Radiation pressure on dust aggregates in circumstellar disks

We apply the ballistic particle-cluster and cluster-cluster aggregation of spherical monomers identical in size and material composition to study the effect of the particle's shape and structure on the radiation pressure force acting on circumstellar dust particles. Furthermore, the influence of the material composition on the radiation pressure is investigated based on the assumption that the constituents of dust aggregates are composed of either silicate or carbon.We show that the ratio of radiation pressure to stellar gravity in the radial direction from a star is weaker for aggregates than for homogeneous spherical grains in the radius range of submicron or less. Therefore fluffy dust particles of submicron radius have a longer dynamical lifetime, compared to compact spherical particles. We also show that the nonradial component of the radiation pressure force can reach the same order of magnitude as the radial component of the radiation pressure reduced by stellar gravity for aggregates of submicron or less in size. This non-radial component of the radiation pressure may yield a component of random motion along the trajectories of the particles.     

Radiation pressure on bacterial clumps in the solar vicinity and their survival between interstellar transits

Radiation pressure cross-sections for clumps of hollow bacterial grains with thin coatings of graphite are calculated using rigorousGüttler formulae. The carbonized skins are expected to form through exposure to solar ultraviolet radiation, but a limiting thickness of about 0.03 μm is determined by opacity effects. The ratios of radiation pressure to gravity P/G are calculated for varying sizes of the clumps and for varying thickness of the graphite coatings. Bacterial clumps and individual desiccated bacteria without coatings of radii in the range 0.3–8 μm have P/G ratios less than unity, whereas particles with coatings of 0.02μmthickness have ratios in excess of unity. Such coatings also provide protection from damaging ultraviolet radiation. Putative cometary bacteria, such as have beenrecently collected in the stratosphere, are thus not gravitationally bound in the solar system provided they possess carbonised exterior coatings. They are rapidly expelled from the solar system reaching nearby protosolar nebulae in timescales of a few million years. Even with the most pessimistic assumptions galactic cosmic rays are unable to diminish viability to an extent that vitiates the continuity of panspermia. This revised version was published online in July 2006 with corrections to the Cover Date.     

Porous dust grains in the shells of Herbig Ae/Be stars

The transfer of polarized radiation in inhomogeneous circumstellar shells with a spheroidal spatial distribution of porous dust particles is computed. The grains are modeled by an MRN mixture of silicate and graphite particles. The optical properties of porous particles (considered separately in the Appendix) are computed by using effective medium theory and Mie theory. The following observational characteristics have been computed for WW Vul, a typical Herbig Ae star with Algol-like minima: the spectral energy distribution from the ultraviolet to the far infrared, the color-magnitude diagrams, the wavelength dependence of linear polarization, and the shell brightness distribution. The effect of grain porosity on the results is considered. It has been found that only moderate particle porosity (the volume fraction of matter is f ～0.5) can explain available observational data in terms of the approach used. Since radiation pressure must rapidly sweep submicron-sized grains out of the vicinity of Herbig Ae/Be stars, we briefly discuss how particle porosity can affect this process.     

Hamiltonian Formulation and Perturbations for Dust Motion Around Cometary Nuclei

In this paper we analyze the dynamical behavior of large dust grains in the vicinity of a cometary nucleus. To this end we consider the gravitational field of the irregularly shaped body, as well as its electric and magnetic fields. Without considering the effect of gas friction and solar radiation, we find that there exist grains which are static relative to the cometary nucleus; the positions of these grains are the stable equilibria. There also exist grains in the stable periodic orbits close to the cometary nucleus. The grains in the stable equilibria or the stable periodic orbits won’t escape or impact on the surface of the cometary nucleus. The results are applicable for large charge dusts with small area-mass ratio which are near the cometary nucleus and far from the Solar. It is found that the resonant periodic orbit can be stable, and there exist stable non-resonant periodic orbits, stable resonant periodic orbits and unstable resonant periodic orbits in the potential field of cometary nuclei. The comet gravity force, solar gravity force, electric force, magnetic force, solar radiation pressure, as well as the gas drag force are all considered to analyze the order of magnitude of these forces acting on the grains with different parameters. Let the distance of the dust grain relative to the mass centre of the cometary nucleus, the charge and the mass of the dust grain vary, respectively, fix other parameters, we calculated the strengths of different forces. The motion of the dust grain depends on the area-mass ratio, the charge, and the distance relative to the comet’s mass center. For a large dust grain (> 1 mm) close to the cometary nucleus which has a small value of area-mass ratio, the comet gravity is the largest force acting on the dust grain. For a small dust grain (< 1 mm) close to the cometary nucleus with large value of area-mass ratio, both the solar radiation pressure and the comet gravity are two major forces. If the a small dust grain which is close to the cometary nucleus have the large value of charge, the magnetic force, the solar radiation pressure, and the electric force are all major forces. When the large dust grain is far away from the cometary nucleus, the solar gravity and solar radiation pressure are both major forces.     

The size distribution of dust grains in single clouds – II. The analysis of extinction using inhomogeneous grains

The extinction curves of single clouds, seen towards the stars HD 147165, 179406 and 202904, have been modelled using various mixtures containing both the bare and inhomogeneous (composite and/or multilayer) grains. It has been shown that the models composed of the bare graphite and silicate grains together with the multilayer grains containing silicates, organic refractory and water ice, are more useful in explaining extinction under the reduced cosmic abundances. The models based on Mathis' composite grains or on Greenberg & Li's core-mantle grains can also provide quite good fits of the extinction and the measured scattering parameters, but still require an excessive amount of carbon which results in too large a C/O ratio. The inhomogeneous grains essentially contribute to the extinction in practically the whole wavelength range of our extinction curves. As a rule, such grains have quite wide size distributions, centred at around 100 nm, although graphite grains are mainly of small sizes.     

Radiation Pressure and the Poynting-Robertson Effect for Fluffy Dust Particles

A correct understanding of the dynamical effect of solar radiation exerted on fluffy dust particles can be achieved with assistance of a light scattering theory as well as the equation of motion. We reformulate the equation of motion so that the radiation pressure and the Poynting-Robertson effect on fluffy grains are given in both radial and nonradial directions from the center of the Sun. This allows numerical estimates of these radiation forces on fluffy dust aggregates in the framework of the discrete dipole approximation, in which the first term of the scattering coefficients in Mie theory determines the polarizability of homogeneous spheres forming the aggregates.The nonsphericity in shape turns out to play a key role in the dynamical evolution of dust particles, while its consequence depends on the rotation rate and axis of the grains. Unless a fluffy dust particle rapidly revolves on its randomly oriented axis, the nonradial radiation forces may prevent, apart from the orbital eccentricity and semimajor axis, the orbital inclination of the particle from being preserved in orbit around the Sun. However, a change in the inclination is most probably controlled by the Lorentz force as a consequence of the interaction between electric charges on the grains and the solar magnetic field. Although rapidly and randomly rotating grains spiral into the Sun under the Poynting-Robertson effect in spite of their shapes and structures, fluffy grains drift inward on time scales longer at submicrometer sizes and shorter at much larger sizes than spherical grains of the same sizes. Numerical calculations reveal that the dynamical lifetimes of fluffy particles are determined by the material composition of the grains rather than by their morphological structures and sizes. The Poynting-Robertson effect alone is nevertheless insufficient for giving a satisfactory estimate of lifetimes for fluffy dust grains since their large ratios of cross section to mass would reduce the lifetimes by enhancing the collisional probabilities. We also show that the radiation pressure on a dust particle varies with the orbital velocity of the particle but that this effect is negligibly small for dust grains in the Solar System.     

Grain growth by radiation pressure induced coagulation

A theoretical treatment is given of the growth of grains as a consequence of their mutual coagulation brought about by relative motions induced by radiation pressure. Analytical and numerical techniques are employed to tackle the relevant coagulation equation. The results are of particular astrophysical significance in the context of forming very small grains following a nucleation process, in the production of grains large enough to allow condensation of volatiles onto their surface, and in any situation where the supply of volatiles has been exhausted. It was found that in interstellar clouds, grains composed of iron, graphite and glassy carbon, being typical examples of three basic types of material, could grow to a size where condensation of the volatiles was possible. On the other hand, olivine, a typical silicate, could not. If a source of radiation existed at the centre of the cloud, then growth could occur if the cloud was turbulent or if the density was high enough; otherwise the grains were driven out of the regions of interest at high velocity. In the latter case, with a high cloud density, re-radiation has to be taken into account.     

Dust Grains in the Comae and Tails of Sungrazing Comets: Modeling of Their Mineralogical and Morphological Properties

Observations of sungrazing comets, all of which belong to the Kreutz family, provide the opportunity of studying the properties of dust in the comae and tails of the comets. On the basis of available information on cometary and interplanetary dust as well as observations of dust in the tails of sungrazers, we model dust in sungrazing comets as fluffy silicate aggregates of submicrometer sizes. To better interpret observational data, we numerically calculate the solar radiation pressure, the equilibrium temperature, and the sublimation and crystallization rates of silicate grains near the Sun. Our results show that the dust tails contain aggregates of submicrometer crystal grains, but not amorphous grains, since amorphous silicates mostly crystallize after release from the comets. The peak in the lightcurves of the dust comae observed either at 11.2 or 12.3 solar radii (R_⊙) seems to result from sublimation of fluffy aggregates consisting of crystalline or amorphous olivines, respectively. We attribute an additional enhancement in the lightcurves inside 7 R_⊙ to increasing out-flow of crystalline and amorphous pyroxenes composed fluffy aggregates. According to our model, the observed lightcurves indicate a high abundance of olivine and a low abundance of pyroxene in the comets, which may bear implications about the dynamical and thermal history of the sungrazers and their progenitor.     

Distribution and properties of fragments and debris from the split Comet 73P/Schwassmann-Wachmann 3 as revealed by Spitzer Space Telescope

During 2006 March-2007 January, we used the IRAC and MIPS instruments on the Spitzer Space Telescope to study the infrared emission from the ensemble of fragments, meteoroids, and dust tails in the more than 3° wide 73P/Schwassmann-Wachmann 3 debris field. We also investigated contemporaneous ground-based and HST observations. In 2006 May, 55 fragments were detected in the Spitzer image. The wide spread of fragments along the comet’s orbit indicates they were formed from the 1995 splitting event. While the number of major fragments in the Spitzer image is similar to that seen from the ground by optical observers, the correspondence between the fragments with optical astrometry and those seen in the Spitzer images cannot be readily established, due either to strong non-gravitational terms, astrometric uncertainties, or transience of the fragments’ outgassing. The Spitzer data resolve the structure of the dust comae at a resolution of ∼1000 km, and they reveal the infrared emission due to large (mm to cm size) particles in a continuous dust trail that closely follows the projected orbit. We detect fluorescence from outflowing CO₂ gas from the largest fragments (B and C), and we measure the CO₂:H₂O proportion (1:10 and 1:20, respectively). We use three dimensionless parameters to explain dynamics of the solid particles: the rocket parameter α is the reaction force from day-side sublimation divided by solar gravity, the radiation pressure parameter β is the force due to solar radiation pressure divided by solar gravity, and the ejection velocity parameter ν is the particle ejection speed divided by the orbital speed of the comet at the time of ejection. The major fragments have ν>α>β and are dominated by the kinetic energy imparted to them by the fragmentation process. The small, ephemeral fragments seen by HST in the tails of the major fragments have α>ν>β and are dominated by rocket forces (until they become devolatilized). The meteoroids along the projected orbit seen by Spitzer have β∼ν?α and are dominated by radiation pressure and ejection velocity, though both influences are much less than gravity. Dust in the fragments’ tails has β?(ν+α) and is dominated by radiation pressure.     

Non-Spherical Dust Grains in the Envelopes of Late-Type Giants

Effects of the grain shape on circumstellar dust dynamics and polarization of stellar radiation are analyzed. The grains are modeled by rotating prolate and oblate spheroids. It is shown that an asymmetry of the geometry of light scattering by non-spherical particles results in a component of the radiation pressure force perpendicular to the wave-vector of incident light. For silicate spheroids, this component can exceed 20 % of . For small metallic grains, the radiation pressure force for a spheroid can be 5–10 times greater than that for a sphere of the same volume. A simple light scattering consideration demonstrates that the distinction in the scattering geometry of aligned non-spherical grains can explain the observed wavelength variations of the positional angle of polarization. This revised version was published online in September 2006 with corrections to the Cover Date.     

Optical properties of spheroidal particles

A new exact solution of the diffraction problem for the homogeneous spheroid on the basis of the method of separation of variables is given. This solution is considerably more efficient than the one of Asano and Yamamoto from the computational point of view. The expressions for various characteristics of the scattered radiation are obtained. The radiation pressure on spheroidal particles is considered taking into account the radial and transversal components. The method of calculations and various tests, which were used to control the computer programs, are described. Numerical results for forward and arbitrary angles scattering by prolate and oblate spheroids with the refractive indices typical for ice and silicates are presented. The dependence of the results on the propagation direction and the polarization of the incident radiation, size of particle and its aspect ratio are examined. The asymptotics for the characteristics of the scattered radiation for the extremely prolate and extremely oblate spheroids are derived. The range of the validity of these approximations is studied. Astrophysical applications include: a) the calculations of the interstellar extinction, interstellar linear and circular polarization curves for the ensemble of partially oriented spheroidal grains, and b) the consideration of the profiles and polarization of the interstellar feature 2200 for the partially oriented graphite spheroids. Appendices contain the expressions for integrals of products of the angular spheroidal functions and the asymptotics for the oblate spheroidal functions.     

A study of the icy tails of the distant comets

The properties of the icy-grain model, formulated recently for the nearly straight, structureless tails of a number of comets with large perihelion distances, are studied. The approach used is based on the comparison of the orientation and general profile of the tails with a set of synchrones, i.e., theoretical trajectories of particles emitted from the cometary nucleus at particular times. A number of features observed in the distant comets, such as a sharply bounded envelope around the nuclear condensation, jet activity in the coma, a slight curvature of the tail, and the absence of its appreciable broadening with increasing distance from the nucleus, are also explained by the icy-grain model. The model is further confronted with the tail-orientation, spectroscopic, and spectrophotometric data available on comets with perihelia beyond 2.2 AU. It is established that the transition region between 2 and 3 AU, where water snow starts evaporating rapidly, has a profound effect on the dynamics of the icy tails. It is suggested that the icy (or solid-hydrate) grains, constituting the tails of the distant comets, may be carriers of fine meteoric-dust particles, of microns and submicron sizes, which are set free once the grains start disintegrating by evaporation.     

Planck mean efficiency factors for six material candidates as interstellar grains

Planck mean absorption cross-sections have been computed for spherical grains composed of graphite, iron, ice, olivine, amorphous quartz and a lunar silicate. Experimentally determined infrared optical constants have been used for all these materials. Ice mantle particles and planetesimal particles (iron core and olivine mantle) have also been considered with values of outer and inner radii covering a wide range of astrophysical conditions.The results given both graphically and in a tabular form are discussed and compared with those of other authors. The relationships of mantle and core properties are also critically discussed.     

Radiation pressure correction to meteor orbits

We present a method to calculate the radiation pressure force to gravity ratio on meteoroids from their atmospheric flight. Radiation pressure corrections to meteor orbits are negligible for fireballs; of the order of or less than the measurement errors (≈ 1%) for photographic meteors; of the order of and in some cases substantially larger than the measurement errors (≈ 10%) for radar meteors.     

The use of Auger spectroscopy for the in situ elemental characterization of sub‐micrometer presolar grains

Abstract— Presolar grains are small samples of stardust that can be found at low abundances in some of the most unaltered types of extraterrestrial materials. While earlier laboratory studies of stardust mainly focused on grain types that can be extracted from bulk meteorites by acid dissolution techniques, such as silicon carbide and graphite, recent analyses of presolar silicates rely on isotope imaging searches for locating these grains in situ. Since presolar silicates are generally less than a micrometer in diameter and represent at best only a few hundred ppm of their host materials (e.g., primitive meteorites or interplanetary dust particles), locating and studying these particles can be analytically challenging. Recently, we began using scanning Auger spectroscopy for the in situ elemental characterization of presolar silicate grains as a complement to NanoSIMS isotopic studies for obtaining spatially matched compositional data. Auger spectroscopy is a well‐established analytical technique for elemental characterizations in the material sciences, but has not been widely used in geological applications. We discuss the application of this technique to sub‐micrometer sized silicate grains and address practical issues such as sample preparation, measurement settings, spatial resolution, data processing, and elemental quantification.     

Temperature-influenced dynamics of small dust particles

The motion of spherical dust particles under the action of gravity, electromagnetic radiation force and Lorentz force (LF) is studied theoretically for materials with temperature-dependent dielectric functions in the visible (VIS) spectral range. Even a weak variation of the optical constants with heliocentric distance may influence predominately a long-term dynamical behaviour of submicron-sized and small micron-sized dust grains. It is shown that the lifetime of carbonaceous or Si particles may change by several tens of per cent because of the temperature dependence of particle refractive indices. The orbital inclination is the most evident difference between the evolution of a dust particle with temperature-dependent optical properties and one without. While carbonaceous 2-μm-sized particles with optical constants independent of temperature may evolve in orbits with inclinations greater than an initial value, grains of the same size with variable refractive indices will be spread along orbits characterized with inclinations lower than the initial one. Here the temperature works as a separation factor for particles having slightly different temperature dependences of the optical constants.     

The temperature of nonspherical circumstellar dust grains

The temperatures of prolate and oblate spheroidal dust grains in the envelopes of stars of various spectral types are calculated. Homogeneous particles with aspect ratios a/b≤10 composed of amorphous carbon, iron, dirty ice, various silicates, and other materials are considered. The temperatures of spherical and spheroidal particles were found to vary similarly with particle size, distance to the star, and stellar temperature. The temperature ratio T _d(spheroid)/T _d(sphere) depends most strongly on the grain chemical composition and shape. Spheroidal grains are generally colder than spherical particles of the same volume; only iron spheroids can be slightly hotter than iron spheres. At a/b≈2, the temperature differences do not exceed 10%. If a/b≥4, the temperatures can differ by 30–40%. For a fixed dust mass in the medium, the fluxes at wavelengths λ≥100 are higher if the grains are nonspherical, which gives overestimated dust masses from millimeter observations. The effect of grain shape should also be taken into account when modeling Galactic-dust emission properties, which are calculated when searching for fluctuations of the cosmic microwave background radiation in its Wien wing.     

Large Dust Grains Around Cometary Nuclei

Large amounts of particles ejected from the nucleus surface are present in the vicinity of the cometary nuclei when comets are near the Sun (at heliocentric distances ≤2 AU). The largest dust grains ejected may constitute a hazard for spatial vehicles. We tried to obtain the bounded orbits of those particles and to investigate their stability along several orbital periods. The model includes the solar and the cometary gravitational forces and the solar radiation pressure force. The nucleus is assumed to be spherical. The dust grains are also assumed to be spherical, and radially ejected. We include the effects of centrifugal forces owing to the comet rotation. An expression for the most heavy particles that can be lifted is proposed. Using the usual values adopted for the case of Halley’s comet, the largest grains that can be lifted have a diameter about 5 cm, and the term due to the rotation is negligible. However, that term increases the obtained value for the maximum diameter of the lifted grain in a significant amount when the rotation period is of the order of a few hours.     

Infrared spectra of quasars and related objects

An attempt is made to explain the infrared radiation observed for several quasars and Seyfert galaxies as thermal radiation of a dust envelope surrounding the cores of these objects. Two kinds of dust particles (graphite and silica) are taken into consideration. It is shown that the observed spectral behaviour and the luminosity in the infrared can be introduced as thermal radiation of silica grains. In the case of 3C 273 one finds that the radius of the dust envelope is about 50 pc and the total mass of dust is about 600M _⊙.     

Large-scale ionization fronts and the nature and distribution of light scattering particles in the orion nebula

Image-tube filter photographs calibrated against photoelectric filter photometry have been used to give maps of M42 in absolute flux units over the central 15 arc min of the nebula in Hα, [Nii] (λ 6584 Å), Hβ and continuum at λ 4700 Å. Maps of the ratios Hα/[Nii] and (for the first time) of continuum/Hβ have been produced with unprecedented spatial resolution. These show that the gas to dust ratio is high near the exciting stars and falls strongly in the vicinity of large scale ionization fronts marked by minima in the Hα/[Nii] ratio. These results are interpreted in terms of detailed shell models containing either ice or graphite or silicate scattering particles. In all models there must be a central hole in the distribution of scattering particles. The effect of neutral globules and intrusions is investigated. It is found that all types of grain are trapped inside neutral intrusions near the centre of the nebula by the pressure of the Lα light surrounding the globule, but in the early evolution of the nebula particles can escape into the ionized medium when fronts are R-type. Ice grains escaping at this time will be destroyed for distances to the exciting stars less than 1 pc. These results can explain both the central hole in dust and the underabundance of oxygen in the ionized gas observed earlier. Arguments depending on colour index of the scattered light indicate that mixtures of scattered light from ice in the globules and from ice in the ionized medium can explain the observations, but that the graphite and silicate particles fail. A schematic model of the Orion Nebula is presented to attempt to explain the large scale phenomena observed here. It demonstrates that simple shell models for this nebula are dubious.