Detecting protoplanets with ALMA

Theoretical investigations show that planet-disk interactions cause structures in circumstellar disks, which are usually much larger in size than the planet itself and thus more easily detectable. The specific result of planet-disk interactions depends on the evolutionary stage of the disk. Exemplary signatures of planets embedded in disks are gaps and spiral density waves in the case of young, gas-rich protoplanetary disks and characteristic asymmetric density patterns in debris disks. Numerical simulations convincingly demonstrate that high-resolution imaging performed with observational facilities which are already available or will become available in the near future will allow to trace these “fingerprints” of planets in protoplanetary and debris disks. These observations will provide a deep insight into specific phases of the formation and early evolution of planets in circumstellar disks. In this context, the Atacama Large Millimeter Array (ALMA) will play a crucial role by allowing to trace features in disks which are indicative for various stages of the formation and early evolution of planets in circumstellar disks.     

探测到气体的碎片盘统计研究

碎片盘通常被描述为贫气盘, 其气尘比显著低于原行星盘, 因此很少在碎片盘中探测到气体. 得益于各种灵敏的望远镜, 探测到气体的碎片盘的数量正在不断增加, 寻找更多的含有气体的碎片盘成为碎片盘研究的重点课题. 然而大范围的搜寻非常耗时且低效, 因而为了更快更好地遴选探测的目标, 需要根据这类源的特性择选更小范围样本. 通过统计探测到气体的碎片盘及其宿主恒星的参数总结出这些源的总体特性. 为此搜集了已经发表的探测到气体的碎片盘的文献, 总计找到37个源, 包括探测到CO等冷气体的12个源、CaII等热气体的14个源以及冷热气体共存的11个源. 通过统计其宿主恒星的光谱类型、年龄、离地球距离和碎片盘的相对光度、气体质量、尘埃质量等信息, 得出了主要结论: 宿主恒星多为B型和A型恒星, 年龄大部分都小于50Myr, 相对光度分布相比于已知碎片盘更为集中, 在$10^{ - 5     

Debris disks: seeing dust, thinking of planetesimals and planets

Debris disks are optically thin, almost gas-free dusty disks observed arounda significant fraction of main-sequence stars older than about 10 Myr. Since the circumstellar dust is short-lived, the very existence of these disks is considered as evi-dence that dust-producing planetesimals are still present in mature systems, in whichplanets have formed – or failed to form – a long time ago. It is inferred that theseplanetesimals orbit their host stars at asteroid to Kuiper-belt distances and continuallysupply ...     

Magnetic tensions can control the dynamics of accretion disks

Magnetic tensions are likely to be the dominant shear force in accretion disks, larger when integrated than turbulent viscosity by an order of magnitude or more. In galactic disks, they guarantee the mass-accretion rate required by the quasar phenomenon. In fast-revolving, clumpy disks, magnetic pressures can exceed static pressures and be amplified towards ram pressures. The inner, near-rigidly rotating parts of galactic disks are suggestive candidates. The gas velocities in such magnetically controlled disks mimic higher central masses than present.     

The role of toroidal magnetic field in the stability of accretion disks with alpha viscosity and other viscosities

The standard thin accretion disk model predicts that the inner regions of alpha model disks, where radiation pressure is dominant, are thermally and viscously unstable. However, observations show that the bright X-ray binaries and AGN accretion disks, corresponding to radiation-pressure thin disks, are stable. In this paper, we reconsider the linear and local instability of accretion disks in the presence of a toroidal magnetic field. In the basic equations, we consider physical quantities such as advection, thermal conduction, arbitrary viscosity, and an arbitrary cooling function also. A fifth order diffusion equation is obtained and is solved numerically. The solutions are compared to non-magnetic cases. The results show that the toroidal magnetic field can make the thermal instability in radiation pressure-dominated slim disks disappear if ? _m ≥0.3. However, it causes a more thermal instability in radiation pressure alpha disks without advection. Also, we consider the thermal instability in accretion disks with other values of the viscosity and obtain a general criterion for thermal instability in the long-wavelength limit and in the presence of a toroidal magnetic field.     

Energy accumulating accretion disk orbiting a Kerr black hole

A geometrically thin, energy accumulating $aL-disk is suggested which orbits a Kerr black hole. With increasing internal forces, the “standard” disks develop into energy accumulating disks. These accumulating disks are geometrically thin as long as their internal forces remain below a certain bound, allowing nearly geodesic orbits.     

On the possibility of studying the dynamics of astrophysical disks in a two-dimensional formulation

A closed system of two-dimensional equations describing the dynamics of rotating, gravitating gas disks is derived. It is an integrodifferential system for barotropic disks and a differential system for polytropic disks. For both barotropic and polytropic disks, these equations differ both from the dynamical equations used in the literature for astrophysical disks and from the traditional equations of two-dimensional hydrodynamics. The sufficient conditions under which the dynamics of a disk can be described in a two-dimensional formulation are obtained. The first condition reflects the thin-disk approximation. The second condition imposes a limit on the characteristic times of processes studied in a two-dimensional formulation. In most cases this condition limits the characteristic frequency of a process to the disk's rotational frequency.Translated from Astrofizika, Vol. 39, No. 3, pp. 441–466, July–September, 1996.     

The origin of Jovian and Saturnian satellites in accretion disks

The properties of gas-dust disks that surrounded Jupiter and Saturn during the final stage of their formation are analyzed. The sizes of the disks are determined by the total planetocentric angular momentum of the matter accreted by planets and correspond to the sizes of the orbits of their largest satellites. The mass of the solid component of disks is limited from below by the total mass of the Galilean satellites of Jupiter (no less than 4 × 10²⁶ g) and the mass of the largest Saturnian satellites (1.4 × 10²⁶ g), whereas the mass of the gaseous component is limited from above by the amount of hydrogen and helium that could have been later lost by the disks. Our analysis of the known mechanisms of dissipation of gas showed that its simultaneous content in the disks relative to the solid component was much lower than the corresponding gas-to-solid ratio in the Sun. A certain amount of solid compounds (including ice) could have been brought into the disks with planetesimals, which had undergone mutual collisions in the neighborhood of giant planets and served as germs of satellites. The bulk of solid matter appears to have been captured into disks when the latter were crossed by smaller and intermediate-sized planetesimals, which then became parts of the satellites.     

Signatures of planets in protoplanetary and debris disks

We discuss selected possibilities to detect planets in circumstellar disks. We consider the search for characteristic signatures in these disks caused by the interaction of giant planets with the disk as the most promising approach. Numerical simulations show that these signatures are usually much larger in size than the planet itself and thus much easier to detect. The particular result of the planet–disk interaction depends on the evolutionary stage of the disk. Primary signatures of planets embedded in disks are gaps in the case of young disks and characteristic asymmetric density patterns in debris disks.We present simulations which demonstrate that high spatial resolution observations performed with instruments/telescopes that will become available in the near future will be able to trace the location and other properties of young and evolved planets. These observations will allow to directly investigate the formation and evolution of planets in protoplanetary and debris disks.     

General relativistic analysis of structure and stability of charged fluid disks around compact objects

In this paper we give a detailed general relativistic formulation of the study of structure and stability of charged fluid disks around compact objects like black holes neglecting the self-gravitation of the disk itself. Having presented the general equations for equilibrium as well as for perturbations we solve explicitly the cases of rigidly and differentially rotating thin disks, with constant charge density and zero pressure, confined to the equatorial plane of the black hole. By using normal mode analysis we have analysed the stability of such disks under purely radial perturbations and find that the disks are generally stable. On leave of absence from Government College, Jagadalpur 494005     

Dynamics of plance stellar systems

The evolution of initially balanced rotating disks of stars is investigated with a computer model for isolated disks of stars. An isolated, initially cold balanced disk is found to be violently unstable. Adding a sufficient amount of velocity dispersion will stabilize all small-scale disturbances. However, the disks are still unstable against slowly growing long wave-length modes and after about two rotations most disks tend to assume a bar-shaped structure. It is found that the final mass distribution over most of the disk can be closely approximated by an exponential variation, irrespective of the initial mass distribution. The gravitational two-stream instability is investigated by means of a modified computer model for infinite doubly periodic stellar systems.     

Relationship between the thickness of stellar disks and the relative mass of a dark galactic halo

We analyze the R-and K _s-band photometric profiles for two independent samples of edge-on galaxies. The thickness of old stellar disks is shown to be related to the relative masses of the spherical and disk components of galaxies. The radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with increasing total mass-to-light ratio of the galaxies, which reflects the relative contribution of the dark halo to the total mass, and with decreasing central deprojected disk brightness (density). Our results are in good agreement with numerical models of collisionless disks that evolved to a marginally stable equilibrium state. This suggests that, in most galaxies, the vertical stellar-velocity dispersion, on which the equilibrium-disk thickness depends, is close to a minimum value that ensures disk stability. The thinnest edge-on disks appear to be low-brightness galaxies in which the dark-halo mass far exceeds the stellar-disk mass.     

Disks Around Post-Main Sequence Binaries

We propose that at least two stars on or near the AGB have long-lived orbiting disks: HD 44179, the central star in the Red Rectangle, and BM Gem, a carbon-rich star with an oxygen-rich circumstellar envelope. The CO emission from both of these disks has a spike with a width near ∼2 km s⁻¹, indicating disk radii of ∼10¹⁶ cm. The dust in such disks is therefore quite cold (near T ∼ 50 K for the Red Rectangle) and may emit primarily at submillimeter wavelengths. The disks around stars where there is also substantial mass loss may not be easily observable; there could be many as yet undiscovered disks around AGB stars This revised version was published online in September 2006 with corrections to the Cover Date.     

非磁化吸积盘中的粘滞性和朗缪尔波湍动应力

粘滞性问题一直是吸积盘理论中十分重要而又难以解决的一个基本理论问题．最近,Balbus和Hawley建议在磁化吸积盘中存在一种局域的磁流体剪切不稳定性机制,它能导致磁化吸积盘中有效的角动量转移,从而可以部分地解决磁化吸积盘中的粘滞性问题．但是Balbus－Hawley机制对非磁化吸积盘仍然是无效的．在本文中,我们研究了一种非磁化吸积盘模型,其中粘滞性机制起源于等离子体朗缪尔波湍动应力,并与标准a吸积盘模型中起源于流体或磁流体湍流的雷诺应力的粘滞性机制进行了比较．结果表明等离子体朗缪尔波湍动应力不仅对非磁化吸积盘中粘滞性的起源有重要的贡献,而且有可能是比流体湍流或磁流体湍流的雷诺应力更加有效的粘滞性起源的物理机制．     

Observational signature of planet formation: The ALMA view

Protoplanetary disks are the most probable sites where planet formation takes place. According to theory, planet formation in protoplanetary disks should show remarkable signatures, such as a gap/hole or a spiral structure. In fact, recent high-angular and high-sensitivity observations in millimeter and submillimeter wavelengths, as well as optical/near-IR wavelengths, have shown such structures in protoplanetary disks. Two particular examples of such disks around AB Aurigae and HD 142527 are discussed here, with an emphasis on results obtained using the Submillimeter Array. These disks—and their probable planet formation—will be very important future targets for ALMA to study the physical process of planet formation in detail.     

Magnetic fields of active galactic nuclei and quasars: Redshift dependence

The observed reduction in the fraction of quasars with polarized radiation as the redshift increases is explained by an increase in the magnetic-field strength in accretion disks at high z. The emerging Faraday rotation causes the radiation to be depolarized. This mechanism allows the magnetic fields in accretion disks to be estimated at several hundred gauss. We give simple asymptotic formulas that describe the Faraday depolarization in optically thick accretion disks.     

Long-term V/R variations of Be stars due to global one-armed oscillations of equatorial disks

We study the long-term variations of Balmer line profiles due to global one-armed oscillations in Be-star disks. In order to examine the qualitative effects of oscillations on line profiles, we assume that the eigenfunctions of one-armed nonlinear oscillations are similar to those of linear oscillations. Computing the line profiles for various values of disk parameters, we find that in small disks or in disks with steep density gradients the one-armed fundamental modes cause remarkable variabilities similar to the observedV/R variations.     

The Color-Magnitude Relations of Late-type Spiral Galaxies and Their Disks

The 395 late-type spiral galaxies brighter than 15^m in r-band are selected from the Data Release 2 of Sloan Digital Sky Survey and the colormagnitude relations of these galaxies and their disks are investigated. It is found that the colors g − r, r − z and g − z of these galaxies and their disks are strongly correlated with the r-band absolute magnitudes, i.e., the more luminous galaxies (or disks) have the redder colors than the less luminous galaxies (or disks). And the correlation of galaxies is stronger than that of their disks.     

Dust in the solar system and in extra-solar planetary systems

Among the observed circumstellar dust envelopes a certain population, planetary debris disks, is ascribed to systems with optically thin dust disks and low gas content. These systems contain planetesimals and possibly planets and are believed to be systems that are most similar to our solar system in an early evolutionary stage. Planetary debris disks have been identified in large numbers by a brightness excess in the near-infrared, mid-infrared and/or submillimetre range of their stellar spectral energy distributions. In some cases, spatially resolved observations are possible and reveal complex spatial structures. Acting forces and physical processes are similar to those in the solar system dust cloud, but the observational approach is obviously quite different: overall spatial distributions for systems of different ages for the planetary debris disks, as opposed to detailed local information in the case of the solar system. Comparison with the processes of dust formation and evolution observed in the solar system therefore helps understand the planetary debris disks. In this paper, we review our present knowledge of observations, acting forces, and major physical interactions of the dust in the solar system and in similar extra-solar planetary systems.     

晚型旋涡星系盘的颜色梯度与结构参数的相关关系

从Sloan数字巡天第2批释放的数据(SDSS DR2)中选择了395个在r波段亮于15等的面向晚型旋涡星系作为样本,研究了盘的颜色梯度与结构参数的关系.结果表明:盘的颜色梯度与盘的绝对星等(质量)无关;盘的颜色梯度与盘的尺度有关,越大的盘颜色梯度越陡;盘的颜色梯度与盘的颜色有关,越蓝的盘颜色梯度越陡;盘的颜色梯度与盘的表面亮度有关,越亮的盘颜色梯度越陡,并简单讨论了盘的颜色梯度与各结构参数的相关关系对晚型旋涡星系盘恒星形成历史的约束.