Accretion of charged matter by collapsing objects

Re-analysis of the X-ray emission from RS CVn-type binaries has revealed that-contrary to the claims by many authors-their coronal activity is not independent of bolometric luminosity (and thus mass). It becomes clear that activity in late-type stars is also powered-just as in the case of early-type stars-mostly by photospheric radiation.     

Accretion disc models around compact objects

We present a survey of accretion disc models around compact objects — in particular the accretion onto white dwarfs, neutron stars, and black holes. We discuss both the thin disc as well as thick disc models and also the feaibility where either of these can be applied in the astrophysical systems. The crucial role of magnetic field in facilitating the formation of accretion discs in neutron stars is indicated. The prime significance of accretion discs in the generation of soft and hard X-rays is also discussed. Thick disc models are found to explain the observations of active galactic nuclei and also collimated and persistent jets in some of the radio sources.     

Accretion disks in luminous young stellar objects

An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and, therefore, predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.     

Rydberg matter in space: low-density condensed dark matter

Here Rydberg matter is proposed as a candidate for the missing dark matter or dark baryonic matter in the Universe. Spectroscopic and other experimental studies give valuable information on the properties of Rydberg matter, especially its very weak interaction with light caused by the very small overlap with low states, and because of the necessary two-electron transitions even for disturbed matter. Recently, the unidentified infrared (UIR) bands have been shown to agree well with calculations and experiments on Rydberg matter. This is the reason for the present, somewhat speculative, proposal that dark matter has, at least partially, the form of Rydberg matter. The UIR bands have also been observed directly in emission from Rydberg matter in the laboratory. The unique space-filling properties of Rydberg matter are described: a hydrogen atom in this matter occupies a volume  5×10¹²  times larger than in its ground state or in a hydrogen molecule.     

Accretion by isolated neutron stars

Accretion of interstellar material by an isolated neutron star is discussed. The point I address here is the interaction between the accretion flow and the stellar magnetosphere. I show that the interchange instabilities of the magnetospheric boundary under the conditions of interest are basically suppressed. The entry of the material into the magnetosphere is governed by diffusion. Due to this reason the persistent accretion luminosity of isolated neutron stars is limited to <4×10²⁶ erg s⁻¹. These objects can also appear as X-ray bursters with the burst durations of ∼30 min and repetition time of ∼10⁵ yr. This indicates that the number of the accreting isolated neutron stars which could be observed with recent and modern X-ray missions is a few orders of magnitude smaller than that previously estimated.      

Chemical evolution of circumstellar matter around young stellar objects

Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed. The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity. The data are discussed with reference to recent theoretical models.     

Revisiting Field Burial by Accretion onto Neutron Stars

The surface magnetic field strength of millisecond pulsars (MSPs) is found to be about 4 orders of magnitude lower than that of garden variety radio pulsars (with a spin of \({\sim }\)0.5–5 s and \(B{\sim }10^{12}\hbox { G}\)). The exact mechanism of the apparent reduction of field strength in MSPs is still a subject of debate. One of the proposed mechanisms is burial of the surface magnetic field under matter accreted from a companion. In this article we review the recent work on magnetic confinement of accreted matter on neutron stars poles. We present the solutions of the magneto-static equations with a more accurate equation of state of the magnetically confined plasma and discuss its implications for the field burial mechanism.     

Stability of Accretion Models

A critical analysis of standard accretion models is presented. We consider the stability of models in the theories of disc accretion onto black holes and spherical/disc accretion onto a magnetosphere. We take into account realistic physics processes and geometry (inner magnetic field in the accreted plasma, finite conductivity, finite length of the field lines, finite rotation of the accreted object, and magnetic shear on the boundary between the magnetosphere and accreted plasma). The influence of these factors leads to radical changes of both the accretion as whole and the energy release in the accreting system. Strong current-sheet and Z-pinch-like structures should arise over the polar region of the accreting object. Particle acceleration in the electric fields of current discharges in these regions may be a source of efficient conversion of energy into nonthermal particles and of the emission observed from many accreting objects.     

Accretion disk evolution

Three aspects of mass transfer instability models of dwarf novae are examined. The hydrodynamic development of instabilities in the secondary are examined within Roche geometry and shown to extend at least a few degrees away from the line of centres. The form of the outburst light curves observed in SS Cygni are shown to be a natural consequence of mass transfer bursts with a duration either less than, or greater than, the disk viscous timescale. Finally the two-dimensional structure of the disc in the plane of the orbit is studied. As with -disks the viscous evolution time following a burst of mass transfer determines the size of viscosity within the disk. Significant deviations from axial symmetry are found to be present.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Accretion disk instabilities

Basic properties of accretion disk instabilities are summarized. We first explain the standard disk model by Shakura and Sunyaev. In this model, the dominant sources of viscosity are assumed to be chaotic magnetic fields and turbulence in gas flow, and the magnitude of viscosity is prescribed by so-called model. It is then possible to build a particular disk model. In the framework of the standard model, accretion disks are stationary, but when some of the basic assumptions are relaxed, various kinds of instabilities appear. In particular, we focus on the thermal limit-cycle instability caused by partial ionization of hydrogen (and helium). We demonstrate that the disk instability model well accounts for the basic observed features of outbursts of dwarf novae and X-ray nova. We then introduce other kinds of instabilities based on the viscosity model. They are suspected to produce time variabilities observed on a wide range of timescales in close binaries and active galactic nuclei.     

Accretion rates of planetesimals by protoplanets embedded in nebular gas

When protoplanets growing by accretion of planetesimals have atmospheres, small planetesimals approaching the protoplanets lose their energy by gas drag from the atmospheres, which leads them to be captured within the Hill sphere of the protoplanets. As a result, growth rates of the protoplanets are enhanced. In order to study the effect of an atmosphere on planetary growth rates, we performed numerical integration of orbits of planetesimals for a wide range of orbital elements and obtained the effective accretion rates of planetesimals onto planets that have atmospheres. Numerical results are obtained as a function of planetesimals’ eccentricity, inclination, planet’s radius, and non-dimensional gas-drag parameters which can be expressed by several physical quantities such as the radius of planetesimals and the mass of the protoplanet. Assuming that the radial distribution of the gas density near the surface can be approximated by a power-law, we performed analytic calculation for the loss of planetesimals’ kinetic energy due to gas drag, and confirmed agreement with numerical results. We confirmed that the above approximation of the power-law density distribution is reasonable for accretion rate of protoplanets with 1-10 Earth masses, unless the size of planetesimals is too small. We also calculated the accretion rates of planetesimals averaged over a Rayleigh distribution of eccentricities and inclinations, and derived a semi-analytical formula of accretion rates, which reproduces the numerical results very well. Using the obtained expression of the accretion rate, we examined the growth of protoplanets in nebular gas. We found that the effect of atmospheric gas drag can enhance the growth rate significantly, depending on the size of planetesimals.     

Turbulence in Accretion Disks

An accretion disk is an inevitable part of the star forming process. Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in astrophysical accretion disks. The key conceptual point is that the combination of a subthermal magnetic field and outwardly decreasing differential rotation is subject to the magnetorotational instability. This rapidly generates magnetohydrodynamical (MHD) turbulence, leading to greatly enhanced angular momentum transport. Purely hydrodynamic disks, on the other hand, are stable. Disks that are too cool to couple effectively to the magnetic field will not be turbulent. Fully global three dimensional MHD simulations are now beginning to probe the properties of accretion disks from first principles.     

Accretion of dust by chondrules in a MHD-turbulent solar nebula

Numerical magnetohydrodynamic (MHD) simulations of a turbulent solar nebula are used to study the growth of dust mantles swept up by chondrules. A small neighborhood of the solar nebula is represented by an orbiting patch of gas at a radius of 3 AU, and includes vertical stratification of the gas density. The differential rotation of the nebular gas is replaced by a shear flow. Turbulence is driven by destabilization of the flow as a result of the magnetorotational instability (MRI), whereby magnetic field lines anchored to the gas are continuously stretched by the shearing motion. A passive contaminant mimics small dust grains that are aerodynamically well coupled to the gas, and chondrules are modeled by Lagrangian particles that interact with the gas through drag. Whenever a chondrule enters a region permeated by dust, its radius grows at a rate that depends on the local dust density and the relative velocity between itself and the dust. The local dust abundance decreases accordingly. Compaction and fragmentation of dust aggregates are not included. Different chondrule volume densities ρ_c lead to varying depletion and rimmed-chondrule size growth times. Most of the dust sweep-up occurs within ～1 gas scale-height of the nebula midplane. Chondrules can reach their asymptotic radius in 10–800 years, although short growth times due to very high ρ_c may not be altogether realistic. If the sticking efficiency Q of dust to chondrules depends on their relative speed δv, such that Q < 10^?2 whenever δv > v_stick ≈ 34 cm/s (with v_stick a critical sticking velocity), then longer growth times result due to the prevalence of high MRI-turbulent relative velocities. The vertical variation of nebula turbulent intensity results in a moderate dependence of mean rimmed-chondrule size with nebula height, and in a ～20% dispersion in radius values at every height bin. The technique used here could be combined with Monte Carlo (MC) methods that include the physics of dust compaction, in a self-consistent MHD-MC model of dust rim growth around chondrules in the solar nebula.     

Accretion process of the moon

Recent geochemical and geophysical data suggest that the initial temperature of the Moon was strongly peaked toward the lunar surface. To explain such an initial temperature distribution, a simple model of accretion process of the Moon is presented. The model assumes that the Moon was formed from the accumulation of the solid particles or gases in the isolated, closed cloud. Two equations are derived to calculate the accretion rate and surface temperature of the accreting Moon. Numerical calculations are made for a wide range of the parameters particle concentration and particle velocity in the cloud. A limited set of the parameters gives the initial temperature profiles as required by geochemical and geophysical data. These models of the proto-moon cloud indicate that the lunar outershell, about 400 km thick, was partially or completely molten just after the accretion of the Moon and that the Moon should have been formed in a period shorter than 1000 yr. If the Moon formed at a position nearer to the Earth than its present one, the Moon might have been formed in a period of less than one year.On leave from Geophysical Institute, University of Tokyo.Contribution No. 2104, Division of Geological and Planetary Sciences, California Institute of Technology.     

Time-Scale for Accretion of Matter

Mass accretion is the key factor for evolution of galaxies. It can occur through secular evolution, when gas in the outer parts is driven inwards by dynamical instabilities, such as spirals or bars. This secular evolution proceeds very slowly when spontaneous, and can be accelerated when triggered by companions. Accretion can also occur directly through merging of small companions, or more violent interaction and coalescence. We discuss the relative importance of both processes, their time-scale and frequency along a Hubble time. Signatures of both processes can be found in the Milky Way. It is however likely that our Galaxy had already gathered the bulk of its mass about 8–10 Gyr ago, as is expected in hierarchical galaxy formation scenarios. This revised version was published online in July 2006 with corrections to the Cover Date.     

On a possible source of energy for the ejection of matter from cosmic objects

The transformation of the energy of an accelerating, expanding universe into internal energy of cosmic objects is discussed. The well known fact that Hubble expansion is observed on scale lengths two or more orders of magnitude smaller than the “cells of homogeneity” is taken into account, along with observational data indicating that this expansion also takes place on the scale of the solar system. Changes in the potential energy of individual model objects are examined on this basis and it is shown that the potential energy increases, thereby threatening the continued existence of these objects. An expression is obtained for the mass which can attain the escape energy within a given energy accumulation time. Some estimates are made for the assumed masses of galactic clusters. Over a period of 10⁷ years a protocluster can accumulate enough energy for ejection of a clump of matter with a mass equal to that of our galaxy.