Accretion disc outbursts: a new version of an old model

We have developed 1D time-dependent numerical models of accretion discs, using an adaptive grid technique and an implicit numerical scheme, in which the disc size is allowed to vary with time. The code fully resolves the cooling and heating fronts propagating in the disc. We show that models in which the radius of the outer edge of the disc is fixed produce incorrect results, from which probably incorrect conclusions about the viscosity law have been inferred. In particular we show that outside-in outbursts are possible when a standard bimodal behaviour of the Shakura–Sunyaev viscosity parameter α is used. We also discuss to what extent insufficient grid resolution has limited the predictive power of previous models. We find that the global properties (magnitudes, etc.) of transient discs can be addressed by codes using a high, but reasonable, number of fixed grid points. However, the study of the detailed physical properties of the transition fronts generally requires resolutions which are out of reach of fixed grid codes. It appears that most time-dependent models of accretion discs published in the literature have been limited by resolution effects, improper outer boundary conditions, or both.     

The [Sii] electron density distribution over the planetary nebula NGC 7009

Electron densities have been measured from [Sii] 6716/6731 Å line ratios for a grid of points over the surface of the planetary nebula NGC 7009 using a photon counting detector. The radial dependence of the electron density has been modelled, and the relationship provides possible evidence that the planetary nebula shell is driven by a strong stellar wind.     

Hemispheric Imaging of Galactic Neutral Hydrogen with a Phased Array Antenna System

The thousand element array (THEA) system is a phased array system consisting of 1 m² tiles having 64 Vivaldi elements each, arranged on a regular 8-by-8 grid, which has been developed as a demonstrator of technology and applicability for SKA. In this paper we present imaging results of Galactic neutral hydrogen with THEA. Measurements have been taken using a dense 2-by-2 array of four tiles as a four tile adder. The results are compared with results from the Leiden-Dwingeloo Survey, showing qualitative agreement, but also indicating that further studies are needed on the instrumental characteristics.     

Average anisotropy characteristics of high energy cosmic ray particles and geomagnetic disturbance index Ap

The average characteristics of the diurnal and semi-diurnal anisotropy of cosmic ray intensity at relativistic energies have been obtained by using data from the worldwide grid of neutron monitor for the period 1989 to 1996. The complex behaviour of the diurnal amplitudes and time of maxima (phase) and its association with the Ap index on a long-term and day-to-day basis have been studied. Even though the general characteristics, on a yearly average basis, have not changed significantly during this period, both the diurnal and semi-diurnal amplitudes and phases vary significantly, besides significant changes being observed for different interplanetary conditions on a short-term basis. It is found that the relationship between the Ap index and the diurnal vector is out of phase during the period 1991 to 1995. On a long-term basis, the correlation of diurnal variation with Ap index has been found to vary during the solar cycle. On a short-term basis, it has been observed that the high Ap days are usually associated with higher amplitudes with phase shifted to earlier hours.     

A comparative analysis of empirical calibrators for nebular metallicity

We present a new analysis of the main empirical calibrators of oxygen abundance for ionized gas nebulae. With that aim we have compiled an extensive sample of objects with emission-line data including the near-infrared [S  iii ] lines and the weak auroral lines which allow for the determination of the gas electron temperature. For all the objects the oxygen abundances have been derived in a homogenous way, using the most recent sets of atomic coefficients and taking into the account the effect of particle density on the temperature of O⁺. The residuals between directly and empirically derived abundances as a function of abundance have been studied. A grid of photoionization models, covering the range of physical properties of the gas, has been used to explain the origin of the uncertainties affecting each abundance calibrator. The range of validity for each abundance parameter has been identified and its average uncertainty has been quantified.     

Automated cloud tracking system for the Akatsuki Venus Climate Orbiter data

Japanese Venus Climate Orbiter, Akatsuki, is cruising to approach to Venus again although its first Venus orbital insertion (VOI) has been failed. At present, we focus on the next opportunity of VOI and the following scientific observations.We have constructed an automated cloud tracking system for processing data obtained by Akatsuki in the present study. In this system, correction of the pointing of the satellite is essentially important for improving accuracy of the cloud motion vectors derived using the cloud tracking. Attitude errors of the satellite are reduced by fitting an ellipse to limb of an imaged Venus disk. Next, longitude–latitude distributions of brightness (cloud patterns) are calculated to make it easy to derive the cloud motion vectors. The grid points are distributed at regular intervals in the longitude–latitude coordinate. After applying the solar zenith correction and a highpass filter to the derived longitude–latitude distributions of brightness, the cloud features are tracked using pairs of images. As a result, we obtain cloud motion vectors on longitude–latitude grid points equally spaced. These entire processes are pipelined and automated, and are applied to all data obtained by combinations of cameras and filters onboard Akatsuki. It is shown by several tests that the cloud motion vectors are determined with a sufficient accuracy. We expect that longitude–latitude data sets created by the automated cloud tracking system will contribute to the Venus meteorology.     

Galactic evolution along the hubble sequence

A generalization of the multiphase chemical evolution model applied to a wide set of theoretical galaxies is shown. This set of models has been computed by using the so-called Universal Rotation Curve from (Persic, Salucci and Steel, 1996) to calculate the radial mass distributions of each theoretical galaxy. By assuming that the molecular cloud and star formation efficiencies depend on the morphological type of each galaxy, we construct a bi-parametric grid of models whose results are valid in principle for any spiral galaxy, of given maximum rotation velocity or total mass, and morphological type. This revised version was published online in August 2006 with corrections to the Cover Date.     

Computer simulation of three-dimensional stellar systems in cylindrical coordinates

TheN-body problem does not have an exact and analytic solution, and computer technique or computer simulation can be a good candidate to solve it. Computing speed in computer simulation is very important. There are many algorithms and computational methods in computer simulation which reduce computer time.In this report a computer simulation model in a cylindrical coordinate, in which the FACR (Fourier Analysis and Cyclic Reduction) method is used, has been proposed and demonstrated the presence of spiral, barred, and ringed galaxy. The method using a cylindrical grid has good symmetrical properties specially for rotating stellar systems.     

A robust determination of halo environment in the cosmic field

A number of methods for studying the large-scale cosmic matter distribution exist in the literature. One particularly common method employed to define the cosmic web is to examine the density, velocity or potential field. Such methods are advantageous since a Hessian matrix can be constructed whose eigenvectors (and eigenvalues) indicate the principal directions (and strength) of local collapse or expansion. Technically this is achieved by diagonalizing the Hessian matrix using a fixed finite grid. The resultant large-scale structure quantification is thus inherently limited by the grid’s finite resolution. Here, we overcome the obstacle of finite grid resolution by introducing a new method to determine halo environment using an adaptive interpolation which is more robust to resolution than the typical “Nearest Grid Point” (NGP) method. Essentially instead of computing and diagonalizing the Hessian matrix once for the entire grid, we suggest doing so once for each halo or galaxy in question. We examine how the eigenvalues and eigenvector direction’s computed using our algorithm and the NGP method converge for different grid resolutions, finding that our new method is convergent faster. Namely changes of resolution have a much smaller effect than in the NGP method. We therefore suggest this method for future use by the community.     

A comprehensive range of X-ray ionized-reflection models

X-ray ionized reflection occurs when a surface is irradiated with X-rays so intense that its ionization state is determined by the ionization parameter  ξ∝ F / n   , where F is the incident flux and n the gas density. It occurs in accretion, on to compact objects including black holes in both active galaxies and stellar-mass binaries, and possibly in gamma-ray bursts. Computation of model reflection spectra is often time consuming. Here we present the results from a comprehensive grid of models computed with our code, which has now been extended to include what we consider to be all energetically important ionization states and transitions. This grid is being made available as an ionized-reflection model, reflion , for xspec .     

Multi-fluid model of comet 1P/Halley

A new three-dimensional magnetohydrodynamic model of the coma of a comet has been developed and applied to simulations of a Halley-class coma using the solar-wind conditions of the Giotto flyby of Halley in 1986. The code developed for high-performance parallel processing computers, combines the high spatial resolution of smaller than 1 km grid spacing near the nucleus, with a large computational domain that enables structures nearly 10 million km down the comet tail to be modeled. Ions, neutrals, and electrons are considered as separate interacting fluids. Significant physical processes treated by the model include both photo and electron impact ionization of neutrals, recombination of ions, charge exchange between solar-wind ions and cometary neutrals, and frictional interactions between the three fluids considered in the model. A variety of plasma structures and physical parameters that are the output of this model are compared with relevant Giotto data from the 1986 Halley flyby.     

Limb-darkening corrections for interferometric uniform disc stellar angular diameters

Stellar angular diameters determined interferometrically are generally established by fitting the observed visibility data with a curve appropriate for a uniformly illuminated disc. The resulting uniform-disc diameters must be corrected for the effects of limb darkening in order to determine the true angular diameters of the stars. An extensive grid of limb-darkening corrections, based directly on the centre-to-limb intensity variations for Kurucz model stellar atmospheres, has been computed without the intermediate step of a parametrized representation of the centre-to-limb variation. The limitations of this method of correction are discussed.     

Improving performance of zoom-in cosmological simulations using initial conditions with customized grids

We present a method for customizing the root grid of zoom-in initial conditions used for simulations of galaxy formation. Starting from the white noise used to seed the structures of an existing initial condition, we cut out a smaller region of interest and use this trimmed white noise cube to create a new root grid. This new root grid contains similar structures as the original, but allows for a smaller box volume and different grid resolution that can be tuned to best suit a given simulation code. To minimally disturb the zoom region, the dark matter particles and gas cells from the original zoom region are placed within the new root grid, with no modification other than a bulk velocity offset to match the systemic velocity of the corresponding region in the new root grid. We validate this method using a zoom-in initial condition containing a Local Group analog. We run collisionless simulations using the original and modified initial conditions, finding good agreement. The dark matter halo masses of the two most massive galaxies at $z=0$ match the original to within 15%. The times and masses of major mergers are reproduced well, as are the full dark matter accretion histories. While we do not reproduce specific satellite galaxies found in the original simulation, we obtain qualitative agreement in the distributions of the maximum circular velocity and the distance from the central galaxy. We also examine the runtime speedup provided by this method for full hydrodynamic simulations with the ART code. We find that reducing the root grid cell size improves performance, but the increased particle and cell numbers can negate some of the gain. We test several realizations, with our best runs achieving a speedup of nearly a factor of two.     

A detailed structural analysis of the Deslandres area of the Moon

The components of the Lunar grid of the Deslandres area of the Moon are described and their azimuths plotted. The grid is shown to be the product of endogenic processes which may also have been important in the early geological evolution of the Earth.     

A grid of chemical evolution models as a tool to interpret spiral and irregular galaxies data

We present a generalization of the multiphase chemical evolution model (CEM) applied to a wide set of theoretical galaxies with different masses and evolutionary rates. This generalized set of models has been computed using the so-called universal rotation curve from Persic, Salucci & Steel to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time-scale or gas infall rate on to the disc. We have adopted 10 sets of values, between 0 and 1, for the molecular cloud and star formation efficiencies, as corresponding to their probability nature, for each one of the radial distributions of total mass. Thus, we have constructed a biparametric grid of models, depending on those efficiency sets and on the rotation velocity, whose results are valid in principle for any spiral or irregular galaxy. The model results provide the time-evolution of different regions of the disc and the halo along galactocentric distance, measured by the gas (atomic and molecular) and stellar masses, the star formation rate (SFR) and chemical abundances of 14 elements, for a total of 440 models. This grid may be used to estimate the evolution of a given galaxy for which only present time information, such as radial distributions of elemental abundances, gas densities and/or star formation, which are the usual observational constraints of chemical evolution models (CEMs), is available.     

CESAM: a free code for stellar evolution calculations

The cesam code is a consistent set of programs and routines which perform calculations of 1D quasi-hydrostatic stellar evolution including microscopic diffusion of chemical species and diffusion of angular momentum. The solution of the quasi-static equilibrium is performed by a collocation method based on piecewise polynomials approximations projected on a B-spline basis; that allows stable and robust calculations, and the exact restitution of the solution, not only at grid points, even for the discontinuous variables. Other advantages are the monitoring by only one parameter of the accuracy and its improvement by super-convergence. An automatic mesh refinement has been designed for adjusting the localisations of grid points according to the changes of unknowns. For standard models, the evolution of the chemical composition is solved by stiffly stable schemes of orders up to four; in the convection zones mixing and evolution of chemical are simultaneous. The solution of the diffusion equation employs the Galerkin finite elements scheme; the mixing of chemicals is then performed by a strong turbulent diffusion. A precise restoration of the atmosphere is allowed for.     

Sputtering of grains in C-type shocks

Sputtering yields are reported for the release of Mg, Fe, Si and O under impact of He, C, O, Si and Fe on grain material composed of Mg- and Fe-bearing silicates. The yields were derived using the trim code, which simulates the results of the transport of ions in matter by means of classical Monte Carlo techniques. The energetics of the sputtering process are a key factor in the sputtering calculations, and so detailed determinations have been made of the energy with which atoms are bound to the lattice, using solid-state simulation programs. The probability of ejection of an atom is computed at a given energy, for a number of angles of incidence, and integrated to obtain the mean yield at that energy. These numerical results are then fitted with a simple function of energy for convenience in subsequent applications.
A grid of C-type shock models has been computed, using our new sputtering yields, for pre-shock densities in the range 10⁴ n _H n (H)+2 n (H₂)10⁶ cm⁻³ and shock speeds 20 v _s45 km s⁻¹. Sputtered fractions can be high, exceeding 50 per cent for shock speeds in excess of approximately 40 km s⁻¹. The column densities of Si and SiO were also computed, for comparison with observations.     

Automatic grid azimuth by hour angle of the sun,a star or a planet using an electronic theodolite Kern E2

The paper deals with the automatic grid azimuth determination of an object on the earth using the electronic theodolite Kern E2 by observing the sun, a star, or a planet. The observation time and the readings of horizontal and vertical circles of the electronic theodolite enter automatically the electronic calculator Hewlett-Packard HP41CX. The calculator computes the grid azimuth of the terrestrial object and directs the observer what to do. Therefore, even a personnel without specific high education can make observations. For the solar observations it is not necessary to have an astronomical almanac, because the program computes the solar coordinates. The general input data, together with the measured ones, can be stored on a magnetic cassette and later on, if necessary, one can correct the general data and recalculate the azimuth. The method proved to be very practical in the field work.     

Confirmation of quiet pulsar model by the boundary element method

A method for solving the force-free surface problem for the pulsar magnetosphere is outlined. The given formulation is extended to an oblique rotator problem. Since we solve equation subject to the boundary values—i.e., the boundary element method (BEM) developed in Paper I is used—we can directly determine the force-free surface. Another merit of this method rests in the fact that we only use a two-dimensional grid, in spite of the problem being three-dimensional. A numerical calculation has been performed to confirm the solution by the particle method (Krause-Polstorff and Michel, 1985).