246 research outputs found
Modelling interstellar extinction and polarization with spheroidal grains
We calculate the wavelength dependence of the ratio of the linear
polarization degree to extinction (polarizing efficiency)
from the ultraviolet to near-infrared. The prolate and
oblate particles with aspect ratios from up to 10 are assumed to be
rotating and partially aligned with the mechanism of paramagnetic relaxation
(Davis--Greenstein). Size/shape/orientation effects are analyzed. It is found
that the wavelength dependence of is mainly determined
by the particle composition and size whereas the values of
depend on the particle shape, degree and direction of
alignment.Comment: 13 pages, 9 figures, aacepted for publication in Journal of
Quantitative Spectroscopy & Radiative Transfer (special issue, X Conference
on Electromagnetic & Light Scattering
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&
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
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
Interstellar extinction and polarization -- A spheroidal dust grain approach perspective
We extend and investigate the spheroidal model of interstellar dust grains
used to simultaneously interpret the observed interstellar extinction and
polarization curves. We compare our model with similar models recently
suggested by other authors, study its properties and apply it to fit the
normalized extinction and the polarizing efficiency
measured in the near IR to far UV region for several
stars seen through one large cloud. We conclude that the model parameter
being the angle between the line of sight and the magnetic field
direction can be more or less reliably determined from comparison of the theory
and observations. This opens a way to study the spatial structure of
interstellar magnetic fields by using multi-wavelength photometric and
polarimetric observations.Comment: 11 pages, 4 figures and 4 tables, To appear in MNRAS (added
- …