From interstellar abundances to grain composition: the major dust constituents Mg, Si and Fe

We analyse observational correlations for three elements entering into the composition of interstellar silicate and oxide grains. Using current solar abundances (Asplund et al. 2009), we convert the gas-phase abundances into dust-phase abundances for 196 sightlines. We deduce a sharp difference in abundances for sightlines located at low (|b|<30\degr) and high (|b|>30\degr) galactic latitudes. For high-latitude stars the ratios Mg/Si and Fe/Si in dust are close to 1.5. For disk stars they are reduced to ${\rm Mg/Si} \sim 1.2$ and ${\rm Fe/Si} \sim 1.05$. The derived numbers indicate that 1) the dust grains cannot be the mixture of silicates with olivine and pyroxene composition only and some amount of magnesium or iron (or both) should be in another population and 2) the destruction of Mg-rich grains in the warm medium is more effective than of Fe-rich grains. We reveal a decrease of dust-phase abundances and correspondingly an increase of gas-phase abundances with distance $D$ for stars with D\ga 400\,pc. We attribute this fact to an observational selection effect: a systematic trend toward smaller observed hydrogen column density for distant stars. We find differences in abundances for disk stars with low (E({\rm B-V}) \la 0.2) and high (E({\rm B-V}) \ga 0.2) reddenings which reflect the distinction between the sightlines passing through diffuse and translucent interstellar clouds. For Scorpius-Ophiuchus we detect an uniform increase of dust-phase abundances of Mg and Si with an increase of the ratio of total to selective extinction $R_{\rm V}$ and a decrease of the strength of the far-UV extinction. This is the first evidence for a growth of Mg-Si grains due to accretion in the interstellar medium.Comment: 16 pages, 16 figures, accepted for publication in Astronomy and Astrophysic

Dust in the diffuse interstellar medium: Extinction, emission, linear and circular polarisation

We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, linear and circular polarisation of light. The model is set-up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500nm. Dust grains with radii larger than 6nm are spheroids. Spheroidal dust particles have a factor 1.5 - 3 larger absorption cross section in the far IR than spherical grains of the same volume. Mass estimates derived from submillimeter observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified, so that absolute element abundances are not direct input parameters. The model is confronted with the average properties of the Milky Way, which seems to represent dust in the solar neighbourhood. It is then applied to four specific sight lines including the reflection nebula NGC2023. For these sight lines, we present linear and circular spectro-polarimetric observations obtained with FORS/VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributor to the polarisation.Comment: accepted by A&A Edito

Dust extinction and absorption: the challenge of porous grains

In many models of dusty objects in space the grains are assumed to be composite or fluffy. However, the computation of the optical properties of such particles is still a very difficult problem. We analyze how the increase of grain porosity influences basic features of cosmic dust -- interstellar extinction, dust temperature, infrared bands and millimeter opacity. Porous grains can reproduce the flat extinction across the 3 - 8 \mkm wavelength range measured for several lines of sight by {\it ISO} and {\it Spitzer}. Porous grains are generally cooler than compact grains. At the same time, the temperature of very porous grains becomes slightly larger in the case of the EMT-Mie calculations in comparison with the results found from the layered-sphere model. The layered-sphere model predicts a broadening of infrared bands and a shift of the peak position to larger wavelengths as porosity grows. In the case of the EMT-Mie model variations of the feature profile are less significant. It is also shown that the millimeter mass absorption coefficients grow as porosity increases with a faster growth occurring for particles with Rayleigh/non-Rayleigh inclusions. As a result, for very porous particles the coefficients given by two models can differ by a factor of about 3. It is found that an increase of porosity leads to an increase of extinction cross sections at some wavelengths and a decrease at others depending on the grain model. However, this behaviour is sufficient to reproduce the extinction curve in the direction of the star $\sigma$ Sco using current solar abundances. In the case of the star $\zeta$ Oph our model requires larger amounts of carbon and iron in the dust-phase than is available.Comment: Astronomy and Astrophysics (accepted; 11 pages, 11 figures

Effective medium theories for irregular fluffy structures: aggregation of small particles

We study the extinction efficiencies as well as scattering properties of particles of different porosity. Calculations are performed for porous pseudospheres with small size (Rayleigh) inclusions using the discrete dipole approximation. Five refractive indices of materials covering the range from $1.20+0.00i$ to $1.75+0.58i$ were selected. They correspond to biological particles, dirty ice, silicate, amorphous carbon and soot in the visual part of spectrum. We attempt to describe the optical properties of such particles using Lorenz-Mie theory and a refractive index found from some effective medium theory (EMT) assuming the particle is homogeneous. We refer to this as the effective model. It is found that the deviations are minimal when utilizing the EMT based on the Bruggeman mixing rule. Usually the deviations in extinction factor do not exceed $\sim 5$ for particle porosity ${\cal P}=0 - 0.9$ and size parameters x_{\rm porous} = 2 \pi r_{\rm s, porous}/\lambda \la 25. The deviations are larger for scattering and absorption efficiencies and smaller for particle albedo and asymmetry parameter. Our calculations made for spheroids confirm these conclusions. Preliminary consideration shows that the effective model represents the intensity and polarization of radiation scattered by fluffy aggregates quite well. Thus, the effective models of spherical and non-spherical particles can be used to significantly simplify computations of the optical properties of aggregates containing only Rayleigh inclusions.Comment: 24 pages, 9 figures, accepted for publication in Applied Optic

Light scattering by an elongated particle: spheroid versus infinite cylinder

Using the method of separation of variables and a new approach to calculations of the prolate spheroidal wave functions, we study the optical properties of very elongated (cigar-like) spheroidal particles. A comparison of extinction efficiency factors of prolate spheroids and infinitely long circular cylinders is made. For the normal and oblique incidence of radiation, the efficiency factors for spheroids converge to some limiting values with an increasing aspect ratio a/b provided particles of the same thickness are considered. These values are close to, but do not coincide with the factors for infinite cylinders. The relative difference between factors for infinite cylinders and elongated spheroids (a/b \ga 5) usually does not exceed 20 % if the following approximate relation between the angle of incidence $\alpha (in degrees)$ and the particle refractive index m=n+ki takes the place: \alpha \ga 50 |m-1| + 5 where 1.2 \la n \la 2.0 and k \la 0.1. We show that the quasistatic approximation can be well used for very elongated optically soft spheroids of large sizes.Comment: 12 pages, 7 figures, Accepted by Measurement Science and Technology (special OPC issue

Multiple scattering of polarized radiation by non-spherical grains: first results

We present the first numerical radiative transfer simulation of multiple light scattering in dust configurations containing aligned non-spherical (spheroidal) dust grains. Such models are especially important if one wants to explain the circular polarization of light, observed in a variety of astronomical objects. The radiative transfer problem is solved on the basis of the Monte Carlo method. Test simulations, confirming the correct numerical implementation of the scattering mechanism, are presented. As a first application, we investigate the linear and circular polarization of light coming from a spherical circumstellar shell. This shell contains perfectly aligned prolate or oblate spheroidal grains. The most remarkable features of the simulated linear polarization maps are so-called polarization null points where the reversal of polarization occurs. They appear in the case when the grain alignment axis is perpendicular to the line of sight. The maps of circular polarization have a sector-like structure with maxima at the ends of lines inclined to the grain alignment axis by \pm 45\degr.Comment: 13 pages, 14 figures, accepted by A&