共查询到20条相似文献，搜索用时 46 毫秒
1.
D. I. Nagirner 《Astrophysics》1994,37(1):69-78
A new method is presented for calculating spectral line radiation transport in a homogeneous circular cylinder with infinitely long axis for isotropic scattering with complete frequency redistribution or no change in frequency. In addition to the cylinder, scattering in a layer symmetric about its midpoint and in a sphere are also studied. Equations are derived which are satisfied by the resolvent of the fundamental integral equation describing these cases of scattering. By applying a Hankel transform over a finite interval, the resolvent is expressed in terms of an auxiliary function. An equation is derived for this auxiliary function which is easily soluble by iteration, with the speed of its convergence increasing with growth in the radius of the scattering region.Translated from Astrofizika, Vol. 37, No. 1, pp. 111–127, January–March, 1994.This study was carried out with financial support from the Russian Fund for Fundamental Studies (project 93-02-2957). 相似文献
2.
A probabilistic model for solving transfer problems in non-homogeneous, isotropic, and non-coherent scattering cylindrical shell media has been proposed. The source function is considered to be frequency independent. The scattering and transmission functions have been defined for the case of complete redistribution in frequency. A tractable integrodifferential equation for the scattering function has been derived. 相似文献
3.
S. I. Grachev 《Astrophysics》2000,43(1):70-86
In the first of the series of papers by Ivanov et al. it was shown that the model problem of the transfer of polarized radiation as a result of resonance scattering from two-level atoms in a homogeneous plane atmosphere in the absence of LTE comes down, in the approximation of complete frequency redistribution, to the solution of an integral matrix equation of the Wiener-Hopf type for a (2 × 2) matrix source function S(τ). In the second paper in this series, devoted to the vector Milne problem, complete asymptotic expansions of the matrix I(z) [which is essentially a Laplace transform of the matrix S(τ)] for the case of a Doppler profile of the coefficient of absorption, and the coefficients of asymptotic expansions of S(τ) (τ » 1) are expressed in terms of coefficients of the expansions of I(z). We show that asymptotic expansions of S(τ) can be found directly from an integral matrix equation of the Wiener-Hopf type for S(τ). We give new recursive equations for the coefficients of these expansions, as well as a new derivation of asymptotic expansions of the matrix I, including its second column, which was considered only briefly by Ivanov et al. 相似文献
4.
Multiple resonance scattering of spectral line radiation is examined in atmospheres with uniformly distributed sources of
unpolarized radiation. It is assumed that the profile of the absorption coefficient is lorentzian and that scattering involves
complete frequency redistribution. The polarization characteristics of the emerging radiation are determined by iterative
solution of a nonlinear Ambartsumyan-Chandrasekhar matrix integral equation. In particular, it is found that for pure scattering
the maximum polarization at the limb of the disk is 5.421%. The polarization characteristics of the emerging radiation are
compared for three different absorption profiles: Lorentz, Doppler, and rectangular (monochromatic radiation).
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Translated from Astrofizika, Vol. 50, No. 2, pp. 199–217 (May 2007). 相似文献
5.
In this paper we describe a self-contained method for performing the spectral-imaging deconvolution of X-ray data on clusters of galaxies observed by the ASCA satellite. Spatially resolved spectral studies of data from this satellite require such a correction, because its optics redistribute photons over regions that are of comparable size to the angular scales of interest in clusters. This scattering is a function not only of spatial position but also of energy. To perform a correction for these effects we employ maximum-likelihood deconvolution of the image (within energy bands of 1 keV) to determine the spatial redistribution, followed by a Monte Carlo energy reassignment of photon energies with position to determine the spectral redistribution. We present tests on simulated cluster data, convolved with the various instrumental characteristics and the X-ray background, which show that our methodology can successfully recover a variety of intrinsic temperature profiles in typical observational circumstances. In Paper II (this issue) we shall apply our spectral-imaging deconvolution procedure to a large sample of galaxy clusters to determine temperature profiles. 相似文献
6.
A comparison of mean number of scatterings and escape probabilities has been made in isotropic scattering and dipole scattering
by using the angle-averaged partial frequency redistribution functionR
I. We have solved the equations of radiative transfer and statistical equilibrium simultaneously in a spherically symmetric
expanding atmosphere. Two cases of atmospheric extension (i.e.)B/A=3 and 10 (whereB andA are the outer and inner radii of the atmosphere) have been treated.
We find that the partial frequency redistribution gives a larger mean number of scatterings compared to that given by complete
redistribution. Velocities tend to reduce the mean number of scatterings and in crease the mean escape probabilities. 相似文献
7.
S. I. Grachev 《Astrophysics》1978,14(1):63-69
Conclusions Besides the problems considered here, one can find comparatively easily asymptotic solutions in a problem with a spherically symmetric exponential distribution of the primary sources, and also in the case when the primary sources are distributed uniformly within a sphere of finite radius. However, we do not give these solutions because they are cumbersome. Note that some of these problems may have direct astrophysical applications. For example, the problem of a point source can arise in the determination of the source function in the problem of the emission of a planetary nebula.The results of the present work, and also of the majority of the other investigations have been obtained under the assumption of complete frequency redistribution for scattering in the comoving coordinate system. In reality, this assumption is not strictly satisfied. However, calculations made recently for moving media [13, 14] have shown that the results found for true and complete redistribution differ little from one another, these differences being less than in the case of a medium which is not in motion.A. A. Zhdanov Leningrad State University. Translated from Astrofizika, Vol. 14, No. 1, pp. 111–121, January–March, 1978. 相似文献
8.
H. A. Harutyunian 《Astrophysics》2006,49(3):405-414
Elementary events of photon scattering by a chaotically moving monoenergetic electron gas are examined to obtain the redistribution
function (RF). It is pointed out that in many problems and, in particular, the interaction of background radiation with the
intergalactic plasmas of rich galactic clusters, it is sufficient to have the angle-averaged RF. Expressions are obtained
for the averaged RF and its asymptotes which describe the redistribution function for low frequency radiation. The question
of determining the frequencydependent absorption coefficient is discussed.
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Translated from Astrofizika, Vol. 49, No. 3, pp. 475–485 (August 2006). 相似文献
9.
V. M. Loskutov 《Astrophysics》2011,54(3):421-428
The limiting polarization of a resonance line is examined for standard radiative transfer of polarized radiation in a semi-infinite
scattering atmosphere with complete frequency redistribution. Two families of profiles of the line absorption coefficient,
which are generalizations of Lorentz and Doppler profiles, are examined. It is shown that for both families this parameter
approaches the Sobolev-Chandrasekhar limit when the fraction of absorption within the frequency interval (expressed in appropriate
units) from −1 to 1 relative to the total absorption in the line approaches unity. 相似文献
10.
We discuss a model for the formation of the chromospheric Ca ii K line which does not make the usual assumption of complete redistribution. Using a physically reasonable scattering model, we find significant departures due to the frequency dependence of the line source function, particularly in the relative intensity and centre-to-limb behaviour of the K1 parts of the line and in the asymmetry produced by differential velocity fields. We conclude that the frequency dependence of the K line source function must be considered in quantitative models for the formation of the K line. 相似文献
11.
A. Peraiah 《Astrophysics and Space Science》1978,58(1):189-205
The effects of partial redistribution of frequency on the formation of spectral lines in a static and spherically symmetric media have been investigated. The partial redistribution functionsR
I andR
II (Hummer, 1962) have been employed to calculate the lines for a two-level atom in non-LTE in a spherically symmetric medium with homogenous physical characteristics whose ratiosB/A (of outer to inner radii) are equal to 2 and 10. These results are compared with those formed in a plane-parallel medium withB/A=1. Two types of atmosphere are treated: (1) a pure scattering medium with =0 and =0, and (2) an atmosphere with a constant source of emission =10–4 and =0, where is the probability per scatter that a photon will be destroyed by collisional de-excitation and is the ratioK
c/K
l of opacity due to continuous absorption per unit interval of frequency to that in the line. Lines formed in complete redistribution also have been calculated for the sake of comparison, and the total optical depth in all cases has been taken to be 103 at the line centre.Vast differences have been found between the lines formed by complete and partial redistribution functions (which, for the sake of simplicity, we shall hereafter refer to as CRD and PRD, respectively). In the case of a purely scattering medium, a small amount of emission is observed in the wings for all cases of scattering functions in the spherical medium as a result of the combined effects of curvature and physical scattering. In the scattering medium, more photons are scattered into the cores of the lines by PRD than in the case of CRD. The lines formed in the medium with internal sources show emission in all cases with small absorption in the cores, except those lines formed by the angle-dependent PRD functions which again depend on the geometrical extension of the medium. 相似文献
12.
Hydrogen Lyman α (Lyα) is our primary emission-line window into high-redshift galaxies. Despite an extensive literature, Lyα radiative transfer in the most realistic case of a dusty, multiphase medium has received surprisingly little detailed theoretical attention. We investigate Lyα resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Lyα surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. Our fitting formulae can be readily adapted for fast radiative transfer in numerical simulations. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. Our simple analytic model shows good agreement with full Monte Carlo simulations. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption, , and we provide fitting formulae for several geometries of astrophysical interest. We consider the following two interesting applications. (i) Equivalent widths ( EWs ). Lyα can preferentially escape from a dusty multiphase interstellar medium if most of the dust lies in cold neutral clouds, which Lyα photons cannot penetrate. This might explain the anomalously high EWs sometimes seen in high-redshift/submillimetre sources. (ii) Multiphase galactic outflows . We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged. 相似文献
13.
It is shown that the complicated form of the line absorption probability function for scattering in subordinate lines, derived by McKenna (1984), is an artifact of adopting a mathematically inconvenient expression for the corresponding redistribution function. It is demonstrated that the absorption probability must be given by the Voigt function. 相似文献
14.
A. I. Shapiro 《Astrophysics》2002,45(2):215-222
A model problem in the theory of line formation in an optically thick, purely scattering, stellar atmosphere is considered. The integral equation of radiation transfer at line frequencies is solved numerically for a two-level atom in the approximation of complete frequency redistribution in scattering. The numerical results are compared with those calculated from equations of the asymptotic theory. On the basis of the asymptotic theory, the positions of intensity maxima in a line are found for different absorption profiles. 相似文献
15.
D. I. Nagirner 《Astrophysics》1995,38(1):39-47
We present a method for computing the spectrum of the integral equation for radiation transfer in a cylinder. This method, as in the previous articles in this series, is based on a Hankel transformation applied to the equation. Calculating the spectrum then reduces to solving the equation for the auxiliary function for each eigenvalue separately. The corresponding eigenfunction is then found by an additional integration. We find asymptotic expressions for the eigenvalues and the eigenfunctions for a cylinder with a large optical radius when there is scattering in a spectral line, with complete redistribution over frequency when the absorption coefficient obeys a power law. We also derive equations determining the quantities entering into these expressions. For the simplest kernel of the equation all quantities can be expressed in terms of Bessel functions and roots of a transcendental equation.Translated from Astrofizika, Vol. 38, No. 1, pp. 75–88, January–March, 1995. 相似文献
16.
V. V. Ivanov 《Astrophysics》2009,52(1):24-39
This is a discussion of V. A. Ambartsumyan’s studies of the mean number of scatterings for photons in scattering media and
of further work and development in this area, especially at Ambartsumyan’s St. Petersburg school. The following questions
are discussed briefly: (a) the traditional method for calculating the number of scatterings from the source function and critiques
of this method. (b) The equation for the number N(τ; τ0 ) of scatterings for a photon born at optical depth τ in a plane layer of optical thickness τ0 and its use for calculating the number of scatterings, averaged over the entire ensemble of photons for a medium with arbitrary internal sources. These questions
are first considered for the case of monochromatic scattering, and then for scattering in a spectral line with complete frequency
redistribution (CFR). (c) The mean path length for a resonance line photon in a scattering medium with CFR and continuum absorption: the basic equations and asymptotic
behavior of an optically thick layer. (d) A review of calculations of and in media that are so thick that the CFR approximation breaks down and the effects of partial frequency redistribution (PFR)
become dominant. The presentation is at a semiquantitative level in many parts of this paper, with stress on physical significance
rather than the mathematics, through the use of approximate and asymptotic solutions.
Translated from Astrofizika, Vol. 52, No. 1, pp. 29–45 (February 2009). 相似文献
17.
A. G. Nikoghossian 《Astrophysics》2000,43(3):337-342
The Lagrangian formalism is used to obtain nonlinear equations for frequently encountered statistical characteristics of radiative
transfer. Under the assumption of complete frequency redistribution, the average number of scatterings and the average time
of stay of a photon in a one-dimensional, semi-infinite atmosphere are analyzed. Two typical problems of multiple scattering
are solved to illustrate the possible applications of the equations obtained.
Translated from Astrofizika, Vol. 43, No. 3, pp. 463-471, July– September, 2000. 相似文献
18.
The conditions for the source functions of a multiplet to be equal are studied for plasmas with and without magnetic fields.
It is found that source function equality holds—in addition to the case of collisional predominance—only when the redistribution
functions are all identical and no interlocking with other lines occurs. When magnetic fields are present, the assumption
of source function equality leads to a violation of the invariance conditions of the scattering matrix and should therefore
not be made. 相似文献
19.
Maarten Baes † Herwig Dejonghe 《Monthly notices of the Royal Astronomical Society》2001,326(2):722-732
Accurate photometric and kinematic modelling of disc galaxies requires the inclusion of radiative transfer models. Because of the complexity of the radiative transfer equation (RTE), sophisticated techniques are required. Various techniques have been employed for the attenuation in disc galaxies, but a quantitative comparison of them is difficult, because of the differing assumptions, approximations and accuracy requirements that are adopted in the literature. In this paper, we present an unbiased comparison of four methods to solve the RTE, in terms of accuracy, efficiency and flexibility. We apply them all to one problem that can serve as a first approximation of large portions of disc galaxies: a one-dimensional plane-parallel geometry, with both absorption and multiple scattering taken into account, with arbitrary vertical distributions of stars and dust and an arbitrary angular redistribution of the scattering. We find that the spherical harmonics method is by far the most efficient way to solve the RTE, whereas both Monte Carlo simulations and the iteration method, which are straightforward to extend to more complex geometries, have a cost that is about 170 times larger. 相似文献
20.
H. Frisch 《Solar physics》1996,164(1-2):49-66
Resonance polarization, which is created by the scattering of an anisotropic radiation field in regions of zero or weak magnetic fields, is strongly dependent on the frequency redistribution taking place during the scatterings. Here we discuss the frequency redistribution matrix relevant to resonance lines, concentrating on linear polarization. First we analyze in detail the redistribution matrix in a zero magnetic field given by the theory of Omont, Smith and Cooper (1972), revisited by Domke and Hubeny (1988). We explain that the linear polarization maxima which may appear in the wings of the Stokes Q profiles of strong resonances lines such as the Ca I 4227 Å line are coherent frequency redistribution effects. Various approximate forms of the frequency redistribution matrix are also examined. For resonance polarization in a weak magnetic field, we suggest a new expression for the redistribution matrix which can be used at all line frequencies, and is consistent with the condition that the Hanle effect acts only in the line core. 相似文献