A Model of an Active Region on the Surface of a Cometary Nucleus

An active region on the surface of a cometarynucleus is considered as a conic hole in the surface dust mantle with icy bottom and dusty side-walls. This conic structureshould concentrate solar energy onto the bottom andtherefore enhance sublimation. Preliminary results of thecalculation of this effect are given. The temperature distributionat the bottom of the crater is calculated for different sets ofits geometrical parameters. Effects of intensified sublimation depending on the geometrical parameters areconsidered for the specific case when a single active region islocated exactly at the pole of the nucleus and the pole is directed tothe Sun.     

On the Light-Absorbing Surface Layer of Cometary Nuclei: I. Radiative Transfer

A small but increasing volume of observations of cometary nuclei has accumulated during the past two decades. This development is accelerating with upcoming space missions such as Stardust, Contour, and Rosetta. In response to the growing need for a theoretical understanding of optical properties of cometary nuclei, we have calculated synthetic reflectance spectra in the wavelength region 0.2-2.0 μm, photometric colors in the Johnson-Kron-Cousins UBVRI system, and visual geometric albedos for a large number of porous ice-dust mixtures with differing composition, regolith grain sizes, and grain morphologies, such as core-mantle grains, dense clusters of such grains, and large irregular particles with internal scatterers. The calculations are based on Mie theory, the discrete dipole approximation, Hapke theory, and a numerical solution to the equation of radiative transfer in particulate media. In addition, wavelength-integrated directional-hemispherical albedos and flux attenuation profiles in the regolith as functions of depth have been calculated in order to improve the energy budget and treatment of energy boundary conditions in thermal models of cometary nuclei.Our results are compared with spectra and colors of observed cometary nuclei. Our main conclusions are that only regolith consisting of relatively large core-mantle grains, or clusters of smaller core-mantle grains, is capable of reproducing the red colors seen in comets; that ice-dust mixtures actually can be darker than ice-free regolith in certain circumstances; and that solar radiation sometimes penetrates to a depth that is comparable to the region in which diurnal temperature variations occur.     

On the Light-Absorbing Surface Layer of Cometary Nuclei: II. Thermal Modeling

The classical way to treat absorption of solar light in thermophysical modeling of cometary nuclei (and other ice-rich bodies such as jovian satellites) has been to assume complete opaqueness of the surface material. However, as shown by Davidsson and Skorov (2002, Icarus156, 223-248), substantial light penetration can occur in porous ice even if it is very dusty, implying that gradual absorption of energy in a surface layer should be accounted for.We present a thorough comparison between a surface energy absorption model and a layer energy absorption model, for various combinations of heliocentric distances, conductivities, opacities, pore sizes, and rotational periods relevant for cometary nuclei, by fully solving the coupled differential equations of heat transfer and gas diffusion. We find substantial differences between the models in terms of gas production rate, thermal lag angle, surface temperature, and the origin of coma molecules. For example, the surface energy absorption model overestimates the total gas production by a factor of 2-7, underestimates the lag angle by a factor of 2-3, and places the origin of coma molecules at the surface, instead of the near-surface interior.     

GSC3658-0076: A Binary System at the Beginning of Mass Transfer

Photometric data of the new discovered binary GSC3658-0076 observed by [González-Rojas et al.: 2003, IBVS, No. 5437.] were analyzed using the latest Wilson-Devinney code. The system turns out to be a detached binary system with the primary component almost filling its Roche lobe, while the secondary one is detached from the critical Roche lobe. According to the mass-radius relation of unevolved (ZAMS) detached binaries given by [Demircan and Kahraman: 1991, Ap&SS 181, 313.], the primary component is more evolved. These properties reveal that GSC3658-0076 may be at the beginning of the mass transfer phase and may evolve from the present detached system into a contact binary or be in the broken-contact phase predicted by TRO theory.     

Gasdynamical Treatment of Mass Transfer in Symbiotic Stars

The results of two-dimensional, non-adiabatic gas dynamical simulations of gas flows in symbiotic stars are presented. It is shown that for a binary system with components that do not fill their Roche lobes, the structure of the gaseous stream is determined not only by the flow from the vicinity of the inner Lagrangian point, but also by the flow of matter caused by the orbital motion of the accretor through the gas of the stellar wind.The calculated gaseous flow structure consists of a set of shocks and tangential discontinuities. It is found that for all considered cases two bow shocks exist. One of the bow shocks is located in front of the accretor in the path of its orbital motion, and the second one is between the components.It is found that matter is accreted in a spiral fashion, forming an accretion disk. In steady-state simulations, a structure with shocks (both trailing and leading) in the disk was observed. Gas dynamical perturbations of the disk's outer edge by the accreting gas inflow lead to the formation of two intensive trailing shocks propagating inward. Gas dynamical perturbations of the inner part of the disk result in the occurrence of leading shocks. One possible mechanism for the generation of spiral shocks is dissipative instability. The existence of spiral shocks results in a redistribution of the angular momentum of the disk material and an increase of the accretion rate.     

Cometary—Meteoric Gas in the Solar Corona: The Dynamics of Ca Ions

Prolonged searches for the cold matter brought into the solar corona from the outskirts of the Solar system have finally been crowned with success. After the first prediction about the possibility of detecting emission from certain atoms and low-charge ions made by Shestakova in 1990, several attempts have been made to detect the resonance emission of Ca ions in the solar corona. The wide-field interferometric observations of the corona in the Ca II H and K lines performed by Gulyaev on Shcheglov's facility revealed an extended emission region to the west of the Sun only during the solar eclipse of February 26, 1998. We attempt to construct a model for the motion of Ca ions after their separation from the parent body. The ions move away from the Sun mainly under light pressure. Reasonably good agreement of the model with the observed radial velocities and the emission configuration is achieved by assuming that the chain of parent bodies moves from the south northward in a parabolic orbit almost perpendicularly to the plane of the ecliptic.     

A Closer View of the Nucleus of NGC 4314

We present a high-resolution kinematical study of the ionized gas in the circumnuclear region of NGC 4314. Our spectra reveal the presence of a central structure (apparently a ring or torus) located at ∼ 2 arcsec from the nucleus, which we identify as the shocked interface between the nuclear bar and the gas inside the HII region ring. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrodynamic Aspects of Modeling of the Mass Transfer and Coagulation Processes in Turbulent Accretion Disks

This paper considers, in the context of modeling the evolution of a protoplanetary cloud, the hydrodynamic aspects of the theory of concurrent processes of mass transfer and coagulation in a two-phase medium in the presence of shear turbulence in a differentially rotating gas–dust disk and of polydisperse solid particles suspended in a carrying flow of solid particles. The defining relations are derived for diffuse fluxes of particles of different sizes in the equations of turbulent diffusion in the gravitational field, which describe the convective transfer, turbulent mixing, and sedimentation of disperse dust grains onto the central plane of the disk, as well as their coagulation growth. A semiempirical method is developed for calculating the coefficients of turbulent viscosity and turbulent diffusion for particles of different kinds. This method takes into account the inverse effects of dust transfer on the turbulence evolution in the disk and the inertial differences between disperse solid particles. To solve rigorously the problem of the mutual influence of the turbulent mixing and coagulation kinetics in forming the gas–dust subdisk, the possible mechanisms of gravitational, turbulent, and electric coagulation in a protoplanetary disk are explored and the parametric method of moments for solving the Smoluchowski integro-differential coagulation equation for the particles' size distribution function is considered. This method takes into account the fact that this distribution belongs to a definite parametric class of distributions.     

Toward a Taxonomy of the Edgeworth–Kuiper Objects: A Multivariate Approach

The principal component (PC) and G-mode multivariate statisticshave been used in analysing the set of the 34 Edgeworth–Kuiper objects (EKOs) – 23 Trans NeptunianObjects (TNO) and 11 Centaurs – for which B, V, R, I, J homogeneous photometry were available.The results obtained show that V-I and V-Js are the key parameters in structuring the sample inhomogeneous groups. The PC1 axis (which contains ～93% of the sample total variance) spans fivetimes more than the PC2 (which contains ～6% of the sample total variance). The extremesof the PC1 axis contain the objects having a flat spectrum (low PC1 values) and a very red spectrumrespectively. Independently, the G-mode analysis allows us to distinguish six homogeneous groups of objectswhich confirm and extend the results obtained with the PC analysis. In addition to these groups, a few objectsremain not included in any group (i.e., does not have significant similitude with other objects) and yet givean indication of a more complex compositional structure of the sample. These preliminary results will haveto be confirmed and completed when a larger sample is available, but they provide some interesting hintsfor understanding the – mainly collisional – evolution of the EKOs.     

Tidal Angular Momentum in Close Binary Systems in presence of Wind Driven Non-conservative Mass Transfer with Uniform Mass Accretion Rate

A very well-known property of close binary stars is that they usually rotate slowly than a similar type single star. Massive stars in close binary systems are supposed to experience an exchange of mass and angular momentum via mass transfer and tidal interaction, and thus the evolution of binary stars becomes more complex than that of individual stars. In recent times, it has become clear that a large number of massive stars interact with binary companions before they die. The observation also reveals that in close pairs the rotation tends to be synchronized with the orbital motion and the companions are naturally tempted to invoke tidal friction. We here introduce the effect of tidal angular momentum in the model of wind driven non-conservative mass transfer taking mass accretion rate as uniform with respect to time. To model the angular momentum evolution of a low mass main sequence companion star can be a challenging task. So, to make the present study more interesting, we have considered initial masses of the donor and gainer stars at the proximity of bottom-line main sequence stars and they are taken with lower angular momentum. We have produced a graphical profile of the rate of change of tidal angular momentum and the variation of tidal angular momentum with respect to time under the present consideration.     

Variability in the Nucleus of NGC 1068

The near-infrared output of the nucleus of NGC 1068 increased by a factor of two over the 18 years between 1976 and 1994 and has recently started to decline. It is not clear whether this event has been the response of an extended dusty region to a single outburst in the central engine or is the result of a continuous change in its XUV output. The integrated energy of the event amounts to well over 1052 erg and thus cannot be due to a single supernova. The variable part of the infrared flux from NGC 1068, observed through an aperture of constant diameter, is found to have a constant spectral shape. Comparison with similar data from other Seyfert galaxies shows that the variable infrared component in NGC 1068 is reddened by about 20 mag in Av if the emission mechanism is similar in all cases. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Question of Calculating the Parameters of Vortices in the Near-Surface Atmosphere of Mars

Solar System Research - The paper is dedicated to the study of dust vortices on the Earth and Mars. The hydrodynamic similarity of convective vortices is considered, and the similarity criteria are...     

Amorphous Magnesiosilicate Smokes Annealed in Vacuum: The Evolution of Magnesium Silicates in Circumstellar and Cometary Dust

The mid-infrared spectral evolution of amorphous metastable eutectic magnesiosilica smokes, obtained by kinetically controlled gas-to-solid condensation of a Mg-SiO-O₂-H₂ vapor, proceeded in three distinct phases as a function of increasing time and temperature. This paper reports the mineralogical and chemical properties of these same samples. We found a previously unanticipated size dependence of the petrologic development of the initially amorphous magnesiosilica smokes that may also be at least partially responsible for the previously reported spectroscopic changes. Condensed grains less than approximately 20 nm in diameter remained amorphous throughout the thermal annealing experiment. Mineralogical changes occurred only after fusion of condensed magnesiosilica grains and chemical homogenization of large amorphous agglomerates and ring structures. Kinetically favored nucleation and growth produced the thermodynamically unstable nanocrystalline assemblage forsterite + tridymite. Further mineralogical development was stalled until continued fusing of agglomerates, rings, and some fraction of condensed grains had produced smooth amorphous magnesiosilica sheets of 42 and 20 wt% MgO. In rare sheets with more than ∼55 wt% MgO large forsterite crystals had grown, while enstatite had nucleated in low-MgO sheets still in the presence of forsterite and tridymite. The mineralogical evolution of the samples is critically dependent on the mass of the structural entities in the condensed sample and seems to be restricted to fused agglomerates and ring structures larger than about 20 nm in diameter and the sheet materials. We discuss the implications of our study for the interpretation of similar astrophysical dust analog studies and for astrophysical applications.     

Analysis of the Hipparcos Measurements of HD 10697: A Mass Determination of a Brown Dwarf Secondary

HD 10697 is a nearby main-sequence star around which a planet candidate has recently been discovered by means of radial velocity measurements (Vogt et al.). The stellar orbit has a period of about 3 yr, the secondary minimum mass is 6.35 Jupiter masses (MJ), and the minimum semimajor axis is 0.36 mas. Using the Hipparcos data of HD 10697 together with the spectroscopic elements of Vogt et al., we found a semimajor axis of 2.1+/-0.7 mas, implying a mass of 38+/-13 MJ for the unseen companion. We therefore suggest that the secondary of HD 10697 is probably a brown dwarf, orbiting around its parent star at a distance of 2 AU.     

Gravitational Instability in the Dust Layer of a Protoplanetary Disk with Interaction between the Layer and the Surrounding Gas

We consider gravitational instability of the dust layer in the midplane of a protoplanetary disk with turbulence and shear stresses between the gas in the disk and that in the dust layer. We solve a linearized system of hydrodynamic equations for perturbations of dust (monodisperse) and gas phases in the incompressible gas approximation. We take into account the gas drag of solid particles (dust aggregates), turbulent diffusion and the velocity dispersion of particles, and the perturbation of the azimuthal velocity of gas in the layer upon the transfer of angular momentum from solid particles to it and from this gas to the surrounding gas in the disk. We obtain and solve the dispersion equation for the layer with the ratio of surface densities of the dust phase and gas being well above unity. The following parameters of gravitational instability in the dust layer are calculated: the critical surface density of solid matter and the Stokes number of particles corresponding to the onset of instability, the wavelength range in which instability occurs, and the rate of its growth as a function of the perturbation wavelength in the circumsolar disk at radial distances of 1 and 10 AU. We show that at 10 AU, the maximum instability growth rate increases due to the transfer of angular momentum of gas in the layer to gas outside it, a new maximum emerges at a longer wavelength, a long-wavelength instability “tail” forms, and the critical surface density initiating instability decreases relative to that determined without the transfer of angular momentum to gas outside the layer. None of these effects are observed at 1 AU, since instability in this region probably develops faster than the transfer of angular momentum to the surrounding gаs of a protoplanetary disk occurs.     

A New Approach to the Electro-optical Tracking of a Space Object

The electro-optical tracking of a space object is often interrupted by the dropped frames under the interference of stars. In this paper, the two stages of the tracking process, namely the state extrapolation and real-time sampling, are studied from a probabilistic point of view. Based on the theory of estimation, a new method is proposed, which improves the robustness of prediction by utiliz- ing the data-fusion property of Kalman ?ltering, and realizes the adjustment of the tracking window by the mean shift technique. The corresponding experiment shows that this method can effectively reduce the impact of interferences, and improve the robustness of the tracking process.     

一种空间目标光电跟踪新方法

针对目前空间目标光电观测的自动跟踪过程中,容易受到恒星干扰等影响丢帧后失跟的问题,从概率观点分析了光电跟踪的两个环节:状态外推和实时采样.根据估计理论,建立了一种新的光电跟踪方法.方法利用卡尔曼滤波数据融合的本质提高外推的稳健性,通过均值漂移方法实现跟踪窗口的调整.仿真试验表明,新方法能够有效的降低跟踪中干扰的影响,提高跟踪的稳健性.     

On the Collision Nature of Two Coronal Mass Ejections: A Review

Observational and numerical studies have shown that the kinematic characteristics of two or more coronal mass ejections (CMEs) may change significantly after a CME collision. The collision of CMEs can have a different nature, i.e. inelastic, elastic, and superelastic processes, depending on their initial kinematic characteristics. In this article, we first review the existing definitions of collision types including Newton’s classical definition, the energy definition, Poisson’s definition, and Stronge’s definition, of which the first two were used in the studies of CME–CME collisions. Then, we review the recent research progresses on the nature of CME–CME collisions with the focus on which CME kinematic properties affect the collision nature. It is shown that observational analysis and numerical simulations can both yield an inelastic, perfectly inelastic, merging-like collision, or a high possibility of a superelastic collision. Meanwhile, previous studies based on a 3D collision picture suggested that a low approaching speed of two CMEs is favorable for a superelastic nature. Since CMEs are an expanding magnetized plasma structure, the CME collision process is quite complex, and we discuss this complexity. Moreover, the models used in both observational and numerical studies contain many limitations. All of the previous studies on collisions have not shown the separation of two colliding CMEs after a collision. Therefore the collision between CMEs cannot be considered as an ideal process in the context of a classical Newtonian definition. In addition, many factors are not considered in either observational analysis or numerical studies, e.g. CME-driven shocks and magnetic reconnections. Owing to the complexity of the CME collision process, a more detailed and in-depth observational analysis and simulation work are needed to fully understand the CME collision process.     

Cometary cryo-volcanism: Source regions and a model for the UT 2005 June 14 and other mini-outbursts on Comet 9P/Tempel 1

Data on the UT 2005 June 14 mini-outburst of Comet 9P/Tempel 1 taken from different viewpoints have been examined for morphological differences and parallax. The data were taken with the Hubble Space Telescope (HST), from the Deep Impact (DI) spacecraft, and from the Calar Alto Observatory, Spain. The mini-outburst source region was found to be located near 218 ± 6E, 6 ± 5N on the Deep Impact nucleus shape model. The mini-outburst occurred at ∼12 pm local solar time. The distribution of light in the mini-outburst is similar to that expected for an ejecta curtain. The method and software used to determine the surface location was checked using position angles of the impact ejecta plume as seen from DI and HST. The general region of impact was recovered and a downrange tilt of the ejecta curtain axis of 10.2 deg from the surface normal was found. We computed tracks of possible source regions for nine other mini-outbursts seen from DI. Five of these tracks converge on the 2005 June 14 event location. Three of the tracks converge at a second location near (60E, 20S), well separated from the first. Multiple mini-outbursts arise at each location either from a single source or from a few sources in close proximity. The mini-outbursts occur both at night and during the day indicating at most weak, if any, control by direct sunlight. The times of outburst are non-random with a preference for early afternoon, dusk and midnight. None of the mini-outbursts occurred near dawn. They occur at low latitudes (between ±40 deg) near the points where the principal axis of minimum moment of inertia cuts the surface. These regions are furthest from the center of figure and have the lowest effective surface gravity. We use these results to develop a conceptual model of the mini-outburst process and make comparisons with the theoretical calculations. We find that the tensile strength of the sub-surface material must be very low (e.g., ) and, on the basis of features imaged on the western facet of the nucleus, suggest that inflation of the sub-surface may be occurring. Our model makes specific predictions about the kind of surface morphology that should result from mini-outburst activity. We show that one of the isolated rimless depressions and the close-packed depressions found in the Deep Impact images have the properties needed and identify them as possible sites of past and current mini-outburst activity.