Collisions of electrons with interstellar grains

view Abstract Citations (239) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Temperature Fluctuations in the Interstellar Grains. I. Computational Method and Sublimation of Small Grains Guhathakurta, P. ; Draine, B. T. Abstract A technique is presented for calculating the temperature distribution of interstellar dust grains in the presence of a radiation field or collisional heating by a hot gas, or both. The distribution functions are computed for grain sizes ranging from 0.02 micron to 2.5 A for graphite and silicate grains. Five different radiation fields are considered: one and three times the 'average' interstellar radiation in the solar neighborhood, the radiation field 0.3 pc from a B3V star appropriate for visual reflection nebulae, and the radiation fields in interstellar clouds at optical depths corresponding to visual extinctions of A(V) = 0.25 mag and A(V) = 0.50 mag. For the interstellar radiation field in the solar neighborhood, a minimum grain size N(crit) of roughly 23 for graphite grains and N(crit) of roughly 37 for silicate grains, where N(crit) is the number of atoms in a grain for which the lifetime against sublimation is 10 to the 13th s. Publication: The Astrophysical Journal Pub Date: October 1989 DOI: 10.1086/167899 Bibcode: 1989ApJ...345..230G Keywords: Computational Astrophysics; Cosmic Dust; Interstellar Matter; Radiation Distribution; B Stars; Distribution Functions; High Temperature Gases; Iterative Solution; Monte Carlo Method; Radiative Transfer; Temperature Distribution; Astrophysics; INTERSTELLAR: GRAINS; RADIATIVE TRANSFER full text sources ADS |

ABSTRACTExisting proposals for converting thin section data to their sieve equivalents are all flawed in various ways, while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using φ units, and of non‐spherical (ellipsoidal) grains using both millimetres and φ units, have not been ‐satisfactorily resolved in the literature. It can be shown mathematically that the mean thin section diameters of spherical grains, or axis lengths of a series of parallel sections through ellipsoidal grains, will underestimate the dimensions of the corresponding central section (i.e. one passing through the grain centre) by 0.2023 φ when measured on a volume frequency basis. In order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains, the dimensions of a series of sections cut in 49 unique directions, symmetrically arranged and evenly spread with respect to the ellipsoidal axes, were calculated. This calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations. The data indicated that the mean true nominal diameters (D̄) of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters (d̄′) and major axes (ā′) of the central sections (derived from the observed values by multiplying the millimetre means by 1·1318 and subtracting 0·2023 from the φ means) using the following equations: image A rough estimate (to within c. 5%) of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millimetres or φ units.

We have computed the cross-section for electron collisions with spherical grains at low energies, using both classical and quantum (wave) mechanics. The calculations were found to agree for grain radii as small as 3 x 10 -6 cm and collision energies E/k = 10 K. On the other hand, the results of the classical calculations for smaller grains exceed those of wave mechanics by typically a factor of 2, owing to interference effects, which are not present in the classical limit. We suggest that the greatest uncertainty in electron attachment rates is in the value of the sticking coefficient, which may be expected to vary considerably with the composition of the grain and its mantle.

Developing a porosity-permeability relationship for ellipsoidal grains: A correction shape factor for Kozeny-Carman's equation

The interaction of dust grains originating from the local interstellar cloud with the environment inside the heliosphere is investigated. As a consequence of this interaction, the spatial distribution of interstellar dust grains changes with time. Since dust grains are charged in the interplanetary plasma and radiation environment, the interaction of small grains with the heliosphere is dominated by their coupling to the solar wind magnetic field. The change of the field polarity with the solar cycle imposes a temporal variation of the spatial distribution and the flux of small (radius smaller than 0.4 μm) interstellar dust grains in the solar system, whereas the flux of large grains is constant because of their negligible coupling to the solar wind magnetic field. The flux variation observed by in situ measurements of the Galileo and Ulysses spacecraft are reproduced by simulating the interaction of interstellar grains with charge‐to‐mass ratios between 0.5 and 1.4 C kg−1 with the interplanetary environment.

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

We have investigated the gaseous and solid state molecular composition of dense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to confront these models with the stringent abundance constraints on CO2, H2O and O2, in both gas and solid phases, that have been set by ISO and SWAS. We also compare our results with the chemical composition of dark molecular clouds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we only include additional processes where they are clearly required to satisfy one or more of the ISO-SWAS constraints. When CO, N2 and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state develops after about one million years. We find that accretion of CO2 and H2O cannot explain the ISO observations; as with previous models, accretion and reaction of oxygen atoms are necessary although a high O atom abundance can still be derived from the CO that remains in the gas. The observational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest abundances seen by SWAS or the highest atomic carbon abundances found in molecular clouds; additional chemical processes are required and possible candidates are given. One prediction of models of this type is that there should be some regions of molecular clouds which contain high gas phase abundances of H2O, O2 and NO. A further consequence, we find, is that interstellar grain mantles could be rich in NH2OH and NO2. The search for these regions, as well as NH2OH and NO2 in ices and in hot cores, is an important further test of this scenario. The model can give good agreement with observations of simple molecules in dark molecular clouds such as TMC-1 and L134N. Despite the fact that S atoms are assumed to be continously desorbed from grain surfaces, we find that the sulphur chemistry independently experiences an "accretion catastrophe" . The S-bearing molecular abundances cease to lie within the observed range after about years and this indicates that there may be at least two efficient surface desorption mechanisms operating in dark clouds -one quasi-continous and the other operating more sporadically on this time-scale. We suggest that mantle removal on short time-scales is mediated by clump dynamics, and by the effects of star formation on longer time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with other models of dark cloud chemistry.

Context. The all-sky survey from the Planck space telescope has revealed that thermal emission from Galactic dust is polarized on scales ranging from the whole sky down to the inner regions of molecular clouds. Polarized dust emission can therefore be used as a probe for magnetic fields on different scales. In particular, the analysis of the relative orientation between the density structures and the magnetic field projected on the plane of the sky can provide information on the role of magnetic fields in shaping the structure of molecular clouds where star formation takes place. Aims. The orientation of the magnetic field with respect to the density structures has been investigated using different methods. The goal of this paper is to explicitly compare two of these: the Rolling Hough Transform (RHT) and the gradient technique (GRAD). Methods. We generated synthetic surface brightness maps at 353 GHz (850 μm) via magnetohydrodynamic simulations. We applied RHT and GRAD to two morphologically different regions identified in our maps. Region 1 is dominated by a dense and thick filamentary structure with some branches, while Region 2 includes a thinner filament with denser knots immersed in a more tenuous medium. Both methods derive the relative orientation between the magnetic field and the density structures, to which we applied two statistics, the histogram of relative orientation and the projected Rayleigh statistic, to quantify the variations of the relative orientation as a function of column density. Results. Both methods find areas with significant signal, and these areas are substantially different. In terms of relative orientations, in all our considered cases the predominant orientation of the density structures is perpendicular to the direction of the magnetic field. When the methods are applied to the same selected areas the results are consistent with each other in Region 2 but show some noticeable differences in Region 1. In Region 1, RHT globally finds the relative orientation becoming more perpendicular for increasing column density, while GRAD, applied at the same resolution as RHT, gives the opposite trend. These disparities are caused by the intrinsic differences in the methods and in the structures that they select. Conclusions. Our results indicate that the interpretation of the relative orientation between the magnetic field and density structures should take into account the specificity of the methods used to determine such orientation. The combined use of complementary techniques such as RHT and GRAD provides more complete information, which can be advantageously used to better understand the physical mechanisms operating in magnetized molecular clouds.

Fine-grained dust rims (FGRs) surrounding chondrules in carbonaceous chondrites encode important information about early processes in the solar nebula. Here, we investigate the effect of the nebular environment on FGR porosity, dust size distribution, and grain alignment, comparing the results for rims comprised of ellipsoidal and spherical grains. We conduct numerical simulations in which FGRs grow by collisions between dust particles and chondrules in both neutral and ionized turbulent gas. The resultant rim morphology is related to the ratio ϵ of the electrostatic potential energy at the collision point to the relative kinetic energy between colliding particles. In general, large ϵ leads to a large rim porosity, large rim grain size, and low growth rate. Dust rims comprised of ellipsoidal monomers initially grow faster in thickness than rims comprised of spherical monomers, due to their higher porosity. As the rims grow and obtain a greater electrostatic potential, repulsion becomes dominant, and this effect is reversed. Grain size coarsening toward the outer regions of the rims is observed for low- and high-ϵ regimes, and is more pronounced in the ellipsoidal case, while for the medium-ϵ regime, small monomers tend to be captured in the middle of the rims. In neutral environments, ellipsoidal grains have random orientations within the rim, while in charged environments ellipsoidal grains tend to align with maximum axial alignment for ϵ = 0.15. The characterization of these FGR features provides a means to relate laboratory measurements of chondrite samples to the formation environment of the parent bodies.

Ultrasonic backscattering in polycrystals with elongated single phase and duplex microstructures

Recent photometric and polarimetric observations of Type Ia supernovae (SNe Ia) show unusually low total-to-selective extinction ratios (R V < 2) and wavelengths of maximum polarization (λ max < 0.4 μm) for several SNe Ia, which indicates peculiar properties of interstellar (IS) dust in the SN-hosted galaxies and/or the presence of circumstellar (CS) dust. In this paper, we use an inversion technique to infer the best-fit grain size distribution and the alignment function of interstellar grains along the lines of sight toward four SNe Ia with anomalous extinction and polarization data (SN 1986G, SN 2006X, SN 2008fp, and SN 2014J). We find that to reproduce low values of R V , a significant enhancement in the mass of small grains of radius a < 0.1 μm is required. For SN 2014J, a simultaneous fit to its observed extinction and polarization is unsuccessful if all the data are attributed to IS dust (model 1), but a good fit is obtained when accounting for the contribution of CS dust (model 2). For SN 2008fp, our best-fit results for model 1 show that in order to reproduce an extreme value of λ max ∼ 0.15 μm, small silicate grains must be aligned as efficiently as big grains. For this case, we suggest that strong radiation from the SN can induce efficient alignment of small grains in a nearby intervening molecular cloud via the radiative torque (RAT) mechanism. The resulting time dependence polarization from this RAT alignment model can be tested by observing at ultraviolet wavelengths.

IN many studies of interstellar grains it has been customary to assume that the properties of these particles can be described by the properties of macroscopic systems with the same composition. Indeed it has been the practice to assume that the particulate nature of interstellar dust is manifested only by the way in which such dust scatters and extinguishes light. Detailed considerations of the properties of interstellar grains as small particles lead, however, to the observation that grains cannot be considered in many instances to behave simply as sub-volumes of a macroscopic solid. Here I show that several pertinent parameters describing interstellar grains cannot be predicted from the bulk properties of an assumed grain material. My discussion is limited to small (r<10−5 cm) spherical grains of refractory materials.

Purpose – The purpose of this paper is to present a new and efficient technique for discrete element modelling using non-convex polyhedral grain shapes. Design/methodology/approach – The efficiency of the technique follows from the use of grains that are dilated versions of the basic polyhedral grain shapes. Dilation of an arbitrary polyhedral grain is accomplished by placing the center of a sphere of fixed radius at every point on the surface. The dilated vertices become sphere segments and the edges become cylinder segments. The sharpness of the vertices and edges can be adjusted by varying the dilation radius. Contacts between two dilated polyhedral grains can be grouped into three categories; vertex on surface, vertex on edge, and edge on edge, or in the grammar of the model, sphere on polygonal surface, sphere on cylinder, and cylinder on cylinder. Simple, closed-form solutions exist for each of these cases. Findings – The speed of the proposed polyhedral discrete element model is compared to similar models using spherical and ellipsoidal grains. The polyhedral code is found to run about 40 percent as fast as an equivalent code using spherical grains and about 80 percent as fast as an equivalent code using ellipsoidal grains. Finally, several applications of the polyhedral model are illustrated. Originality/value – Few examples of discrete element modeling studies in the literature use polyhedral grains. This dearth is because of the perceived complexity of the polyhedral coding challenges and the slow speed of the codes compared to codes for other grain shapes. This paper presents a much simpler approach to discrete element modeling using polyhedral grain shapes.

We present a novel method to measure the strength of interstellar magnetic fields based on ultraviolet (UV) polarization of starlight, which is in part produced by weakly aligned, small interstellar grains. We begin with calculating degrees of alignment of small (size $a\sim 0.01\mu$m) and very small ($a\sim 0.001\mu$m) grains in the interstellar magnetic field due to the Davis-Greenstein paramagnetic relaxation and resonance paramagnetic relaxation. We compute the degrees of paramagnetic alignment with the ambient magnetic field $B$ using Langevin equations. In this paper, we take into account various processes essential for the dynamics of small grains, including infrared (IR) emission, electric dipole emission, plasma drag and collisions with neutral and ionized species. We find that the alignment of small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and IR wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model requires a higher degree of alignment of small grains for the case with the peak wavelength of polarization $\lambda_{\max}<0.55\mu$m, which exhibits an excess UV polarization relative to the Serkowski law, compared to the typical case $\lambda_{\max}=0.55\mu$m. We interpret the correlation between the systematic increase of the UV polarization relative to maximum polarization (i.e. of $p(6\mu m^{-1})/p_{\max}$) with $\lambda_{\max}^{-1}$ by appealing to the higher degree of alignment of small grains. We identify paramagnetic relaxation as the cause of the alignment of small grains and utilize the dependence of the degree of alignment on the magnetic field strength $B$ to suggest a new way to measure $B$ using the observable parameters $\lambda_{\max}$ and $p(6\mu m^{-1})/p_{\max}$.[Abridged]

A simple model of surface roughness on the interstellar spherical grains has been considered. A test case with dirty ice spherical grains reveals that the effect of surface roughness is to significantly enhance the extinction in the far ultraviolet compared to the equivalent smooth spheres.