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1.
2.
It is shown that knowing the energy flux density of the radio emission, the rate of increase in the period, and the distance
of a pulsar enables one to calculate all the rest of its most important characteristics (the solid angle of the radio emission
beam, the radio luminosity, the solid angle of the beam of γ rays, the energy flux density of the g-ray emission, and the
magnetic moment, moment of inertia, and mass of the neutron star). Equations from which these pulsar characteristics can be
calculated are given at the end of the paper. The results of calculations for a number of pulsars are given in Tables 2 and
3 as an illustration.
Translated from Astrofizika, Vol. 43, No. 2, pp. 277-291, April–June, 2000. 相似文献
3.
M. A. Kazarian 《Astrophysics》2000,43(2):136-140
The results of spectroscopic observations of the nucleus of the S component of the galaxy Kaz 163, carried out on the 6-m
telescope of the Special Astronomical Observatory in 1982, are presented. The equivalent widths, relative intensities, and
half-widths of emission lines were determined. The electron temperature, electron density, and mass of the gaseous component
of the nucleus were also determined. The half-widths and equivalent widths varied almost twofold over the time of observations
from October 31, 1981, to May 28, 1982. During the same time the intensity of the [O III] λ 4363 auroral line increased fairly
strongly.
Translated from Astrofizika, Vol. 43, No. 2, pp. 183-189, April–June, 2000. 相似文献
4.
Gelys Trancho Bryan Miller Begoa García-Lorenzo Sebastin F. Snchez 《New Astronomy Reviews》2006,49(10-12):613
Studies of globular cluster systems play a critical role in our understanding of galaxy formation. Imaging with the Hubble Space Telescope has revealed that young star clusters are formed copiously in galaxy mergers, strengthening theories in which giant elliptical galaxies are formed by the merger of spirals [e.g. Whitmore, B.C., Schweizer, F., Leitherer, C., Borne, K., Robert, C., 1993. Astronomical Journal. 106, 1354; Miller, B.W., Whitmore, B.C., Schweizer, F., Fall, S.M., 1997. Astronomical Journal. 114, 2381; Zepf, S.E., Ashman, K.M., English, J., Freeman, K.C., Sharples, R.M., 1999. Astronomical Journal. 118, 752; Ashman, K.M., Zepf, S.E., 1992. Astrophysical Journal. 384, 50]. However, the formation and evolution of globular cluster systems is still not well understood. Ages and metallicities of the clusters are uncertain either because of degeneracy in the broad-band colors or due to variable reddening. Also, the luminosity function of the young clusters, which depends critically on the metallicities and ages of the clusters, appears to be single power-laws while the luminosity function of old clusters has a well-defined break. Either there is significant dynamical evolution of the cluster systems or metallicity affects the mass function of forming clusters. Spectroscopy of these clusters are needed to improve the metallicity and age measurements and to study the kinematics of young cluster systems. Therefore, we have obtained GMOS IFU data of 4 clusters in NGC1275. We will present preliminary results like metallicities, ages, and velocities of the star clusters from IFU spectroscopy. 相似文献
5.
太阳系探索和地外生命搜索 总被引：1，自引：0，他引：1
综述了国际上进行太阳系空间探测的现状，着重介绍了探测月球，火星，不星和外行星的意义，目的，手段和成就，择要介绍了美国宇般局，欧洲空间局，俄罗斯和日本近年来和下世纪初的空间计划，论及地外生命搜索，主要介绍了火星上水和生命搜索进展，地外生命搜索的的SETI计划及其发民菜，物理学家与生物学家有关搜索成功的可能性的争论，以及太阳系外行星系统的发现，太阳系外行星系统的发现是当代天文学最时髦的，也将是未来21世纪成果最丰富的研究领域之一，目前，已经发现了大约50个太阳系外行星系统，太阳系外行星系统的发现与地外生命搜索研究是密不可分的，这项研究近20年来发展较快，它的研究也促进了航天学，宇宙化学，天文生物学乃至哲学等其化学科的发展。 相似文献
6.
SS433是银河系内一个著名的高能天体,W50是它周围的超新星遗迹,自20世纪70年代末SS433的运动模型建立以来,已经受到了越来越多的关注,取得了丰富的多波段观测资料。但是,直到现在,关于这一系统的一些基本性质和参数还存在相当大的争论。该文介绍了关于SS433研究的某些新进展,主要包括SS433的运动模型和喷流的膨胀冷却模型,SS433的物质损失,各种时标的光变和喷流的结构,并对关于SS433研究的热点问题作了总结与展望。 相似文献
7.
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
8.
Ultraviolet and near-infrared observations of auroral emissions from the footprint of Io's magnetic Flux Tube (IFT) mapping to Jupiter's ionosphere have been interpreted via a combination of the unipolar inductor model [Goldreich, P., Lynden-Bell, D., 1969. Astrophys. J. 156, 59-78] and the multiply-reflected Alfvén wave model [Belcher, J.W., 1987. Science 238, 170-176]. While both models successfully explain the general nature of the auroral footprint and corotational wake, and both predict the presence of multiple footprints, the details of the interaction near Io are complicated [Saur, J., Neubauer, F.M., Connerney, J.E.P., Zarka, P., Kivelson, M.G., 2004. In: Bagenal, F., Dowling, T.E., McKinnon, W.B. (Eds.), Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press, Cambridge, UK, pp. 537-560; Kivelson, M.G., Bagenal, F., Kurth, W.S., Neubauer, F.M., Paranicas, C., Saur, J., 2004. In: Bagenal, F., Dowling, T.E., McKinnon, W.B. (Eds.), Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press, Cambridge, UK, pp. 513-536]. The auroral footprint brightness is believed to be a good remote indicator of the strength of the interaction near Io, indicating the energy and current strength linking Io with Jupiter's ionosphere. The brightness may also depend in part on local auroral acceleration processes near Jupiter. The relative importance of different physical processes in this interaction can be tested as Jupiter's rotation and Io's orbital motion shift Jupiter's magnetic centrifugal equator past Io, leading to longitudinal variations in the plasma density near Io and functionally different variations in the local field strength near Jupiter where the auroral emissions are produced. Initial HST WFPC2 observations found a high degree of variability in the footprint brightness with time, and some evidence for systematic variations with longitude [Clarke, J.T., Ben Jaffel, L., Gérard, J.-C., 1998. J. Geophys. Res. 103, 20217-20236], however the data were not of sufficient quality to determine functional relationships. In this paper we report the results from a second, more thorough study, using a series of higher resolution and sensitivity HST STIS observations and a model for the center to limb dependence of the optically thin auroral emission brightness based on measurements of the auroral curtain emission distribution with altitude. A search for correlations between numerous parameters has revealed a strong dependence between Io's position in the plasma torus and the resulting footprint brightness that persists over several years of observations. The local magnetic field strength near Jupiter (i.e. the size of the loss cone) and the expected north/south asymmetry in auroral brightness related to the path of currents generated near Io through the plasma torus en route to Jupiter appear to be less important than the total plasma density near Io. This is consistent with the near-Io interaction being dominated by collisions of corotating plasma and mass pickup, a long-standing view which has been subject to considerable debate. The brightness of the auroral footprint emissions, however, does not appear to be proportional to the incident plasma density or energy, and the interpretation of this result will require detailed modeling of the interaction near Io. 相似文献
9.
We analyze velocity fields of the Great Red Spot (GRS) and Oval BA that were previously extracted from Cassini, Galileo, and Hubble Space Telescope images (Asay-Davis, X.S., Marcus, P.S., Wong, M., de Pater, I. [2009]. Icarus 203, 164-188). Our analyses use reduced-parameter models in which the GRS, Oval BA, and surrounding zonal (east-west) flows are assumed to have piece-wise-constant potential vorticity (PV), but with finite-sized transition regions between the pieces of constant PV rather than sharp steps. The shapes of the regions of constant PV are computed such that the flow is a steady, equilibrium solution of the 2D quasigeostrophic equations when viewed in a frame translating uniformly in the east-west direction. All parameter values of the models, including the magnitudes of the PV, areas of the regions with constant PV, locations of the transition regions, widths of the transition regions, and the value of the Rossby deformation radius, are found with a genetic algorithm such that the velocity produced by the equilibrium solution is a “best-fit” to the observed velocity fields. A Monte Carlo method is used to estimate the uncertainties in the best-fit parameter values.The best-fit results show that there were significant changes (greater than the uncertainties) in the PV of the GRS between Galileo in 1996 and Hubble in 2006. In particular, the shape of the PV anomaly of the GRS became rounder, and the area of the PV anomaly of the GRS decreased by 18%, although the magnitudes of PV in the anomaly remained constant. In contrast, neither the area nor the magnitude of the PV anomaly of the Oval BA changed from 2000, when its cloud cover was white, to 2006, when its cloud cover was red. The best-fit results also show that the areas of the PV anomalies of the GRS and of the Oval BA are smaller than the areas of their corresponding cloud covers at all times. Using the best-fit values of the Rossby deformation radius, we show that the Brunt-Väisälä frequency is 15% larger at 33°S than at 23°S. As expected (Marcus, 1993), the best-fit results show that the PV of the zonal flow has “jumps” at the latitudes of the maxima of the eastward-going jet streams. However, a surprising result is that a large “jump” in the PV of the zonal flow occurs at the location of a maximum of the westward going jet stream neighboring the GRS. Another surprise is that the jumps in the PV of the zonal flow do not all have the same sign, which implies that there is not a monotonic “staircase” of zonal PV from north to south as was anticipated (
[Marcus, 1993] and [McIntyre, 2008]). 相似文献
10.
G. H. Born E. J. Christensen A. J. Ferrari J. F. Jordan S. J. Reinbold 《Celestial Mechanics and Dynamical Astronomy》1974,9(3):395-414
This paper presents a comprehensive analysis of the Mars orbital phase of the Mariner 9 trajectory as determined from Earth based radio data. Both the method and accuracy of the orbit determination process are reviewed. Analysis is presented to show the effects of Mars gravity model and node in the plane of the sky errors on the accuracy of orbit determination. In addition the long term evolution of the orbit from insertion through the first 500 revolutions is presented, and decomposed into effects from the Mars garvity field,n-body perturbations, and solar radiation pressure. Since the orbit period is nearly commensurable with the Mars rotational period, the orbit experiences significant resonance perturbations. The primary perturbation is in-track with a maximum amplitude of 1000 km and a wavelength of 39 spacecraft revolutions.This paper was presented at the AIAA/AAS Astrodynamics Conference, Palo Alto, California, September 11 and 12, 1972. At this time Mariner 9 operations were still underway. The operational life of Mariner 9 ended October 27, 1972, when the supply of nitrogen gas, used for attitude stabilization, was depleted. This paper represents one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, under NASA Contract No. NAS 7-100. 相似文献
11.
Abstract— Historically, there have been a range of diameter estimates for the large, deeply eroded Vredefort impact structure within the Witwatersrand Basin, South Africa. Here, we estimate the diameter of the transient cavity at the present level of erosion as ～124–140 km, based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted Transvaal outliers. Taking erosion into account (<6 km) and scaling to original final rim diameter, an estimate of close to 300 km for the rim diameter is obtained. Independent estimates of the final rim diameter, based on an empirical relation of central uplift diameter to rim diameter, spatial distribution of pseudotachylites, and concentric large scale structural patterns, give a similar estimate of close to 300 km for the original final rim diameter. An impact structure of this size is expected to have had an original multi-ring form. At this size, the Vredefort impact structure encompasses the bulk of the Witwatersrand Basin, which appears to owe its preservation to the Vredefort impact. In addition, the Vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area, which, in recent interpretations, is believed to be a component in the creation of the world-class gold deposits of the Witwatersrand Basin. 相似文献
12.
13.
G. Cayrel de Strobel 《Astronomy and Astrophysics Review》1996,7(3):243-288
Summary. This review is primarily directed to the question whether photometric solar analogues remain such when subjected to detailed
spectroscopic analyses and interpreted with the help of internal stucture models. In other words, whether the physical parameters:
mass, chemical composition, age (determining effective temperature and luminosity), chromospheric activity, equatorial rotation,
lithium abundance, velocity fields etc., we derive from the spectral analysis of a photometric solar analogue, are really
close to those of the Sun. We start from 109 photometric solar analogues extracted from different authors. The stars selected
had to satisfy three conditions: i) their colour index must be contained in the interval: –0.69, ii) they must possess a trigonometric parallax, iii) they must have undergone a high resolution detailed spectroscopic
analysis. First, this review presents photometric and spectrophotometric researches on solar analogues and recalls the pionneering
work on these stars by the late Johannes Hardorp. After a brief discussion on low and high resolution spectroscopic researches,
a comparison is made between effective temperatures as obtained, directly, from detailed spectral analyses and those obtained,
indirectly, from different photometric relations. An interesting point in this review is the discussion on the tantalilizing
value of the of the Sun, and the presentation of a new reliable value of this index. A short restatement of the kinematic properties of
the sample of solar analogues is also made. And, finally, the observational diagram, obtained with 99 of the initially presented 109 analogues, is compared to a theoretical diagram. This latter has been constructed with a grid of internal structure models for which, (very important for this investigation),
the Sun was used as gauge. In analysing the position, with respect to the Sun, of each star we hoped to find a certain number
of stars tightly neighbouring the Sun in mass, chemical composition and state of evolution. The surprising result is that
the stars occupy in this HR Diagram a rather extended region around the Sun, many of them seem more evolved and older than
the Sun, and only 4 of the evolved stars seem younger. The age of some stars in the sample is also discussed in terms of chromospheric
activity and Li-content. Our conclusion is much the same as that contained in previous papers we have written on the subject:
in spite of a much larger number of stars, we have not been able to nominate a single star of the sample for a “perfect good
solar twin”. Another aim in beginning, 25 years ago, this search for solar analogues, was to have ready a bunch of stars resembling
the Sun and analysed spectroscopically in detail, in order that, when planets hunters of solar type stars, finally would have
found such a specimen, we would have been able to immediately compare the physical parameters of this star to those of the
Sun. We have been lucky enough: one of the good solar analogues we present herewith, is 51 Pegasi (HD 217014) which, according
to the very recent observations by Mayor and Queloz (1995), has a planet orbiting around it. And what is more: two other
stars possessing planets: 47 Ursae Majoris (HD 95128) and 70 Virginis (HD 117176), have just been discovered by Marcy and
Butler (187 Meeting of the AAS, January 1996). One of them, 47 Ursae Majoris, is also included in the list of photometric solar analogues.
The other star, 70 Virginis, has only been included after the “Planets News”, because the colour index of this star is slightly higher than the prescribted limit of the selection, (, instead, 0.69). It would have been a pity to leave the third ” planet star out of the competition. 相似文献
14.
In the present paper, the piston model of the coronal transient (see Hu. 1983a, b is discssed in detail, and the quantitative results of unsteady gasdynamics are applied to the coronal transient processes. The piston model explains the major features of the transient observations, such as the density profile, the geometric configuration, the kinetic process and the classifications of the coronal transient. Based on the idea of piston model, the bright feature and the dark feature of the transient are the gasdynamical response of the dense plasma ejecting into the corona, and associate with the compressed and rarefied flows, respectively. The quantitative results show that the density increment in the compressed region and the density decrement in the rarefied region are one order of magnitude larger and smaller, respectively, to the density in the quiet corona, it agrees quantitatively with the observations, and both the bright feature and dark feature are explained at the same time. 相似文献
15.
16.
Energetic mass outflows have been detected in molecular line observations towards young stellar objects. In this review we take the Orion-KL as an example to discuss the overall structure of a high-velocity outflow and its environment. The kinematics of the high-velocity molecular emission show clear evidence of a bipolar jet which originates in the vicinity of IRc2, a massive protostar. Towards the ends of the jet, 0.05 pc away from the origin of the flow, the interaction between the high-velocity flow and the ambient molecular gas excites shocks. The protostar is encircled by a disc of dense molecular gas, the inner 0.04 pc of which is expanding while the outer part shows signs of rotation and contraction. A comparison between the dynamical timescales of the disk and the bipolar jet may suggest that the disk itself, or some mechanism of disk formation, is also responsible for the bipolar nature of the high-velocity flow.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.NRO, a branch of the Tokyo Astronomical observatory, is a cosmic-radio observing facility open to outside users. 相似文献
17.
We present here the first study of the X-ray properties of an evolutionary sample of merging galaxies. Both ROSAT PSPC and HRI data are presented for a sample of eight interacting galaxy systems, each believed to involve a similar encounter between two spiral discs of approximately equal size. The mergers span a large range in age, from completely detached to fully merged systems.
A great deal of interesting X-ray structure is seen, and the X-ray properties of each individual system are discussed in detail. Along the merging sequence, several trends are evident: in the case of several of the infrared bright systems, the diffuse emission is very extended, and appears to arise from material ejected from the galaxies. The onset of this process seems to occur very soon after the galaxies first encounter one another, and these ejections soon evolve into distorted flows. More massive extensions (perhaps involving up to 1010 M⊙ of hot gas) are seen at the 'ultraluminous' peak of the interaction, as the galactic nuclei coalesce.
The amplitude of the evolution of the X-ray emission through a merger is markedly different from that of the infrared and radio emission, however. Although the X-ray luminosity rises and falls along the sequence, the factor by which the X-ray luminosity increases, relative to the optical, appears to be only about a tenth of that seen in the far-infrared. This, we believe, may well be linked with the large extensions of hot gas observed.
The late, relaxed remnants appear relatively devoid of gas, and possess an X-ray halo very different from that of typical ellipticals, a problem for the 'merger hypothesis', whereby the merger of two disc galaxies results in an elliptical galaxy. However, these systems are still relatively young in terms of total merger lifetime, and they may still have a few Gyr of evolution to go through before they resemble typical elliptical galaxies. 相似文献
A great deal of interesting X-ray structure is seen, and the X-ray properties of each individual system are discussed in detail. Along the merging sequence, several trends are evident: in the case of several of the infrared bright systems, the diffuse emission is very extended, and appears to arise from material ejected from the galaxies. The onset of this process seems to occur very soon after the galaxies first encounter one another, and these ejections soon evolve into distorted flows. More massive extensions (perhaps involving up to 10
The amplitude of the evolution of the X-ray emission through a merger is markedly different from that of the infrared and radio emission, however. Although the X-ray luminosity rises and falls along the sequence, the factor by which the X-ray luminosity increases, relative to the optical, appears to be only about a tenth of that seen in the far-infrared. This, we believe, may well be linked with the large extensions of hot gas observed.
The late, relaxed remnants appear relatively devoid of gas, and possess an X-ray halo very different from that of typical ellipticals, a problem for the 'merger hypothesis', whereby the merger of two disc galaxies results in an elliptical galaxy. However, these systems are still relatively young in terms of total merger lifetime, and they may still have a few Gyr of evolution to go through before they resemble typical elliptical galaxies. 相似文献
18.
BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. This implies the existence of a parent population of sources with a misaligned jet that have been identified with low-power radiogalaxies. The spectrum of BL Lacs, dominated by non-thermal emission over the whole electromagnetic range, together with bright compact radio cores, high luminosities, rapid and large amplitude flux variability at all frequencies and strong polarization makes these sources an optimal laboratory for high energy astrophysics. A most distinctive characteristic of the class is the weakness or absence of spectral lines, that historically hindered the identification of their nature and ever thereafter proved to be a hurdle in the determination of their distance. In this paper, we review the main observational facts that contribute to the present basic interpretation of this class of active galaxies. We overview the history of the BL Lac objects research field and their population as it emerged from multi-wavelength surveys. The properties of the flux variability and polarization, compared with those at radio, X-ray and gamma-ray frequencies, are summarized together with the present knowledge of the host galaxies, their environments, and central black hole masses. We focus this review on the optical observations, which played a crucial role in the early phase of BL Lacs studies, and in spite of extensive radio, X-ray, and recently gamma-ray observations, could represent the future major contribution to the unveiling of the origin of these sources. In particular, they could provide a firm conclusion on the long debated issue of the cosmic evolution of this class of active galactic nuclei and on the connection between formation of supermassive black holes and relativistic jets. 相似文献
19.
Saturn’s rings host two known moons, Pan and Daphnis, which are massive enough to clear circumferential gaps in the ring around their orbits. Both moons create wake patterns at the gap edges by gravitational deflection of the ring material (Cuzzi, J.N., Scargle, J.D. [1985]. Astrophys. J. 292, 276-290; Showalter, M.R., Cuzzi, J.N., Marouf, E.A., Esposito, L.W. [1986]. Icarus 66, 297-323). New Cassini observations revealed that these wavy edges deviate from the sinusoidal waveform, which one would expect from a theory that assumes a circular orbit of the perturbing moon and neglects particle interactions. Resonant perturbations of the edges by moons outside the ring system, as well as an eccentric orbit of the embedded moon, may partly explain this behavior (Porco, C.C., and 34 colleagues [2005]. Science 307, 1226-1236; Tiscareno, M.S., Burns, J.A., Hedman, M.M., Spitale, J.N., Porco, C.C., Murray, C.D., and the Cassini Imaging team [2005]. Bull. Am. Astron. Soc. 37, 767; Weiss, J.W., Porco, C.C., Tiscareno, M.S., Burns, J.A., Dones, L. [2005]. Bull. Am. Astron. Soc. 37, 767; Weiss, J.W., Porco, C.C., Tiscareno, M.S. [2009]. Astron. J. 138, 272-286). Here we present an extended non-collisional streamline model which accounts for both effects. We describe the resulting variations of the density structure and the modification of the nonlinearity parameter q. Furthermore, an estimate is given for the applicability of the model. We use the streamwire model introduced by Stewart (Stewart, G.R. [1991]. Icarus 94, 436-450) to plot the perturbed ring density at the gap edges.We apply our model to the Keeler gap edges undulated by Daphnis and to a faint ringlet in the Encke gap close to the orbit of Pan. The modulations of the latter ringlet, induced by the perturbations of Pan (Burns, J.A., Hedman, M.M., Tiscareno, M.S., Nicholson, P.D., Streetman, B.J., Colwell, J.E., Showalter, M.R., Murray, C.D., Cuzzi, J.N., Porco, C.C., and the Cassini ISS team [2005]. Bull. Am. Astron. Soc. 37, 766), can be well described by our analytical model. Our analysis yields a Hill radius of Pan of 17.5 km, which is 9% smaller than the value presented by Porco (Porco, C.C., and 34 colleagues [2005]. Science 307, 1226-1236), but fits well to the radial semi-axis of Pan of 17.4 km. This supports the idea that Pan has filled its Hill sphere with accreted material (Porco, C.C., Thomas, P.C., Weiss, J.W., Richardson, D.C. [2007]. Science 318, 1602-1607). A numerical solution of a streamline is used to estimate the parameters of the Daphnis-Keeler gap system, since the close proximity of the gap edge to the moon induces strong perturbations, not allowing an application of the analytic streamline model. We obtain a Hill radius of 5.1 km for Daphnis, an inner edge variation of 8 km, and an eccentricity for Daphnis of 1.5 × 10−5. The latter two quantities deviate by a factor of two from values gained by direct observations (Jacobson, R.A., Spitale, J., Porco, C.C., Beurle, K., Cooper, N.J., Evans, M.W., Murray, C.D. [2008]. Astron. J. 135, 261-263; Tiscareno, M.S., Burns, J.A., Hedman, M.M., Spitale, J.N., Porco, C.C., Murray, C.D., and the Cassini Imaging team [2005]. Bull. Am. Astron. Soc. 37, 767), which might be attributed to the neglect of particle interactions and vertical motion in our model. 相似文献
20.
Mercury's neutral sodium exosphere is simulated using a comprehensive 3D Monte Carlo model following sodium atoms ejected from Mercury's surface by thermal desorption, photon stimulated desorption, micro-meteoroid vaporization and solar wind sputtering. The evolution of the sodium surface density with respect to Mercury's rotation and its motion around the Sun is taken into account by considering enrichment processes due to surface trapping of neutrals and ions and depletion of the sodium available for ejection from the surfaces of grains. The change in the sodium exosphere is calculated during one Mercury year taking into account the variations in the solar radiation pressure, the photo-ionization frequency, the solar wind density, the photon and meteoroid flux intensities, and the surface temperature. Line-of-sight column densities at different phase angles, the supply rate of new sodium, average neutral and ion losses over a Mercury year, surface density distribution and the importance of the different processes of ejection are discussed in this paper. The sodium surface density distribution is found to become significantly nonuniform from day to night sides, from low to high latitudes and from morning to afternoon because of rapid depletion of sodium atoms in the surfaces of grains mainly driven by thermal depletion. The shape of the exosphere, as it would be seen from the Earth, changes drastically with respect to Mercury's heliocentric position. High latitude column density maxima are related to maxima in the sodium surface concentration at high latitudes in Mercury's surface and are not necessarily due to solar wind sputtering. The ratio between the sodium column density on the morning side of Mercury's exosphere and the sodium column density on the afternoon side is consistent with the conclusions of Sprague et al. (1997, Icarus 129, 506-527). The model, which has no fitting parameters, shows surprisingly good agreement with recent observations of Potter et al. (2002, Meteor. Planet. Sci. 8, 3357-3374) successfully explaining their velocity and column density profiles vs. heliocentric distance. Comparison with this data allows us to constrain the supply rate of new sodium atoms to the surface. We also discuss the possible origins of the strong high latitude emissions (Potter and Morgan, 1990, Science 248, 835-838; 1997a, Adv. Space Res. 19, 1571-1576; 1997b, Planet. Space Sci. 45, 95-100; Sprague et al., 1998, Icarus 135, 60-68) and the strong variations of the total content of the sodium exosphere on short (Potter et al., 1999, Planet. Space Sci. 47, 1441-1449) and long time scales (Sprague et al., 1997, Icarus 129, 506-527). 相似文献