfer their energy to Rossby planetary waves which, in turn, transmit it to a zonal flow. Based on this hypothesis and the existing experimental data, the time period over which superrotation is established, as well as the vertical and horizontal eddy viscosity coefficients, is estimated. In parallel, arguments in favor of the proposed hypothesis are obtained. 相似文献
11.
Keiji Ohtsuki 《Icarus》2006,183(2):373-383
We derive an equation for the evolution of rotational energy of Keplerian particles in a dilute disk due to mutual collisions. Three-dimensional Keplerian motion of particles is taken into account precisely, on the basis of Hill's approximation. The Rayleigh distribution of particles' orbital eccentricities and inclinations, and the Gaussian distribution of their rotation rates are also taken into account. Performing appropriate variable transformation, we show that the equation can be expressed with two terms. The first term, which we call collisional stirring term, represents energy exchange between rotation and random motion via collisions. The second term, which we call rotational friction term, tends to equalize the mean rotational energy of particles with different sizes. The equation can describe the evolution of rotational energy of Keplerian particles with an arbitrary size distribution. We analytically evaluate the rates of stirring and friction for the random kinetic energy and rotational energy due to inelastic collisions, for non-gravitating particles in a dilute disk. Using these results, we discuss equilibrium states in a disk of spinning, non-gravitating Keplerian particles. 相似文献
12.
W. Bednarek 《Monthly notices of the Royal Astronomical Society》2009,397(3):1420-1425
This paper focuses on neutron stars (NS) of the magnetar type inside massive binary systems. We determine the conditions under which the matter from the stellar wind can penetrate the inner magnetosphere of the magnetar. At a certain distance from the NS surface, the magnetic pressure can balance the gravitational pressure of the accreting matter, creating a very turbulent, magnetized transition region. It is suggested that this region provides good conditions for the acceleration of electrons to relativistic energies. These electrons lose energy due to the synchrotron process and inverse Compton (IC) scattering of the radiation from the nearby massive stellar companion, producing high-energy radiation from X-rays up to ∼TeV γ-rays. The primary γ-rays can be further absorbed in the stellar radiation field, developing an IC e± pair cascade. We calculate the synchrotron X-ray emission from primary electrons and secondary e± pairs and the IC γ-ray emission from the cascade process. It is shown that quasi-simultaneous observations of the TeV γ-ray binary system LSI +61 303 in the X-ray and TeV γ-ray energy ranges can be explained with such an accreting magnetar model. 相似文献
13.
A. Malizia J. B. Stephen L. Bassani A. J. Bird F. Panessa P. Ubertini 《Monthly notices of the Royal Astronomical Society》2009,399(2):944-951
We study the N H distribution in a complete sample of 88 active galactic nuclei (AGN) selected in the 20–40 keV band from INTEGRAL /Imager on Board the Integral Satellite (IBIS) observations. We find that the fraction of absorbed ( N H ≥ 1022 cm2 ) sources is 43 per cent while the Compton thick AGN comprise 7 per cent of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported by Risaliti, Maiolino & Salvati using an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account, we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore, we show that the measured fraction of absorbed sources in our sample decreases from 80 to ∼20–30 per cent as a function of redshift with all Compton thick AGN having z ≤ 0.015 . If we limit our analysis to this distance and compare only the type 2 objects in our sample with the Risaliti et al. objects below this redshift value, we find a perfect match to their N H distribution. We conclude that in the low-redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in types 1 and 2 AGN, we estimate the fraction of Compton thick objects to be ≥24 per cent. 相似文献
14.
The precise location of the water ice condensation front (‘snow line’) in the protosolar nebula has been a debate for a long time. Its importance stems from the expected substantial jump in the abundance of solids beyond the snow line, which is conducive to planet formation, and from the higher ‘stickiness’ in collisions of ice-coated dust grains, which may help the process of coagulation of dust and the formation of planetesimals. In an optically thin nebula, the location of the snow line is easily calculated to be around 3 AU, subject to brightness variations of the young Sun. However, in its first 5-10 myr, the solar nebula was optically thick, implying a smaller snowline radius due to shielding from direct sunlight, but also a larger radius because of viscous heating. Several models have attempted to treat these opposing effects. However, until recently treatments beyond an approximate 1 + 1D radiative transfer were unfeasible. We revisit the problem with a fully self-consistent 3D treatment in an axisymmetric disk model, including a density-dependent treatment of the dust and ice sublimation. We find that the location of the snow line is very sensitive to the opacities of the dust grains and the mass accretion rate of the disk. We show that previous approximate treatments are quite efficient at determining the location of the snow line if the energy budget is locally dominated by viscous accretion. Using this result we derive an analytic estimate of the location of the snow line that compares very well with results from this and previous studies. Using solar abundances of the elements we compute the abundance of dust and ice and find that the expected jump in solid surface density at the snow line is smaller than previously assumed. We further show that in the inner few AU the refractory species are also partly evaporated, leading to a significantly smaller solid state surface density in the regions where the rocky planets were formed. 相似文献
15.
We discuss the problem of ultra high energy nuclei propagation in extragalactic background radiations. The present paper is the continuation of the accompanying paper I where we have presented three new analytic methods to calculate the fluxes and spectra of ultra high energy cosmic ray nuclei, both primary and secondary, and secondary protons. The computation scheme in this paper is based on the analytic solution of coupled kinetic equations, which takes into account the continuous energy losses due to the expansion of the universe and pair-production, together with photo-disintegration of nuclei. This method includes in the most natural way the production of secondary nuclei in the process of photo-disintegration of the primary nuclei during their propagation through extragalactic background radiations. In paper I, in order to present the suggested analytical schemes of calculations, we have considered only the case of the cosmic microwave background radiation, in the present paper we generalize this computation to all relevant background radiations, including infra-red and visible/ultra-violet radiations, collectively referred to as extragalactic background light. The analytic solutions allow transparent physical interpretation of the obtained spectra. Extragalactic background light plays an important role at intermediate energies of ultra high energy cosmic ray nuclei. The most noticeable effect of the extragalactic background light is the low-energy tail in the spectrum of secondary nuclei. 相似文献
16.
James E. Dale Melvyn B. Davies Ross P. Church Marc Freitag 《Monthly notices of the Royal Astronomical Society》2009,393(3):1016-1033
We show that collisions with stellar-mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al. We show that the missing objects are low-mass giants and asymptotic giant branch stars in the range 1–3 M⊙ . Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of smoothed particle hydrodynamic calculations, restricted three-body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main-sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the 10.5 < K < 12 band. However, depleting the brighter ( K < 10.5) objects out to the required radius would require a large population of black hole impactors which would in turn deplete the 10.5 < K < 12 giants in a region much larger than is observed. We conclude that collisions with stellar-mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar-mass black holes in the Galactic Centre. 相似文献
17.
Dependence of the acceleration of the outer edge of a stellar envelope on supernova explosion energy
Based on numerical simulations of a supernova explosion, we investigate the shock-wave breakout through the stellar surface. The computations have been performed in a wide range of explosion energies and presupernova masses. The results are compared with the classical Gandelman-Frank-Kamenetsky self-similar solution. We have determined the dependence of an arbitrary coefficient in the self-similar solution on the explosion energy and presupernova structure. The derived analytical approximation formula for this coefficient can be used to estimate the supernova explosion energy from such parameters of the ejected envelope determined from astronomical observations as its maximum expansion velocity and the density distribution along its outer edge. The formula may prove to be also useful in studying the X-ray and gamma-ray bursts that accompany the shock-wave breakout through the surface of compact presupernovae. 相似文献
18.
Ernazar B. Abdikamalov Bobomurat J. Ahmedov John C. Miller 《Monthly notices of the Royal Astronomical Society》2009,395(1):443-461
Just as a rotating magnetized neutron star has material pulled away from its surface to populate a magnetosphere, a similar process can occur as a result of neutron-star pulsations rather than rotation. This is of interest in connection with the overall study of neutron star oscillation modes but with a particular focus on the situation for magnetars. Following a previous Newtonian analysis of the production of a force-free magnetosphere in this way Timokhin et al., we present here a corresponding general-relativistic analysis. We give a derivation of the general relativistic Maxwell equations for small-amplitude arbitrary oscillations of a non-rotating neutron star with a generic magnetic field and show that these can be solved analytically under the assumption of low current density in the magnetosphere. We apply our formalism to toroidal oscillations of a neutron star with a dipole magnetic field and find that the low current density approximation is valid for at least half of the oscillation modes, similarly to the Newtonian case. Using an improved formula for the determination of the last closed field line, we calculate the energy losses resulting from toroidal stellar oscillations for all of the modes for which the size of the polar cap is small. We find that general relativistic effects lead to shrinking of the size of the polar cap and an increase in the energy density of the outflowing plasma. These effects act in opposite directions but the net result is that the energy loss from the neutron star is significantly smaller than suggested by the Newtonian treatment. 相似文献
19.
M. A. Hassan B. A. Korany R. Misra I. A. M. Issa M. K. Ahmed F. A. Abdel-Salam 《Astrophysics and Space Science》2012,339(2):355-364
We undertake a spectral study of a sample of bright X-ray sources taken from six XMM-Newton fields at high galactic latitudes,
where AGN are the most populous class. These six fields were chosen such that the observation had an exposure time more than
60 ksec, had data from the EPIC-pn detector in the full-Frame mode and lying at high galactic latitude |b|>25∘. The analysis started by fitting the spectra of all sources with an absorbed power-law model, and then we fitted all the
spectra with an absorbed power-law with a low energy black-body component model.The sources for which we added a black body
gave an F-test probability of 0.01 or less (i.e. at 99% confidence level), were recognized as sources that display soft excess. We
perform a comparative analysis of soft excess spectral parameters with respect to the underlying power-law one for sources
that satisfy this criterion. Those sources, that do not show evidence for a soft excess, based on the F-test probability at a 99% confidence level, were also fitted with the absorbed power-law with a low energy black-body component
model with the black-body temperature fixed at 0.1 and 0.2 keV. We establish upper limits on the soft excess flux for those
sources at these two temperatures. Finally we have made use of Aladdin interactive sky atlas and matching with NASA/IPAC Extragalactic
Database (NED) to identify the X-ray sources in our sample. For those sources which are identified in the NED catalogue, we
make a comparative study of the soft excess phenomenon for different types of systems. 相似文献
20.
James M. Lattimer 《Astrophysics and Space Science》2011,336(1):67-74
Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary
astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative
stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near
the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion
collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including
mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized.
Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear
explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density
relation of extremely dense matter. 相似文献