122 research outputs found
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 and . 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 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 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
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&
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
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
to 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 for particle porosity 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
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 Sco using current solar abundances.
In the case of the star 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
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 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
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