Radiation transfer in dense interstellar clouds

Summary The scattering properties of interstellar dust in the far ultraviolet as derived from the observations are reviewed. The transfer of ultraviolet radiation within a spherically symmetric inhomogeneous cloud is analysed using the method of successive scattering. The effects of different assumptions, both on the albedo and asymmetry factor of interstellar dust, and also of the extinction law on UV radiation penetration into clouds are studied. The results are used to determine the lifetime against photodestruction of interstellar formaldehyde in dense clouds. Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.     

Numerical solution of the radiative transfer equation in spherically symmetric dust shells

An efficient numerical method is presented for solving problems of radiative transfer in dust shells with spherical symmetry. The method can be applied to a wide variety of radiative transfer problems in planetary atmospheres, circumstellar dust envelopes and extended stellar atmospheres. The computational procedure and practical considerations for implementing the method are described in detail. As an illustration, the method is used to determine the grain temperature distribution and far i.r. emission from interstellar dust clouds associated with compact HII regions. Generalizations to problems involving cylindrical geometry and multi-temperature dust shells are discussed in the appendices.     

Hydroclimatology: Perspectives and Applications,by Marlyn L. Shelton

The interstellar space of the spiral arms of our galaxy contains dust clouds in which some 20 kinds of molecule are now known to exist. Many of the molecules are polyatomic. Only five years ago the general opinion was that such molecules could not survive the ultraviolet radiation thought to be present. At that time the diatomic CH, CH + and CN were known from the optical work of the 1930's, and the OH line had been studied for five years by radio-telescopes. But in 1968 observation showed that the climate of interstellar space was not hostile to even fairly complex molecules. They are there within rolatively cool dense clouds of molecular hydrogen that contains the dust particles whose surface may facilitate formation of the molecules, and whose presence certainly protects them from the ultraviolet radiation of stars.

This review tells how the emission and absorption of spectral lines, revealing the presence of molecules, can also furnish estimates of their numbers, densities and isotopic abundances. Departures from thermal equilibrium, including maser and reverse maser action are mentioned. The radio-telescopes and their spectrometers are described.     

It''s a dusty Universe: surface science in space

We live in a dusty Universe! Dust is not only found in our solar system among the planets but is found in a wide variety of objects throughout the Universe, mainly in those regions between the stars known as interstellar clouds. Interstellar dust particles, which consist of cores of silicates and carbonaceous material often surrounded by icy mantles, are most probably highly irregular in shape with a size distribution from micro- to nanometers. Interstellar dust is important for many reasons, including the template it provides for surface chemical reactions that form, among other species, the most important interstellar molecule––H₂. In this article, we discuss the evidence for interstellar dust, its physical and chemical properties, its role in interstellar surface chemistry, and what remains to be learned.     

Magnetic fields in spiral galaxies

Radio polarization observations have revealed large-scale magnetic fields in spiral galaxies. The average total field strength most probably increases with the rate of star formation. The uniform field generally follows the orientation of the optical spiral arms, but is often strongest outside the arms. Long magnetic-field filaments are seen, sometimes up to a 30 kpc length. The field seems to be anchored in large gas clouds and is inflated out of the disk, e.g. by a galactic wind. The field in radio halos around galaxies is highly uniform in limited regions, resembling the structure of the solar corona. The detection of Faraday rotation in spiral galaxies excludes the existence of large amounts of antimatter. The distribution of Faraday rotation in the disks shows two different large-scale structures of the interstellar field; axisymmetric-spiral and bisymmetric-spiral, which are interpreted as two modes of the galactic dynamo driven by differential rotation     

Differentially Rotating Disks of Dust

We present a three-parameter family of solutions to the stationary axisymmetric Einstein equations that describe differentially rotating disks of dust. They have been constructed by generalizing the Neugebauer—Meinel solution of the problem of a rigidly rotating disk of dust. The solutions correspond to disks with angular velocities depending monotonically on the radial coordinate; both decreasing and increasing behaviour is exhibited. In general, the solutions are related mathematically to Jacobi's inversion problem and can be expressed in terms of Riemann theta functions. A particularly interesting two-parameter subfamily represents Bäcklund transformations to appropriate seed solutions of the Weyl class.     

Cosmic chemistry: Molecules in space

Molecular clouds contain most of the mass of gas in interstellar space and are the source of matter from which new stars are formed. The clouds can be studied by means of the radio emission in rotational transitions of the molecules that they contain. We describe the chemical routes by which these molecules are formed, and their ultimate incorporation into icy mantles frozen onto dust grains. The role that chemistry plays in tracing and controlling processes in star-forming regions and in near-stellar environments is briefly indicated.     

年轻恒星形成过程的分子谱线诊断

恒星是宇宙的基本天体,理解恒星的起源和早期演化是天体物理最有举的课题之一,目前了解的恒星形成的一个基本图象是,恒星是由一个足够大质量的星际分子气体云在其其自引力作用下塌缩而形成。在早期塌缩过程中形成一颗原恒星（胚胎）,并且伴随该原恒星的吸积盘,原恒星吸积星际介质中的分子物质,通过吸积盘转移到原恒星表面,原恒星的质量随着增大。在吸积过程中也同时伴随物质向外的剧烈喷发,由于盘的存在,这种喷发通常沿垂直于盘的方向进行,出现双极的空间分布。在天文观测上,一种典型的表现是分子气体的外向流,观测表现为从原恒星双极的红蓝移气体运动^[1]。由于恒星形成了星际分子中,因此,对星际分子的谱线探测是研究恒星形成的强有力手段,现在已经发现和证认的星际分子总数有一百多种,其中丰富度最高的气体是H2。在所有的星际分子中,丰度仅次于H2的CO分子的转动跃迁谱易激发,相对于星际介质的不透明度小,观测上易于实现,因此,CO分子就成为探索原恒星形成的有力探针,在毫米波段出现的J＝1－0和J＝2－1跃迁变线以及CO同位素的谱线是最常使用的观测探针,八十年代以来又陆续发现了环形分子SiC2,C3H,C3H2及含磷分子PN和CN等,给宇宙中有机世界的探测提供了丰富的线索。我们对恒形成区内存在的低温原恒星天体进行毫米 波射电谱线观测,测量该原恒星周围分子气体的分布,其物理化学参数,以及速度场分布。根据这些测量结果并运用成熟的数量方法,分析正在发生的物理化学过程,特别是原恒星质量外流的过程。根据观测分析结果得出所观测天体的形成和演化状况。     

Vibration and critical speeds of composite-ring disks for data storage

A new concept for data storage disks is proposed to increase operating speed with minimum changes in the geometry and design of conventional data storage disks. The disk—named a composite-ring disk—is composed of a storage material inside and a thin composite ring outside. Stress distributions are found for the rotating disk composed of two annular disks, of which the inside is made of isotropic material and the outside is made of orthotropic material. The dynamic equation for a composite-ring disk in rotation is formulated to calculate its natural frequencies and critical speeds. For the solution of transverse vibration, a rotational symmetry condition is applied in the circumferential direction and a finite element interpolation with Hermite polynomials is performed in the radial direction. The results show that reinforcing a disk at the rim increases critical speeds drastically, and can cause buckling in mode (0,0) which occurs above the lowest critical speed.     

Investigation on non-LTE radiation emitted from a laser-irradiated Au disk

1 Introduction The laser-target coupling physics is a key topic in indirect-driven inertial confinement fusion (ICF) and X-ray application research[1―3]. When intense laser light irradiates the solid target, the plasmas are produced rapidly on the surface of the target. The laser en-ergy is mainly absorbed by inverse bremsstrahlung absorption, and a coronal region with high-temperature and low-density plasma is formed. Electron thermal conduction proc-ess transfers energy into over-dense re…     

Differentially Rotating Disks of Dust: Arbitrary Rotation Law

In this paper, solutions to the Ernst equation are investigated that depend on two real analytic functions defined on the interval [0,1]. These solutions are introduced by a suitable limiting process of Bäcklund transformations applied to seed solutions of the Weyl class. It turns out that this class of solutions contains the general relativistic gravitational field of an arbitrary differentially rotating disk of dust, for which a continuous transition to some Newtonian disk exists. It will be shown how for given boundary conditions (i.e. proper surface mass density or angular velocity of the disk) the gravitational field can be approximated in terms of the above solutions. Furthermore, particular examples will be discussed, including disks with a realistic profile for the angular velocity and more exotic disks possessing two spatially separated ergoregions.     

天体物理学讲座第三讲 恒星形成：从星际分子云到原始行星系统

对行星系统起源问题的研究在近年来已经取得很大进展,一方面是星际分子云与恒星形成的天文研究取得了令人瞩目的成就,研究表明,包括太阳在内的恒星是由冷暗的星际分子云收缩形成的。在形成恒星的过程中,普遍伴随出现围绕恒星的盘状的原始行星系统,在这种原始行星系统中间,尘埃颗粒逐渐生长,并凝聚成行星,彗星等,从分子云到原始行星系统大约经历百万年,而木星这样的大行星有可能在恒星收缩阶段同时形成,并且在行星系统的发展过程中逐渐发展,行星系统起源的另一方面进展来自包括彗星,太阳系行星际探测,从而对太阳系这样的行星系统的组成和性质,对行星大气等有了更精确的了解,以此为参考,能够测定那些离太阳系的其他新近发现的行星系统的物理化学性质,从而提供了一系列互相联系和对比的样本,通过上述进展,现代天文学正逐渐揭开行星系统形成之谜。     

Dynamic Superposition and Levitation Capability of Two Confronting Ultrasonic Waves

The superposition dynamics of two confronting ultrasonic waves and their levitation capability for centimeter-sized thin disks are investigated by numerical analyses and validated by experiments. The sound pressure simulation reveals that two opposite ultrasonic waves provide a more effective standing-wave field than a single ultrasonic wave when the diameter of disk-shaped object approaches the wavelength scale. The dynamic superposition of two confronting beams facilitates the acoustic levitation of the clay disk and aluminum disk with diameters of 0.97 and 0.90. The acoustic radiation forces exerting on these thin disks are measured experimentally, which exhibit a better levitation stability for the centimeter-sized thin disks. The equilibrium levitation positions of the two disks are located near the sound pressure node, and the maximum acoustic radiation pressure on their surfaces is less than one percent of the maximum sound pressure.     

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

Recent laboratory experiments using a pulsed Laval nozzle apparatus have shown that reactions between a neutral molecule and the radical OH can occur efficiently at low temperatures despite activation energy barriers if there is a hydrogen-bonded complex in the entrance channel which allows the system to tunnel efficiently under the barrier. Since OH is a major radical in the interstellar medium, this class of reactions may well be important in the chemistry that occurs in the gas phase of interstellar clouds. Using a new gas-grain chemical network with both gas-phase reactions and reactions on the surfaces of dust particles, we studied the role of OH–neutral reactions in dense interstellar clouds at 10, 50, and 100 K. We determined that at least one of these reactions can be significant, especially at the lowest temperatures studied, where the rate constants are large. It was found in particular that the reaction between CH₃OH and OH provides an effective and unambiguous gas-phase route to the production of the gaseous methoxy radical (CH₃O), which has been recently detected in cold, dense interstsellar clouds. The role of other reactions in this class is explored.     

对原行星盘中易挥发物质缺失问题的简要综述

年轻恒星周围存在盘状的气体和尘埃分布,称为原行星盘,行星在其中形成. 为了认识恒星和行星的形成和演化以及构成行星的原材料,对这些盘的观测是必须的. 数值模型有助于从观测数据提取出重要的物理参数,包括盘的尘埃和气体的质量. 这些参数可以作为进一步模拟的输入参数,预期热化学模拟能复现各种分子的观测数据. 但是,对于许多原行星盘,模型算出的多种分子的发射强度高于观测值. 真实的盘中这些分子的丰度比理论预期低,这是易挥发物质的缺失问题. 本文指出在这个问题上理论与观测的差异意味着尘埃的演化对气体化学有重要影响,提示在这些盘中行星形成的早期阶段已经开始了.     

Viscoresistive MHD configurations of plasma in accretion disks

We present a discussion of two-dimensional magneto-hydrodynamics (MHD) configurations, concerning the equilibria of accretion disks of a strongly magnetized astrophysical object. We set up a viscoresistive scenario which generalizes previous two-dimensional analyses by reconciling the ideal MHD coupling of the vertical and the radial equilibria within the disk with the standard mechanism of the angular momentum transport, relying on dissipative properties of the plasma configuration. The linear features of the considered model are analytically developed and the non-linear configuration problem is addressed, by fixing the entire disk profile at the same order of approximation. Indeed, the azimuthal and electron force balance equations are no longer automatically satisfied when poloidal currents and matter fluxes are included in the problem. These additional components of the equilibrium configuration induce a different morphology of the magnetic flux surface, with respect to the ideal and simply rotating disk.     

Quasistatic waves of hydrogravity generated in the galactic interstellar medium by a pulsating neutron star

Based on principles of classical hydrodynamics and Newtonian gravity, the theory of hydrogravity, formulated in the manner of hydromagnetic theory, is developed to account for the gravitational effect of global pulsations of a star on the motions of the ambient gas-dust interstellar medium. Analytic derivation of the dispersion relation for canonical gravity waves at the free surface of an incompressible in viscid liquid is presented, illustrating practical usefulness of the proposed approach, heavily relying on the concept of classical gravitational stress introduced long ago by Fock and Chandrasekhar, and accentuating the shear character of this mode. Particular attention is given to gas-dynamical oscillations of a similar physical nature generated by a pulsating neutron star in an unbounded spherical shell of gas and dust promoted by circumstellar gravitational stresses and damped by viscosity of the interstellar matter. Computed in the long-wavelength approximation, the periods of these gravity-driven shear modes, referred to as quasistatic modes of hydrogravity, are found to be proportional to periods of the gravity modes in the neutron star bulk. Given that collective oscillations of cosmic plasma in the wave under consideration should be accompanied by electromagnetic radiation and taking into account that only the radio waves of this radiation can freely travel through the galactic gas-dust clouds, it is conjectured that the considered effect of gravitational coupling between seismic vibrations of a neutron star and fluctuations of the galactic interstellar medium should manifest itself in the radio range of pulsar spectra. Some useful implications of the theory developed here to a number of current problems of asteroseismology are briefly discussed.     

Waves and instabilities in dark interstellar molecular clouds containing ferromagnetic dust grains

The propagation characteristics of magnetization waves, as well as the instabilities of sound waves in a self-gravitating dark interstellar molecular cloud containing ferromagnetic dust grains and baryonic gas clouds, have been theoretically investigated by including the dynamics of both ferromagnetic dust grains and baryonic gases. It has been shown that there exist two types of subsonic or supersonic (depending on the field strength of the magnetization) transverse magnetization waves, which can be regarded as counterparts of Alfvén waves (for the parallel propagation) and magnetosonic waves (for the perpendicular propagation) in a magnetoactive plasma. It has also been found that, in addition to the usual Jeans instability, the sound waves suffer a new type of instability, which is due to the combined effects of the baryonic gas dynamics and self-gravitational field in both weakly and highly collisional regimes.     

Full-torus impurity transport simulation for optimizing plasma discharge operation using a multi-species impurity powder dropper in the large helical device

The transport of impurities supplied by a multi-species impurity powder dropper (IPD) in the large helical device (LHD) is investigated using a three-dimensional peripheral plasma fluid code (EMC3-EIRENE) coupled with a dust transport simulation code (DUSTT). The trajectories of impurity powder particles (Boron, Carbon, Iron, and Tungsten) dropped from the IPD and the impurity transport in the peripheral plasma are studied in a full-torus geometry. The simulation reveals an appropriate size of the impurity powder particles and an optimum operational range of the dust drop rates for investigating the impurity transport without inducing radiation collapse. The simulation also predicts a favourable plasma discharge condition for wall conditioning (boronization) using the IPD in order to deposit boron to high plasma flux and neutral particle density areas in the divertor region in the inboard side of the torus.     

Exact relativistic models of conformastatic charged dust thick disks

We construct relativistic models of charged dust thick disks for a particular conformastatic spacetime through a Miyamoto–Nagai transformation used in Newtonian gravity to model disk like galaxies. Two simple families of thick disk models and a family of thick annular disks based on the field of an extreme Reissner–Nordström black hole and a Morgan–Morgan-like metric are considered. The electrogeodesic motion of test particles around the structures are analyzed. Also the stability of the particles against radial perturbation is studied using an extension of the Rayleigh criteria of stability of a fluid in rest in a gravitational field. The models built satisfy all the energy conditions.