L.R.S. Bianchi Type I string dust magnetized cosmological models

L.R.S. Bianchi Type I string dust cosmological models with and without magnetic field following the techniques used by Letelier and Stachel, is investigated. To get a determinate solution, we assume a conditionσ is proportional to scalar of expansion θ where σ is shear and θ is scalar of expansion and which leads to A=ℓ B ⁿwhere n is a constant and ℓ is proportionality constant. Some special models are also investigated by introducing the transformation, , which leads to Riccati type differential equation. The physical and geometrical aspects of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cometary dust trail associated with Rosetta mission target: 67P/Churyumov-Gerasimenko

A thin, bright dust cloud, which is associated with the Rosetta mission target object (67P/Churyumov-Gerasimenko), was observed after the 2002 perihelion passage. The neckline structure or dust trail nature of this cloud is controversial. In this paper, we definitively identify the dust trail and the neckline structure using a wide-field CCD camera attached to the Kiso 1.05-m Schmidt telescope. The dust trail of 67P/Churyumov-Gerasimenko was evident as scattered sunlight in all images taken between September 9, 2002 and February 1, 2003, whereas the neckline structure became obvious only after late 2002. We compared our images with a semi-analytical dynamic model of dust grains emitted from the nucleus. A fading of the surface brightness of the dust trail near the nucleus enabled us to determine the typical maximum size of the grains. Assuming spherical compact particles with a mass density of 10³ kg m⁻³ and an albedo of 0.04, we deduced that the maximum diameter of the dust particles was approximately 1 cm. We found that the mass-loss rate of the comet at the perihelion was on or before the 1996 apparition, while the mass-loss rate averaged over the orbit reached . The result is consistent with the studies of the dust cloud emitted in the 2002/2003 return. Therefore, we can infer that the activity of 67P/Churyumov-Gerasimenko has showed no major change over the past dozen years or so, and the largest grains are cyclically injected into the dust tube lying along the cometary orbit.     

The dust grains in the coma of Comet West

The observed variation of reddening as function of the heliocentric distance and the spatial variation of reddening within the coma of Comet West in the visual wavelength range have been considered to infer the properties of the cometary dust grains. The relevant model incorporates the variation in the size distribution function as well as the composition of the spherical grains. The real part of the complex index of refraction (m = m – im) is chosen such thatm = 1.6. The imaginary part is required to vary from m = 0.2 to 0.05 over the wavelength range 0.4 to 0.7 m. This choice of refractive index corresponds to dirty silicate grains. As a by-product, the model also satisfies the observed polarization and albedo for the Comet West.     

Accelerating dust temperature calculations with graphics-processing units

When calculating the infrared spectral energy distributions (SEDs) of galaxies in radiation-transfer models, the calculation of dust grain temperatures is generally the most time-consuming part of the calculation. Because of its highly parallel nature, this calculation is perfectly suited for massively parallel general-purpose graphics-processing units (GPUs). This paper presents an implementation of the calculation of dust grain equilibrium temperatures on GPUs in the Monte-Carlo radiation transfer code sunrise, using the CUDA API. The GPU can perform this calculation 69 times faster than the eight CPU cores, showing great potential for accelerating calculations of galaxy SEDs.     

Infrared emission from P/Halley's dust coma during March 1986

2 to 20 micrometers photometry of the inner dust coma of comet Halley was obtained at the NASA IRTF on Mar 6.85, 12.8, 13.75, 17.7, and 24.8. Positions offset 10" were measured as well as the central brightness. The strength of the 10 micrometers emission feature was observed to vary with location in the coma. The infrared emission is in general agreement with the dust size distribution measured from the Vega and Giotto spacecraft. Mar 6.8, 17.7, and 24.8 corresponded to strong dust jet activity. The strength of the 10 micrometers silicate emission is shown to be a sensitive indicator of grain size and thus of jet activity. Dust production rate on March 13.75, 6 h before Giotto encounter, was approximately 10(7) gm s-1.     

The dust coma of Comet P/Giacobini-Zinner in the infrared

We present 1-20 micrometers photometry of P/Giacobini-Zinner obtained at the NASA Infrared Telescope Facility, during 1985 June-September (r = 1.57-1.03 AU). A broad, weak 10 micrometers silicate emission feature was detected on August 26.6; a similar weak emission feature could have been hidden in the broadband photometry on other dates. The total scattering and emitting cross section of dust in the inner coma was similar to that in other short-period comets, but a factor of 10 (r = 1.56 AU) to 100 (r = 1.03 AU) lower than the amount of dust in Comet Halley. The thermal emission continuum can be fit with models weighted toward either small or large absorbing grains. The dust production rate near perihelion was approximately 10(5) g/s (small-grain model) to approximately 10(6) g/s (large-grain model). The corresponding dust/gas mass ratio on August 26 was approximately 0.1-1. A silicate-rich heterogeneous grain model with an excess of large particles is compatible with the observed spectrum of Giacobini-Zinner on August 26. Thus, weak or absent silicate emission does not necessarily imply an absence of silicates in the dust, although the abundance of silicate particles < or = 1 micrometer radius must have been lower than in Comet Halley.     

The dust coma of periodic comet Churyumov-Gerasimenko (1982 VIII)

The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ～2 × 10⁵ g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ～0.04 at 1.25 μm and ～0.05 at 2.2 μm.     

Temporal deconvolution of the hydrogen coma I. A hybrid model

This paper describes a semi-analytical model, tentatively called the time-resolved model (TRM), for calculating the proper temporally resolved column density and Lyman-alpha intensity profiles of the extended cometary hydrogen coma, with potentially arbitrary number of photodissociation sources with different initial velocity profiles. The non-analytical part consists of a true 3D particle simulator whose results are used to obtain the necessary corrective parameters for the analytical, spherically symmetric formulas for the column density and intensity profiles. The hybrid approach provides a physically accurate and computationally effective method to recreate the extended coma profiles and supports a new analysis method that can provide hydrogen lifetime estimates and a much enhanced temporal resolution for the water production curve, for which a method is described. The ability to recreate the water production profile by different analysis methods is studied with a combination of real and simulated data. This is the first part of a two part publication with the second part dedicated to real data analysis.     

Monte-Carlo and multifluid modelling of the circumnuclear dust coma II. Aspherical-homogeneous, and spherical-inhomogeneous nuclei

We use our newly developed Dust Monte-Carlo (DMC) simulation technique [Crifo, J.F., Lukianov, G.A., Rodionov, A.V., Zakharov, V.V., 2005. Icarus 176, 192-219] to study the dynamics of dust grains in the vicinity of some of the benchmark aspherical, homogeneous cometary nuclei and of the benchmark spherical, inhomogeneous nuclei studied by us precedingly. We use the interim unrealistic simplifying assumptions of grain sphericity, negligible nucleus rotation rate, and negligible tidal force, but take accurately into account the nucleus gravitational force, gas coma aerodynamic force, and solar radiation pressure force, and consider the full mass range of ejectable spherical grains. The resulting complicated grain motions are described in detail, as well as the resulting complicated and often counter-intuitive dust coma structure. The results are used to answer several important questions: (1) When computing coma dust distributions, (a) is it acceptable to take into consideration only one or two of the above mentioned forces (as currently done)? (b) to which accuracy must these forces be known, in particular is it acceptable to represent the gravity of an aspherical nucleus by a spherically symmetric gravity? (c) how do the more efficient but less general Dust Multi-Fluid (DMF) computations compare with the DMC results? (2) Are there simple structural relationships between the dust coma of a nucleus at small heliocentric distance rh, and that of the same nucleus at large rh? (3) Are there similarities between the gas coma structures and the associated dust coma structures? (4) Are there dust coma signatures revealing non-ambiguously a spherical nucleus inhomogeneity or an homogeneous nucleus asphericity? (5) What are the implications of the apparently quite general process of grain fall-backs for the evolution of the nucleus surface, and for the survival of a landed probe?     

Irradiation of dust in molecular clouds. I. UV doses

The radiation fluxes inside molecular clouds owing to a neighboring class A star or to isotropic interstellar irradiation are calculated. Radiation within the interval 912 ? < λ < 2067 ? is found to penetrate deeply enough to ensure a radiation dose for water ice on the order of 100 eV/amu or more over the lifetime of the clouds, whether a star formation region is present or not. The possibility is discussed of using these results for an astrophysical interpretation of published data from laboratory experiments on irradiation of ices of the type H₂O:CH₃OH:NH₃:CO. The resulting radiation-chemical transformation of complex organic materials may play an important role in the prebiological evolution of the dust component of molecular clouds. Translated from Astrofizika, Vol. 52, No. 2, pp. 311–324 (May 2009).     

Three-component dust models for interstellar extinction

Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R _V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R _V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R _V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.     

Interplanetary dust dynamics: I. Long-term gravitational effects of the inner planets on zodiacal dust

Whereas the inner planets' perturbations on meteoroids' and larger interplanetary bodies' orbits have been studied extensively, they are usually neglected in studies of the dynamics of smaller particles producing the zodiacal light through scattering of sunlight. Forces acting on these dust particles are fairly well known and include radiation forces and interaction with the solar wind. This article is the first in a series aimed at improving our knowledge of the dynamical evolution of dust in interplanetary space by studying the combined effects of these perturbations including gravitational perturbations by the planets Venus, Earth, Mars, and Jupiter. The necessity of including effects of the inner planets in dust dynamics investigations is established. Sample trajectories are presented to illustrate commonly occurring phenomenae, such as nonmonotonic changes in semimajor axis, eccentricity, inclination, and in the line of nodes. These perturbations are shown to be due to the inner planets as opposted to Jupiter or nongravitational forces.     

Investigations of the pre-Deep Impact morphology of the dust coma of Comet Tempel-1

The pre-Deep Impact images of Comet Tempel-1 obtained at the Indian Astronomical Observatory are used to investigate the morphology of the dust coma of the comet. We show that the trajectory of a cometary grain under the influence of solar radiation pressure is a reliable diagnostic to estimate its initial velocity. Four main active regions at mean latitudes +45° ± 5°(D), 0° ± 5° (E),−30° ± 5°(A) and−60° ± 5°(F) are found to explain the morphology of the dust coma in the ground-based and published images obtained by the High Resolution Instrument(HRI) cameras aboard the Deep Impact flyby spacecraft. From a χ² fit of the intensity distribution in the observed and the simulated images, we derive the fraction of the productivity of the active vents to the total dust emission of the comet to be 27%. Of this the southern source alone accounts for 19.8%. The grains are found to be ejected with a velocity distribution with an upper limit of 70 ± 7 m s⁻¹. However, the broad region ‘A’ appears to eject slower grains with an upper limit of 24 ± 2.5 m s⁻¹. This source, that is active throughout the cycle is likely to be driven by CO₂ sublimation. We compute the dependence of the percentage contribution of the southern source on the heliocentric distance and show that this ratio varies over the apparition and reaches a maximum at around 260 days before perihelion. The published images of the nucleus of Comet Tempel-1 show significant departure from sphericity. Therefore, the torque exerted by the enhanced activity of the southern region may be significant enough to produce changes in the rotational state of the nucleus before each perihelion passage.     

A study of some stars with circumstellar dust envelopes. I

We present the results of a study of circumstellar dust envelopes of 36 stars of early(O-B-A) types in the directions of the associations Cas OB1, Cas OB2, Per OB1, and Ori OB1. We determine the absorption at 1640 Å, the linear radius of the dust envelopes, the mean value of the coefficient k, and the masses of the envelopes. They differ significantly from one another.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

The composition and size distribution of the dust in the coma of Comet Hale-Bopp

We discuss the composition and size distribution of the dust in the coma of Comet Hale-Bopp. We do this using a model fit for the infrared emission measured by the Infrared Space Observatory (ISO) and the measured degree of linear polarization of scattered light at various phase angles and wavelengths. The effects of particle shape on the modeled optical properties of the dust grains are taken into account. Both the short wavelength (7-44 μm) and the long wavelength (44-120 μm) infrared spectrum are fitted using the same dust parameters, as well as the degree of linear polarization at twelve different wavelengths in the optical to near-infrared domains. We constrain our fit by forcing the abundances of the major rock forming chemical elements to be equal to those observed in meteorites. The infrared spectrum at long wavelengths reveals that large grains are needed in order to fit the spectral slope. The size and shape distribution we employ allows us to estimate the sizes of the crystalline silicates. The ratios of the strength of various forsterite features show that the crystalline silicate grains in Hale-Bopp must be submicrometer-sized. On the basis of our analysis the presence of large crystalline silicate grains in the coma can be excluded. Because of this lack of large crystalline grains combined with the fact that we do need large amorphous grains to fit the emission spectrum at long wavelengths, we need only approximately 4% of crystalline silicates by mass (forsterite and enstatite) to reproduce the observed spectral features. After correcting for possible hidden crystalline material included in large amorphous grains, our best estimate of the total mass fraction of crystalline material is ∼7.5%, which is significantly lower than deduced in previous studies in which the typical derived crystallinity is ∼20-30%. The implications of this low abundance of crystalline material on the possible origin and evolution of the comet are discussed. We conclude that the crystallinity we observe in Hale-Bopp is consistent with the production of crystalline silicates in the inner Solar System by thermal annealing and subsequent radial mixing to the comet forming region (∼30 AU).     

Infrared signatures and models of circumstellar dust disks

This contribution describes theoretical models for the temperature and density structure of circumstellar disks and what properties of disks may be determined from the analysis of disk images and spectral energy distributions. I will summarize the contributions from several groups that have developed publicly available radiation transfer codes and grids of models of disk spectra. In addition, tools for fitting spectra from large datasets, such as the growing Spitzer archive, are now available.     

Magnetostatic models filled with dust and disordered radiation

Magnetostatic models filled with dust and disordered radiation in which the distribution is that of perfect fluid, are obtained. Some physical and geometrical aspects of the models are investigated. In the last section of the paper, the Newtonian analogues of forces are also obtained.     

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.