共查询到20条相似文献，搜索用时 31 毫秒
1.
We propose a scheme to classify planetary nebulae (PNe) according to their departure from axisymmetric structure. We consider only departure along and near the equatorial plane, i.e. between the two sides perpendicular to the symmetry axis of the nebula. We consider six types of departure from axisymmetry: (1) PNe where the central star is not at the centre of the nebula; (2) PNe having one side brighter than the other; (3) PNe having unequal size or shape of the two sides; (4) PNe where the symmetry axis is bent, e.g. the two lobes in a bipolar PN are bent toward the same side; (5) PNe where the main departure from axisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe that show no departure from axisymmetry, i.e. any departure, if it exists, is on scales smaller than the scale of blobs, filaments and other irregularities in the nebula. PNe that possess more than one type of departure are classified by the most prominent type. We discuss the connection between departure types and the physical mechanisms that may cause them, mainly resulting from the influence of a stellar binary companion. We find that ∼50 per cent of all PNe in the analysed sample possess large-scale departure from axisymmetry. This number is larger than that expected from the influence of binary companions, namely ∼25–30 per cent. We argue that this discrepancy comes from many PNe where the departure from axisymmetry, mainly unequal size, shape or intensity, results from the presence of long-lived and large (hot or cool) spots on the surface of their asymptotic giant branch progenitors. Such spots locally enhance the mass-loss rate, leading to a departure from axisymmetry, mainly near the equator, in the descendent PN. 相似文献
2.
A method is suggested for empirical NLTE analyses of solar spectral lines. Special depthdependent departure coefficients (x) are introduced and the formulas are given for further application. From test calculations it is shown that the separation of the departure coefficients for the upper and the lower level from each other and from the uncertainties in several input parameters (oscillator strength times abundance, turbulent velocities and damping constant) is greatly facilitated when spectral lines are analysed on the disk, around the limb, as well as in the flash spectrum. Therefore it is necesssary that all accessible line data are used, from accurate line profiles to equivalent widths or integrated intensities. To reduce the number of independent variables the analysis should include many multiplets between a few spectroscopic terms.We discuss the advantages of translating the departure coefficients into a set of temperatures. Some published LTE and NLTE analyses are compared in terms of these temperatures. The results do not show inexplicable contradictions. For several atomic spectra the departures from LTE seem large enough to be established quantitatively by the detailed method suggested in this paper. 相似文献
3.
Hirayama (1927) studied secular perturbations between a retrograde body and a prograde body by considering that the mean motion of the retrograde body is negative. In this paper we discuss the same problem by measuring angle variables from the departure point and keeping the mean motions positive for both the retrograde body and the prograde body, and compare the analytical solutions with numerically integrated orbits. 相似文献
4.
《Chinese Astronomy and Astrophysics》1984,8(1):94-96
We give a formal expression for the dependence of hydrogen line oscillator strength on departure from LTE and propose a method for determining this departure in the various sub-levels of the hydrogen atom in solar prominences. Results for 10 solar prominences are given. Finally, we discuss the possible implications of a smaller population in the 2p level than in the 2s level. 相似文献
5.
《Chinese Astronomy and Astrophysics》2021,45(3):382-401
There are abundant emission and absorption lines superimposed on the continuum spectra of the different solar atmospheric layers. The chemical composition and physical state of the solar atmosphere can be probed by the inversion of the profiles of these spectral lines. Due to the low density and large temperature difference in the chromosphere and transition region of the Sun, it is hard to establish the thermal dynamic equilibrium. It is necessary to adopt the theory of Non-Local Thermodynamic Equilibrium (N-LTE) to construct the corresponding atmospheric model. In this paper, the departure from the Local Thermodynamic Equilibrium (LTE) in the middle and lower atmosphere of the sun is investigated with the well-defined relative departure factor and the relevant calculations. We first make an inversion of the spectral lines formed at the different heights in the chromosphere and transition region during a total solar eclipse, to obtain the parameters of the observed spectral lines, such as the continuum source function, line source function, Doppler width, and thus the equivalent kinetic temperature. According to these line parameters obtained by the inversion, we calculate the quantitative results about the departure from the LTE at each space sampling point in the 2D field of view. Secondly, we reconstruct the 2D distributions of the radiation intensity, equivalent kinetic temperature, and relative departure factor according to the alignment of the optical fiber array in the integrated field unit (IFU) used by the telescope. The results show that there is a stronger correlation in the distributions of the temperature and relative departure factor existed in local small regions, but without evident correlation with the distribution of radiation intensity. There is an obvious difference between the distributions of the equivalent temperature and relative departure factor derived from two spectral lines, which shows a strong structurization and complexity existed in some local small regions of the solar atmosphere, and provides a new perspective for us to further understand the physics of the middle and lower atmosphere of the Sun. 相似文献
6.
在太阳大气不同层次连续光谱中叠加有丰富的发射线或吸收线,对这些谱线轮廓进行反演分析可以探测太阳大气的化学成分和物理状态.太阳大气的色球及过渡区由于其密度低难以建立热动平衡,建立相应的大气模型需要采用非局部热动平衡(Non-Local Thermodynamic Equilibrium,N-LTE)理论.根据相对偏离因子计算来研究太阳中低层大气偏离局部热动平衡(Local Thermodynamic Equilibrium,LTE)分布的情况.首先对日全食观测过程中得到色球和过渡区不同高度形成的两条光谱数据进行反演,得到确定观测谱线的参数信息,如连续谱源函数、谱线源函数、多普勒宽度和由此推出的等效动力学温度;根据这些反演出的谱线参量计算出二维视场内每个空间采样点偏离LTE状态的定量结果;其次,根据用于观测的积分视场单元光纤排布阵列重构出辐射强度、等效动力学温度和相对偏离因子二维分布.结果显示:在局部小区域,温度分布和相对偏离因子的分布存在较强相关性,而与辐射强度分布无明显相关.从两条谱线导出的等效温度和相对偏离因子分布存在明显的差异.这两种二维分布揭示出太阳大气某些小尺度区域具有较强的结构性和复杂性,为进一步理解太阳中低层大气物理提供了一种新的视角. 相似文献
7.
An indirect optimization procedure is applied to find the mission opportunities for a manned or round-trip mission to Mars. Both the conjunction-class and opposition-class high-thrust trajectories are considered, taking into account simple legs (with only departure and arrival impulses), three-impulse legs (departure, midcourse and arrival impulses) and flyby legs, where the non-propelled flyby of the planet Venus is used instead of the midcourse impulse to reduce the propellant consumption. The absolute positions of all the relevant planets repeat almost perfectly after 32 years: therefore, only the mission opportunities in a 32-year syzygistic cycle are analyzed. The two-body problem formulation is sufficient for preliminary analyses of interplanetary missions and the trajectory is approximated by heliocentric conic orbits passing through the centres of the planets. The mission opportunities correspond to the local minima of the characteristic velocity, that is, the sum of the actual velocity changes obtained by expending the propellant. Numerical results are presented to show that the same mathematical approach can be applied to different classes of missions, to emphasize the indications suggested by Pontryagin's Maximum Principle, to point out some periodicities in the solutions and to discuss the problem of providing initial guesses at the solutions. 相似文献
8.
Roger A. Kopp 《Solar physics》1980,68(2):307-316
In this paper we point out the existence of a special class of solutions to the nonlinear hydrodynamic equations describing the time-dependent solar wind, namely that for which the velocity profile is time-invariant but the density at each point of the corona changes exponentially with time. Theoretical velocity curves are calculated for the case of isothermal expansion and compared with the Parker model for steady-state expansion. These solutions can be used to obtain quantitative estimates for the degree of departure from the latter of a real corona undergoing evolution on a finite time scale.On leave from Los Alamos Scientific Laboratory, Los Alamos, N.M., U.S.A. 相似文献
9.
Hao-jian Yan 《Chinese Astronomy and Astrophysics》1998,22(4):200-496
In this paper we used the method of generator function to gave an improved mapping function of astronomical refraction, separately for optical and radio frequencies. We included a complete consideration of the corrections introduced by the physical and geophysical factors required in astronomy and space techniques. We used sounding balloon data to assess the actual accuracy of our corrected refraction formula. The result is 5″ at elevation 2° and 1″ at elevation 5°. We believe that the main factor that limits the accuracy is departure of the model atmosphere from the real atmosphere. 相似文献
10.
Shinko Aoki 《Celestial Mechanics and Dynamical Astronomy》1987,42(1-4):309-354
A relation between the Celestial Reference System (CRS) and the Terrestrial Reference System is established theoretically by solving the equations of motion of a rigid Earth under the influence of the Sun and the Moon up to the second order perturbation. The solutions include not only nutation including Oppolzer terms but also the right ascension of the dynamical departure point (DP), as well as the wobble matrix.We have found that the kinematical definition of the Non-Rotating Origin NRO (for which our term is DP) given by Capitaine, Guinot and Souchay (1987) is not entirely equivalent to that included in the solutions of the equations of motion but shows perturbation, in particular when this is taken on the instantaneous equator. Besides this serious fault, we feel little merit in taking the DP as reference: (1) Unnecessary spurious mixed secular terms appear which come from the geometrical configuration that the DP leaves far and far from the ecliptic. (2) the DP moves secularly as well as oscillating with respect to space; this literally contradicts the term NRO, or is at least misleading. (3) It does not free us from the precession uncertainty to adopt DP as reference, since we cannot avoid virtual proper motions in terms of the current CRS. (4) No terms ignored hitherto are introduced, even if we take the DP properly chosen, i.e., on the equator of the celestial ephemeris pole. The transformation is only mathematical. There is no sufficient reason to take it instead of the equinox, which is observable in principle, as reference at the cost of the labor of changing all the textbooks, ephemerides, data and computer software now existing. 相似文献
11.
A.F. Zakharov 《Astronomische Nachrichten》2002,323(6):538-541
We discuss basic ideas which were fundamental for the black hole concept. The major goal of the historical part is an attempt to explain the long way to the birth of the black hole concept, since the black hole solution was already found in 1916 by K. Schwarzschild, but the black hole concept was only introduced in 1967 by J.A. Wheeler. We discuss the basic notations of the black hole theory and observational manifestations of black holes. We analyse the possibility to interpret the very peculiar distortion of the Fe Kα‐line in such a way. 相似文献
12.
We have developed a method of stray-light correction which is applicable to filter-based magnetograph observations. Stray-light-corrected Stokes images are obtained by performing the deconvolution of observed Stokes images by the point spread function which is determined from the Stokes I image. For image deconvolution, the maximum entropy principle is used to guarantee that intensity should be positive and polarization degrees should be less than unity. We present an iterative algorithm for the maximum entropy method, which seeks the solution in Fourier space and thus accomplishes fast convergence. We find that our method is effective in correcting stray light which has a spread angle greater than the full width at half maximum of the point spread function. We also discuss the effect of stray light on magnetograph calibration. 相似文献
13.
Harold Zirin 《Solar physics》1996,169(2):313-326
We discuss many aspects of the solar chromosphere from an observational point of view, and show that most existing models are in direct contradiction to radio and eclipse measurements. We plead for attention to the actual observed radio temperatures and density gradients, as well as images of the chromosphere. We find that the chromosphere is not in hydrostatic equilibrium and suggest that the support is due to the tangled intranetwork fields.Dedicated to Cornelis de Jager 相似文献
14.
L. Tenorio A. H. Jaffe S. Hanany C. H. Lineweaver 《Monthly notices of the Royal Astronomical Society》1999,310(3):823-834
We consider wavelets as a tool to perform a variety of tasks in the context of analysing cosmic microwave background (CMB) maps. Using spherical Haar wavelets, we define a position and angular-scale-dependent measure of power that can be used to assess the existence of spatial structure. We apply planar Daubechies wavelets for the identification and removal of point sources from small sections of sky maps. Our technique can successfully identify virtually all point sources that are above 3 and more than 80 per cent of those above 1 . We discuss the trade-offs between the levels of correct and false detections. We denoise and compress a 100 000-pixel CMB map by a factor of 10 in 5 s, achieving a noise reduction of about 35 per cent. In contrast to Wiener filtering, the compression process is model-independent and very fast. We discuss the usefulness of wavelets for power spectrum and cosmological-parameter estimation. We conclude that at present wavelet functions are most suitable for identifying localized sources. 相似文献
15.
D.R. Flower J. Le Bourlot G. Pineau des Forêts S. Cabrit 《Astrophysics and Space Science》2003,287(1-4):183-186
The shock waves associated with molecular outflows may be of continuous (C) type or jump (J) type, depending on conditions in the preshock gas, notably the degree of ionization. Intermediate situations also exist, in which a J-discontinuity terminates or is embedded in a C-type flow. We consider the results of recent modelling of several important cases where shock waves will comprise both C and J components, namely: (1) in the evolution of a planar J into a planar C type shock wave, prior to the attainment of a stationary state; (2) at shock speeds exceeding the maximum value for a planar C type shock, when the cooling flow encounters a sonic point; (3) in a curved bowshock, when the shock speed at the apex exceeds the maximum value for a C type shock while the bow wings remain of C type. We show that proper allowance for the departure of the chemistry from equilibrium, particularly the dissociation/reformation of H2, is crucial for an accurate treatment of the C to J transition. All the possible modes of shock propagation need to be considered when interpreting observations of molecular outflow sources. An important diagnostic tool in this context is the H2 excitation diagram, which plots the logarithm of the column densities of the H2 rotational levels, divided by the statistical weights, against their excitation energies. There can be large differences between the dynamical and physical conditions implied by J type shocks (with and without a precursor), C type shocks, and bowshocks which best fit the observed excitation diagram. We discuss, by means of examples, the use of the excitation diagram, in conjunction with our sophisticated shock model, to constrain conditions in shocks propagating in molecular outflows. 相似文献
16.
The extensive, homogeneous and automatically obtained OAO-2UV data on VV Ori at six different wavelengths, as presented by Eaton (1975), are subjected to detailed numerical analysis, and the extent of derivable information on this star from such photometry is critically assessed.An economic (analytical integration) fitting function which can depend on up to 16 independently specifiable parameters is introduced. The use of frequency domain techniques along previously described lines is also discussed.The determination of sought parameter values is examined in some generality, and it is shown that a simultaneous unique optimal specification of more than six or seven such parameters is highly unlikely. An eight-parameter simultaneous specification for the quantities considered by Eaton would not be unique, rendering the conclusions of that author on relatively indeterminate parameters to be considered sceptically. By adopting means from all the solution sets of a five-element specification for basic parameters and carrying out statistical tests for self consistency it is possible to then investigate gravity darkening and reflection coefficients. We find that there is no great departure in the corresponding values from simple standard theory. 相似文献
17.
A new way to measure the departure from thermodynamic equilibrium is proposed based on the departure factor which evaluates the deviation from a Boltzmann level distribution, used by Short and Hauschildt (2005) and others. The way is based on an explicit relationship describing the departure factor as a function of line to continuum source, dynamic temperature and line photon frequency, under three assumptions that the scattering can be neglected, the background continuum can be treated as a Planck function, and finally the complete redistribution can be true. It has the advantage that the departure can be very conveniently evaluated from the spectral analysis with only the radiative transfer involved. Some physical insights are recovered for some extreme cases.Some example calculations of the departure are presented for the quiet Sun, faint solar flare and strong solar flare for the generally used solar chromospheric lines: Hα, Hβ,CaII H, K and triplet. It is revealed that in the case of solar flares, the departure is less than thermodynamic equilibrium along the larger depth range than in the quiet sun due to chromospheric condensation. It becomes hard to distinguish the departures for the different lines of the same atom or ion. It is expected that this investigation can be constructive for studying stellar atmospheres in cases where the three assumptions are close to reality. 相似文献
18.
Lei Zhu Yue-Fang Wu Yue Wei Department of Astronomy Peking University Beijing 《中国天文和天体物理学报》2006,6(1)
We report on a study of the molecular cloud S64 with observations at millimeter wavelengths of multiple molecular lines of CO isotopes.A weak outflow is found,and its physical parameters are estimated.The departure of the core of S64 from the S64 HII region indicates that there are still other star formation activities in that region. 相似文献
19.
We discuss the point spread function for the Spectro instrument based on realistic effects of the time delayed integration,
transverse image motion and pixelization, but assuming rather small wave front errors. Stars brighter than 10 mag, which will
saturate the CCD, may be observed photometrically using the image wings, but the central wavelengths will be slightly shifted
towards the red.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
We study the influence of the regular component of the Galactic magnetic field (GMF) on the arrival directions of ultra-high energy cosmic rays (UHECRs). We find that, if the angular resolution of current experiments has to be fully exploited, deflections in the GMF cannot be neglected even for E = 1020 eV protons, especially for trajectories along the Galactic plane or crossing the Galactic center region. On the other hand, the GMF could be used as a spectrograph to discriminate among different source models and/or primaries of UHECRs, if its structure would be known with sufficient precision. We compare several GMF models introduced in the literature and discuss for the example of the AGASA data set how the significance of small-scale clustering or correlations with given astrophysical sources are affected by the GMF. We point out that the non-uniform exposure to the extragalactic sky induced by the GMF should be taken into account estimating the significance of potential (auto-) correlation signals. 相似文献