192 research outputs found
Carbonates in space - The challenge of low temperature data
Carbonates have repeatedly been discussed as possible carriers of stardust
emission bands. However, the band assignments proposed so far were mainly based
on room temperature powder transmission spectra of the respective minerals.
Since very cold calcite grains have been claimed to be present in protostars
and in Planetary Nebulae such as NGC 6302, the changes of their dielectric
functions at low temperatures are relevant from an astronomical point of view.
We have derived the IR optical constants of calcite and dolomite from
reflectance spectra - measured at 300, 200, 100 and 10K - and calculated small
particle spectra for different grain shapes, with the following results: i) The
absorption efficiency factors both of calcite and dolomite are extremely
dependent on the particle shapes. This is due to the high peak values of the
optical constants of CaCO3 and CaMg[CO3]2. ii) The far infrared properties of
calcite and dolomite depend also very significantly on the temperature. Below
200K, a pronounced sharpening and increase in the band strengths of the FIR
resonances occurs. iii) In view of the intrinsic strength and sharpening of the
44 mum band of calcite at 200-100K, the absence of this band -- inferred from
Infrared Space Observatory data -- in PNe requires dust temperatures below 45K.
iv) Calcite grains at such low temperatures can account for the '92' mum band,
while our data rule out dolomite as the carrier of the 60-65 mum band. The
optical constants here presented are publicly available in the electronic
database http://www.astro.uni-jena.de/Laboratory/OCDBComment: 20 pages, 10 figures, accepted by ApJ, corrected typo
Infrared properties of SiC particles
We present basic laboratory infrared data on a large number of SiC
particulate samples, which should be of great value for the interpretation of
the 11.3 micron feature observed in the spectra of carbon-rich stars. The
laboratory spectra show a wide variety of the SiC phonon features in the 10-13
micron wavelength range, both in peak wavelength and band shape. The main
parameters determining the band profile are morphological factors as grain size
and shape and, in many cases, impurities in the material. We discovered the
interesting fact that free charge carriers, generated e.g. by nitrogen doping,
are a very common characteristics of many SiC particle samples. These free
charge carriers produce very strong plasmon absorption in the near and middle
infrared, which may also heavily influence the 10-13 micron feature profile via
plasmon-phonon coupling.
We also found that there is no systematic dependence of the band profile on
the crystal type (alpha- vs. beta-SiC). This is proven both experimentally and
by theoretical calculations based on a study of the SiC phonon frequencies.
Further, we give optical constants of amorphous SiC. We discuss the
implications of the new laboratory results for the interpretation of the
spectra of carbon stars.Comment: 17 pages, 12 figures. To appear in A&
Infrared Spectra of Meteoritic SiC Grains
We present here the first infrared spectra of meteoritic SiC grains. The
mid-infrared transmission spectra of meteoritic SiC grains isolated from the
Murchison meteorite were measured in the wavelength range 2.5--16.5 micron, in
order to make available the optical properties of presolar SiC grains. These
grains are most likely stellar condensates with an origin predominately in
carbon stars. Measurements were performed on two different extractions of
presolar SiC from the Murchison meteorite. The two samples show very different
spectral appearance due to different grain size distributions. The spectral
feature of the smaller meteoritic SiC grains is a relatively broad absorption
band found between the longitudinal and transverse lattice vibration modes
around 11.3 micron, supporting the current interpretation about the presence of
SiC grains in carbon stars. In contrast to this, the spectral feature of the
large (> 5 micron) grains has an extinction minimum around 10 micron. The
obtained spectra are compared with commercially available SiC grains and the
differences are discussed. This comparison shows that the crystal structure
(e.g., beta-SiC versus alpha-SiC) of SiC grains plays a minor role on the
optical signature of SiC grains compared to e.g. grain size.Comment: 7 pages, 6 figures. To appear in A&
Constraints on the structure of hot exozodiacal dust belts
Recent interferometric surveys of nearby main-sequence stars show a faint but significant near-infrared excess in roughly two dozen systems, i.e. around 10–30 per cent of stars surveyed. This excess is attributed to dust located in the immediate vicinity of the star, the origin of which is highly debated. We used previously published interferometric observations to constrain the properties and distribution of this hot dust. Considering both scattered radiation and thermal re-emission, we modelled the observed excess in nine of these systems. We find that grains have to be sufficiently absorbing to be consistent with the observed excess, while dielectric grains with pure silicate compositions fail to reproduce the observations. The dust should be located within ∼0.01–1 au from the star depending on its luminosity. Furthermore, we find a significant trend for the disc radius to increase with the stellar luminosity. The dust grains are determined to be below 0.2--0.5μm, but above 0.02--0.15μm in radius. The dust masses amount to (0.2–3.5) × 10⁻⁹ M⊕. The near-infrared excess is probably dominated by thermal re-emission, though a contribution of scattered light up to 35 per cent cannot be completely excluded. The polarization degree predicted by our models is always below 5 per cent, and for grains smaller than ∼0.2μm even below 1 per cent. We also modelled the observed near-infrared excess of another 10 systems with poorer data in the mid-infrared. The basic results for these systems appear qualitatively similar, yet the constraints on the dust location and the grain sizes are weaker
On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21mu emission feature seen in 12 proto-planetary nebulae
(PPNe) remains unidentified since its first detection in 1989. Over a dozen of
candidate materials have been proposed within the past decade, but none of them
has received general acceptance. Very recently, silicon carbide (SiC) grains
with impurities were suggested to be the carrier of this enigmatic feature,
based on recent laboratory data that doped SiC grains exhibit a resonance at
\~21mu. This proposal gains strength from the fact that SiC is a common dust
species in carbon-rich circumstellar envelopes. However, SiC dust has a strong
vibrational band at ~11.3mu. We show in this Letter that in order to be
consistent with the observed flux ratios of the 11.3mu feature to the 21mu
feature, the band strength of the 21mu resonance has to be very strong, too
strong to be consistent with current laboratory measurements. But this does not
yet readily rule out the SiC hypothesis since recent experimental results have
demonstrated that the 21mu resonance of doped SiC becomes stronger as the C
impurity increases. Further laboratory measurements of SiC dust with high
fractions of C impurity are urgently needed to test the hypothesis of SiC as
the carrier of the 21mu feature.Comment: 14 pages, 3 figures, accepted for publication in ApJ
Dust absorption and scattering in the silicon K-edge
The composition and properties of interstellar silicate dust are not well
understood. In X-rays, interstellar dust can be studied in detail by making use
of the fine structure features in the Si K-edge. The features in the Si K-edge
offer a range of possibilities to study silicon-bearing dust, such as
investigating the crystallinity, abundance, and the chemical composition along
a given line of sight. We present newly acquired laboratory measurements of the
silicon K-edge of several silicate-compounds that complement our measurements
from our earlier pilot study. The resulting dust extinction profiles serve as
templates for the interstellar extinction that we observe. The extinction
profiles were used to model the interstellar dust in the dense environments of
the Galaxy. The laboratory measurements, taken at the Soleil synchrotron
facility in Paris, were adapted for astrophysical data analysis and implemented
in the SPEX spectral fitting program. The models were used to fit the spectra
of nine low-mass X-ray binaries located in the Galactic center neighborhood in
order to determine the dust properties along those lines of sight. Most lines
of sight can be fit well by amorphous olivine. We also established upper limits
on the amount of crystalline material that the modeling allows. We obtained
values of the total silicon abundance, silicon dust abundance, and depletion
along each of the sightlines. We find a possible gradient of
dex/kpc for the total silicon abundance versus the Galactocentric distance. We
do not find a relation between the depletion and the extinction along the line
of sight.Comment: 18 pages, 16 figures. Accepted for publication in Astronomy and
Astrophysic
Laboratory-based grain-shape models for simulating dust infrared spectra
Analysis of thermal dust emission spectra for dust mineralogy and physical
grain properties depends on laboratory-measured or calculated comparison
spectra. Often, the agreement between these two kinds of spectra is not
satisfactory because of the strong influence of the grain morphology on the
spectra. We investigate the ability of the statistical light-scattering model
with a distribution of form factors (DFF model) to reproduce experimentally
measured infrared extinction spectra for particles that are small compared to
the wavelength. We take advantage of new experimental spectra measured for free
particles dispersed in air with accompanying information on the grain
morphology. For the calculations, we used DFFs that were derived for aggregates
of spherical grains, as well as for compact grain shapes corresponding to
Gaussian random spheres. Irregular particle shapes require a DFF similar to
that of a Gaussian random sphere with sigma=0.3, whereas roundish grain shapes
are best fitted with that of a fractal aggregate of a fractal dimension
2.4-1.8. In addition we used a fitting algorithm to obtain the best-fit DFFs
for the various laboratory samples. In this way we can independently derive
information on the shape of the grains from their infrared spectra. For
anisotropic materials, different DFFs are needed for the different
crystallographic axes. This is due to a theoretical problem, which is inherent
to all models that are simply averaging the contributions of the
crystallographic directions.Comment: 8 pages, 8 figures, accepted by Astronomy and Astrophysic
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