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1.
Chen and Shawhan (1978) observed the brightness distribution of solar radio bursts at 26 MHz and fitted it to a double gaussian model. The small angle and large angle components were labelled core and halo. As an alternative to the scattering theory by coronal inhomogeneities, the authors introduced an interpretation of these components as the primary source and its reflected image on lower layers of the corona. We stress the difficulties with this interpretation because the corona is indeed very far from being spherically symmetric; the observed source structure may be due to a coronal scattering process involving both weak and strong inhomogeneities. But first of all we point out a relation between the halo dimensions and the local time which casts some doubt on the solar origin of the halo; we argue that it might result at least partially from instrumental or more probably from ionospheric effects.Laboratoire Associé du CNRS No. 264. 相似文献
2.
M. G. Abrahamyan 《Astrophysics》2002,45(2):205-214
The stability of Riemann S ellipsoids inside an oblate halo with respect to the second form of oscillations is investigated. It is shown that some ellipsoids with reverse internal circulation of matter, which are stable inside a spherical halo or in its absence, become unstable with respect to second odd forms of oscillation inside an oblate halo. Here there is asymmetry between conjugate ellipsoids from the standpoint of their stability. Only those conjugate ellipsoids that correspond to higher frequencies of reverse circulation of matter than their corresponding angular rotation rates are unstable. The domains of instability of light and heavy conjugate embedded ellipsoids are obtained as a function of the oblateness measure and relative density of the halo. 相似文献
3.
The influence of the chromospheric condensation on H line profiles for the thermal model of a solar flare has been empirically studied in this paper. The so-called thermal model here means that there is no temperature increase relative to the quiet-Sun chromosphere but with a chromospheric condensation in the lower part of its transition region, which case is assumed to represent the early stage of the impulsive phase. The main results include: when the temperature within the condensation region is assumed to be equal to that in front of it, the influence is to create an additional absorption profile overlapping on the original one; by increasing the condensation strength, the H line profile changes from a little line-center increase to broadened red asymmetry, then to the reversed red asymmetry, and finally to two independent absorption profiles; the thickness of the condensation determines the absorption of the additional profile; descending the transition region has no obvious effect on the basic characteristic of the H line profile except a little increase in the line center. Assuming that the temperature within the condensation is higher than that ahead of the condensation, the calculated H line profiles may be strong enough to be comparable with the observations. This means that if the condensation in the purely thermal model can reach a higher temperature, we may also use only thermal origin to explain a chromospheric flare.Alexander von Humboldt Research Fellow, on leave from Purple Mountain Observatory, Nanjing, China. 相似文献
4.
We present some results of our study on non-equilibriumN-body systems that undergo initial collapse. This collapse is followed by global pulsations of the system. During these pulsations, the mean gravitational field fluctuates violently. Some particles pick up enough energy to be ejected from the system into the halo or even fly off to infinity. We discuss the region in phase space from which these mean field escapers originate and their energy frequency distribution for the illustrative case of a uniform spherical initial state. The pulsations lead to the production of shells in the halo.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain. 相似文献
5.
A. Böhme F. Fürstenberg J. Hildebrandt O. Saal A. Krüger P. Hoyng G. A. Stevens 《Solar physics》1977,53(1):139-155
A two-component (core-halo) emission model has been applied reconciling hard and soft X-ray burst emissions with the microwave burst radiation. The core region is represented by a nonthermal energy distribution (Maxwellian+power law tail) and assumed to be surrounded by a thermal halo. Parameters characterizing the energy distribution and emission measures have been derived numerically from soft and hard X-ray measurements. Using an artificial magnetic field model the microwave flux spectrum has been calculated on the basis of gyro-synchrotron emission and absorption by solving the equation of radiation transfer along the ray trajectories. Open parameters were used to adapt the spectrum to the radio measurements.Thus probable informations about the most appropriate magnetic field parameters as well as about the time- and frequency- dependent source diameters (yielding growth velocities of the core region during the impulsive phase) are deduced for the burst of 1972 May 18 as an example. A fit of the observed spectrum at the burst maximum is consistent with a magnetic field of 150O G at the core centre decreasing up to about 40 G at the top of the halo at a height of 50 000 km above the centre, a core density of 1010 cm–3 decreasing to 109 cm–3 at the outer halo boundary, and a core diameter of 15 000 km (]20).Due to the simple geometry and emission process adopted,- the model refers primarily to special impulsive bursts. For the representation of broad band microwave bursts, e.g. type IV , events, a more complex source geometry and/or other variants of the emission mechanism must be invoked. 相似文献
6.
Jon M. Wallace 《Astrophysics and Space Science》1980,68(1):27-48
A diffusion model for the propagation of relativistic nuclear cosmic rays in the Galaxy is developed. The model has two nonstandard features: The escape of cosmic-ray particles from the Galaxy is simulated by a term in the diffusion equations, rather than the imposition of boundary conditions on the diffusion solution at the surface of the confinement region. And an age-dependent, locally-averaged effective gas distribution is employed in the diffusion equations. The model simulates free-particle outflow at the Galactic boundary. The model is fit to chemical composition data in the 0.3–5 GeV per nucleon range. It is then consistent with the large-scale Galactic -ray data, radio halo data, energy constraints on the assumed supernova sources, and, when extended to very high energies, cosmic-ray anisotrophy data. From the fit we conclude that the cosmic rays are confined in a large flattened or quasis-pherical halo with a scale height in the range 3–6 kpc and an average Galactic escape time of 108 yr. 相似文献
7.
G.B. Sholomitskii 《Astrophysics and Space Science》1997,252(1-2):177-195
It is shown that the high redshift intergalactic gas that probably contains most of the baryonic density b = 0.05(Ho = 75kms-1 Mpc-1) in the standard Cold Dark Matter model can be detected through "true" intergalactic Thompson-scattered halos or light echoes around isotropic radio-loud quasars and radio galaxies. These cosmological halos, which form in the uniform ionized medium around "old" non-evolving sources, have a very large degree of polarization, up to pmax 44 percent at a projected distance half the light-age radius, and a plateau in the annular polarized flux density within 1/3 of the light-age radius, which is about 4 arc min for sources older than 107 years at z 4. The optimal wavelength range for halo polarimetric observations is 20 - 30 cm, depending on the Galactic rotation measure in the direction of the source, the spectral index of the source, and the specific maximized parameter of the halo. If observed with large single-dish radiotelescopes, the 21 cm polarized brightness temperatures of some inner halos, for example the halo around the quasar OQ 172 (S1400 = 2.5 Jy,z = 3.53), are of the same order of magnitude as the current total-intensity limits for cosmic background fluctuations on arc min scales. The halo test can be extended to larger volumes of space by concentric co-adding of source-normalized halos around the much more numerous isotropic radio galaxies. The expected ensemble-averaged profile of the polarized flux density around the symmetry center is calculated by integrating over the halo ages, assuming a uniform source-age distribution. In addition to the electron density or a lower limit for b based on the halo brightness, the characteristic life time of the radio emission can be derived based on its angular size. If there is a moderate deviation of the density evolution of the intergalactic gas from the conservative cubic law due to galaxy formation, high redshift radio halos may be on the rising branch of their brightness beyond its minimum, similar to the well known nonmonotonic behaviour of angular sizes. 相似文献
8.
On the basis of a globular cluster study a crude estimate of the total mass of the galactic halo within 20 kpc from the centre is done. It gives a minimal halo mass of the order of
, yielding possibilities for a mass as large as
. The content of the interstellar matter in the halo is estimated too. It is found that the gas content is a few percents the minimal mass, the gas temperature is very high — about 1×106 K, the magnetic field weak — about 0.25 nT. A weak nonthermal radio emission might be expected from such a halo. 相似文献
9.
F. De Paolis G. Ingrosso Ph. Jetzer M. Roncadelli 《Astrophysics and Space Science》1996,235(2):329-335
In addition to Massive Astrophysical Compact Halo Objects (MACHOs), as detected by microlensing, cold molecular clouds (mainly ofH
2) may well contribute substantially to the galactic halo dark matter. Here, we argue that the existence of such halo clouds can be inferred from the observation of absorption lines (due to heavy molecules located in the clouds themselves) towards stars of the Large Magellanic Cloud, which lie very close, within 1, to a previously microlensed one.The possibility of clusters of MACHOs has been investigated by several authors (see e.g. Carr and Lacey, 1987; Ashman, 1990; Eichler and Silk, 1992 and Wasserman and Salpeter, 1994). 相似文献
10.
We propose a model of chemical evolution of the galactic halo which consists of a succession of two different evolutionary stages; each stage is characterized by different outflow rate of gas from the star-forming region so that different metal-enrichment rate is resulted. The low-metal stars with [Fe/H]<–0.8 are formed mainly during the first 3×108 yr, and most of the high-metal stars with [Fe/H]–0.8 are formed during the succeeding 2×109 yr. This model naturally explains the metallicity distribution of globular clusters in the galactic halo including both the metal-rich and the metal-poor clusters. We also discuss the implications of the present model on the formation and evolution of the galactic halo. 相似文献
11.
It is shown that Mie theory predictions of extinction for pure water-ice with the optical constant measured at 100 K do not fit in detail the observed ice absorption feature in infrared objects, although we attempt to explain the observations by considering size distribution and shape of the grains.In addition, based on a similarity between the ice band and the absorption band found in carbon stars, we feel it is questionable whether or not the ice band can really be attributed to the interstellar water-ice. 相似文献
12.
The discussion of the problems of observation of tenuous intergalactic and/or halo clouds in recombination emission is presented. We elaborate the possibility that clouds are illuminated only by metagalactic ultraviolet radiation flux, and assume some plausible values for the parameters determining the physical conditions in such objects. A simple, semiqualitative approach may give specific observational predictions for the two major signposts of fluorescent emission: Ly and H lines. We conclude that H detection of Lyman-limit systems at low redshift is within present observational capabilities. Open questions for further research are put in a convenient framework. 相似文献
13.
We recently found that the halo of the Milky Way contains a large reservoir of warm-hot gas that accounts for large fraction of the missing baryons from the Galaxy. The average physical properties of this circumgalactic medium (CGM) are determined by combining average absorption and emission measurements along several extragalactic sightlines. However, there is a wide distribution of both, the halo emission measure and the O?vii column density, suggesting that the Galactic warm-hot gaseous halo is anisotropic. We present Suzaku observations of fields close to two sightlines along which we have precise O?vii absorption measurements with Chandra. The column densities along these two sightlines are similar within errors, but we find that the emission measures are different: 0.0025±0.0006 cm?6?pc near the Mrk 421 direction and 0.0042±0.0008 cm?6?pc close to the PKS 2155-304 sightline. Therefore the densities and pathlengths in the two directions must be different, providing a suggestive evidence that the warm-hot gas in the CGM of the Milky Way is not distributed uniformly. However, the formal errors on derived parameters are too large to make such a claim. In the Mrk 421 direction we derive the density of \(1.6^{+2.6}_{-0.8} \times 10^{-4}~\mbox{cm}^{-3}\) and pathlength of \(334^{+685}_{-274}~\mbox{kpc}\) . In the PKS 2155-304 direction we measure the gas density of \(3.6^{+4.5}_{-1.8} \times10^{-4}~\mbox{cm}^{-3}\) and path-length of \(109^{+200}_{-82}~\mbox{kpc}\) . Thus the density and pathlength along these sightlines are consistent with each other within errors. The average density and pathlength of the two sightlines are similar to the global averages, so the halo mass is still huge, over 10 billion solar masses. With more such studies, we will be able to better characterize the CGM anisotropy and measure its mass more accurately. We can then compare the observational results with theoretical models and investigate if/how the CGM structure is related to the larger scale environment of the Milky Way. We also show that the Galactic disk makes insignificant contribution to the observed O?vii absorption; a similar conclusion was also reached by Henley and Shelton (2013) about the emission measure. We further argue that any density inhomogeneity in the warm-hot gas, be it from clumping, from the disk, or from a non-constant density gradient, would strengthen our result in that the Galactic halo path-length and the mass would become larger than what we estimate here. As such, our results are conservative and robust. 相似文献
14.
The relativistic spherical accretion onto black holes is considered. It is shown that electron heat conductivity and ion viscosity increase the ion temperature by roughly two orders of magnitude over the electron temperature at Schwarzschild radius. As a result, the knee arise in gamma-radiation spectrum at 10–30 MeV. The role of magnetic field, electron-positron pairs production and generation of absorption lines in spectrum briefly discussed. 相似文献
15.
Models of spherically-symmetric static systems made up of self-gravitating, completely degenerate neutral fermions containing a core are constructed within the framework of general relativity and the effects of different core masses and compactness on the properties of the system are examined. For the specific case where the fermions are massive neutrinos (10 eV) we find, for example, that it is possible to have a neutrino halo with a normal Galaxy, or a cluster of galaxies, as the core, with the right values of mass and radius required of the invisible halo in the missing mass problem. The suggestive nature of these results calls for further studies using a more realistic equation of state. 相似文献
16.
Gernot Westphalen Peter M.W. Kalberla Dap Hartmann W. Butler Burton 《Astrophysics and Space Science》1997,252(1-2):289-300
The Leiden/Dwingeloo Survey (LDS) offers new possibilities for analyzing Galactic Hi with an outstanding sensitivity. The survey data have been carefully corrected for side-lobe contamination of the antenna and for baseline effects. At present this survey is the most reliable database for analysis of faint, large-scale Hi features. Together with the longstanding dispute whether the Galactic halo is hot or cold, this motivated our investigations which are described in this paper.We have improved the stray-radiation correction procedure significantly by including reflections from the ground. Hi-gas with an unusually large velocity dispersion (LVD) is revealed when these enhanced LDS Data are massively integrated. Gaussian decomposition of more than 250 integrated profiles for b > 20° yields a complete set of 8500 Hi-components representing the north galactic sky on 10° × 10° fields. LVD components were found in every direction of the sky having a characteristic dispersion of 60 kms-1 and column densities of 1.4 · 1019 cm-2. We do not detect the Hi-gas which is associated with the "Lockman-Layer" ( 35 kms-1) and conclude therefore that his analysis was biased by instrumental effects. Correction of this bias in the Bell Survey data set makes the "Lockman-Layer" disappear and does show the LVD component reported here.The LVD gas exhibits minimal sub-rotation and extends several kpc into the Galactic halo. Its scale height is calculated to be 2 kpc and a non-uniform distribution with respect to the distance from the Galactic center is found. The distribution of the LVD gas is presented. Theoretical spectra are calculated from a simple model of the LVD halo and compared to the real data. The LVD gas seems to be a very sensitive indicator of violent disk phenomena. Two possible Galactic chimneys and a peculiar local velocity field are identified. Implications for the modelling of a Galactic halo with various components are discussed. Strict application of the principle, that the turbulent gas pressure plus magnetic and cosmic ray pressure equals the gravitational pull, leads to a stable halo, extending up to z = 3.3 kpc. 相似文献
17.
A model of the galaxy is constructed and evolved in which the integrated influence of stellar and supernova nucleosynthesis on the composition of the interstellar gas is traced numerically. Our detailed assumptions concerning the character of the matter released from evolving stars and supernovae are guided by the results of recent stellar evolutionary calculations and hydrodynamic studies of supernova events. Stars of main sequence mass in the range 4M8M
are assumed to give rise to supernova events, leaving remnants we identify with neutron stars and pulsars and forming both the carbon-to-iron nuclei and ther-process heavy elements in the explosive ejection of the core material. For more massive stars, we assume the core implosion will result in the formation of a Schwarzschild singularity, that is, a black hole or collapsar. The straightforward assumptions (1) that the gas content of the galaxy decreases exponentially with time to its present level of 5% and (2) that the luminosity function characteristic of young clusters and the solar neighborhood is appropriate throughout galactic history, lead to the prediction that 20% of the unevolved stars of approximately one solar mass (M
) in the galaxy today should have metal compositionsZ0.1Z
. As Schmidt has argued from similar reasoning, this is quite inconsistent with current observations; an early generation dominated by more massive stars—which would by now have evolved—is suggested by this difficulty. Many of these massive stars, according to our assumptions, will end their lives as collapsed black hole remnants. It is difficult to visualize an epoch of massive star formation in the collapsing gas cloud which formed our galaxy which would enrich the gas rapidly enough to account for the level of heavy element abundances in halo population stars; we have therefore proposed a stage of star formation which is entirely pregalactic in character. We suggest that the Jeans' length-sized initial condensations in the expanding universe discussed by Peebles and Dicke may provide the appropriate setting for this first generation of stars. Guided by these considerations, and by the need for a substantial quantity of unseen mass to bind our local group of galaxies, we have constructed a model of the galaxy in which this violent early phase of massive star formation produces both (1) approximately 25% of the level of heavy elements observed in the solar system and (2) an enormous unseen mass in the form of black holes. The implications of our model for other features of the galaxy, including supernova nucleosynthesis, the cosmic ray production of the light elements, and cosmochronology, are discussed in detail. 相似文献
18.
R. H. Miller 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):307-324
Numerical experiments undertaken to investigate the longevity and behavior of dark-lane elliptical galaxies are described. This is dynamically the same problem as a disk galaxy in a massive halo. Spiral galaxies are disks from a dynamical point of view. A disk of particles embedded in a self-consistent galaxy provides the basic model used for the experiments. This model is applicable to ordinary disk galaxies if the disk is interpreted as the visible galaxy and the galaxy is interpreted as the massive halo thought to be present around disk galaxies. Fully three-dimensional fully self-consistentn-body computer programs that can handle 100,000 particles are used for the experiments. The background galaxy is oblate, and the disk is inclined to the axis of the oblate galaxy, so the disk precesses differentially to produce a warp. A surprising result is that the galaxy center shifted, leaving the disk center orbiting around the galaxy center. This produces interesting phenomena reminiscent of observations in the region of the Galactic center. 相似文献
19.
The broad 2800 Å absorption feature discovered by Karim, Hoyle, and Wickramasingle is instrumental in origin. The feature is produced by IUE detector saturation effects in overexposed spectra. 相似文献
20.
Study of the transfer from the Earth to a halo orbit around the equilibrium pointL
1 总被引：2，自引：0，他引：2
G. Gómez A. Jorba J. Masdemont C. Simó 《Celestial Mechanics and Dynamical Astronomy》1993,56(4):541-562
The purpose of this paper is to study a transfer strategy from the vicinity of the Earth to a halo orbit around the equilibrium pointL
1 of the Earth-Sun system. The study is done in the real solar system (we use the DE-118 JPL ephemeris in the simulations of motion) although some simplified models, such as the restricted three body problem (RTBP) and the bicircular problem, have been also used in order to clarify the geometrical aspects of the problem. The approach used in the paper makes use of the hyperbolic character of the halo orbits under consideration. The invariant stable manifold of these orbits enables the transfer to be achieved with, theoretically, only one manoeuvre: the one of insertion into the stable manifold. For the total v required, the figures obtained are similar to the ones given by the standard procedures of optimization. 相似文献