18 research outputs found
Experimental mid-infrared spectroscopic extinction measurements of agglomerate dust grains in aerosol
Zur Zeit sind Labormessungen an realen Staubteilchenzusammensetzungen nicht in der Lage,exakte Bandprofile fĂŒr einen direkten Vergleich zu liefern, weil die Proben von einem Medium (KBr) umgeben sind, welches Einfluss nimmt auf die elektromagnetische Polarisation. Ich habe im Rahmen dieser Arbeit eine neue Methode entwickelt, mit der Extinktionsspektren ohne den elektromagnetischen Einfluss umgebender Medien gemessen werden können. Mit Hilfe der Aerosoltechnik habe ich Extinktionsspektren von amorphen wie kristallinen Silikaten, Quarz und Spezies mit hoher Kondensationstemperatur in mittleren Infrarot aufgenommen, die direkt mit beobachteten Spektren verglichen werden können. Ich untersuchte die Unterschiede zu den mit der KBr-Methode aufgenommenen Daten, indem ich die Lage der maxima der Bandenprofile bestimmte und verglich. Desweiteren habe ich die Staubkörner elektronenmikroskopisch untersucht, um morphologische EinflĂŒsse (GröĂe, Form, Agglomeration) auf das gemessene Profil zu untersuchen
Mid-infrared characterization of thiophene-based thin polymer films
AbstractOptical properties of seven regioregular poly(3-alkylthiophene) with different alkyl side chain lengths which are poly(3-butylthiophene-2,5-diyl) (P3BT), poly(3-pentylthiophene-2,5-diyl) (P3PT), poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-heptylthiophene-2,5-diyl) (P3hept), poly(3-octylthiophene-2,5-diyl) (P3OT), poly(3-decylthiophene-2,5-diyl) (P3DT), and poly(3-dodecylthiophene-2,5-diyl) (P3DDT) have been studied in the mid-infrared (IR) spectral region by means of Fourier Transformation Infrared (FTIR) spectroscopy and IR spectroscopic ellipsometry (IRSE). Absorbance spectra obtained in this fingerprint region are potential to characterize the structures formed by organic molecules in thin films due to molecular vibrations in detail. In consequence, the vibrational absorption bands of these seven samples demonstrated that P3PT, P3HT, and P3hept exhibited very similar band profiles, in contrast, the stretching vibration of thiophene rings (â1465cmâ1: CC) underwent a blue shift in P3BT, P3OT, P3DT and P3DDT. The highest value of the real part (É1) of the complex dielectric constant was obtained from P3HT on both indium thin oxide (ITO) and silicon (Si) substrates whereas the imaginary part (Δ2) was directly affected by increasing in the alkyl side chain lengths in a frequency range around 3000cmâ1. The optical properties of P3PT in the mid-IR region developed an affinity with those of P3HT. Thus, P3PT is particularly a suitable polymer active material candidate for high-performance devices
Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells. Dust growth in outflows from supergiants
We investigate the dust growth in oxygen-rich stellar outflows for a set of
nine well-observed massive supergiants with optically thin dust shells. Models
of the infrared emission from their circumstellar dust shells are compared to
their observed infrared spectra so as to derive the essential parameters that
govern dust formation in the extended envelope of these stars. The results
obtained from the comparative study are also compared with the predictions of a
model for silicate dust condensation solely based on laboratory data and basic
stellar properties. The infrared emission in the wavelength range between 6 and
25 mu can be reproduced rather well by a mixture of non-stoichiometric
iron-bearing silicates, alumina, and metallic iron dust particles for all nine
objects. The observed spectra obtained from three objects, mu Cep, RW Cyg, and
RS Per, can be reproduced by a stationary and (essentially) spherically
symmetric outflow which enables a direct comparison with predictions from a
theoretical dust growth model. The temperature at the onset of massive silicate
dust growth is of the order of 920 K and the corresponding outflow velocity of
the order of the sound velocity for these objects. The condensation temperature
suggests that the silicate dust grows on the corundum dust grains that are
formed well in the interior of the silicate dust shell at a much higher
temperature. Our results propose that regarding the two major problems of dust
formation in stellar outflows: (i) formation of seed nuclei; (ii) their growth
to macroscopic dust grains, we are gradually coming close to a quantitative
understanding of the second item.Comment: 29 pages, 9 figures, accepted by Astronomy & Astrophysic
The twofold debris disk around HD 113766 A - Warm and cold dust as seen with VLTI/Midi and Herschel/Pacs
Warm debris disks are a sub-sample of the large population of debris disks,
and display excess emission in the mid-IR. Around solar-type stars, very few
objects show emission features in mid-IR spectroscopic observations, that are
attributed to small, warm silicate dust grains. The origin of this warm dust
can possibly be explained either by a collision between several bodies or by
transport from an outer belt. We present and analyse new far-IR Herschel/Pacs
observations, supplemented by ground-based data in the mid-IR (VLTI/Midi and
VLT/Visir), for one of these rare systems: the 10-16 Myr old debris disk around
HD 113766 A. We improve an existing model to account for these new
observations, and better constrain the spatial distribution of the dust and its
composition. We underline the limitations of SED modelling and the need for
spatially resolved observations. We find that the system is best described by
an inner disk located within the first AU, well constrained by the Midi data,
and an outer disk located between 9-13 AU. In the inner dust belt, our previous
finding of Fe-rich crystalline olivine grains still holds. We do not observe
time variability of the emission features over at least a 8 years time span, in
a environment subjected to strong radiation pressure. The time stability of the
emission features indicates that {\mu}m-sized dust grains are constantly
replenished from the same reservoir, with a possible depletion of
sub-{\mu}m-sized grains. We suggest that the emission features may arise from
multi-composition aggregates. We discuss possible scenarios concerning the
origin of the warm dust. The compactness of the innermost regions as probed by
Midi, as well as the dust composition, suggest that we are witnessing the
outcomes of (at least) one collision between partially differentiated bodies,
in an environment possibly rendered unstable by terrestrial planetary
formation
Recent Results of Solid-State Spectroscopy
Solid state spectroscopy continues to be an important source of information
on the mineralogical composition and physical properties of dust grains both in
space and on planetary surfaces. With only a few exceptions, artificially
produced or natural terrestrial analog materials, rather than 'real' cosmic
dust grains, are the subject of solid state astrophysics. The Jena laboratory
has provided a large number of data sets characterizing the UV, optical and
infrared properties of such cosmic dust analogs. The present paper highlights
recent developments and results achieved in this context, focussing on
'non-standard conditions' such as very low temperatures, very high temperatures
and very long wavelengths.Comment: 15 pages, 10 figures. Contribution to an IAU Conference "The
Molecular Universe" held in Toledo in June 201
Infrared extinction by homogeneous particle aggregates of SiC, FeO and SiO2: comparison of different theoretical approaches
Particle shape and aggregation have a strong influence on the spectral
profiles of infrared phonon bands of solid dust grains. Calculating these
effects is difficult due to the often extreme refractive index values in these
bands. In this paper, we use the Discrete Dipole Approximation (DDA) and the
T-matrix method to compute the absorption band profiles for simple clusters of
touching spherical grains. We invest reasonable amounts of computation time in
order to reach high dipole grid resolutions and take high multi-polar orders
into account, respectively. The infrared phonon bands of three different
refractory materials of astrophysical relevance are considered - Silicon
Carbide (SiC), Wustite (FeO) and Silicon Dioxide (SiO2). We demonstrate that
even though these materials display a range of material properties and
therefore different strengths of the surface resonances, a complete convergence
is obtained with none of the approaches. For the DDA, we find a strong
dependence of the calculated band profiles on the exact dipole distribution
within the aggregates, especially in the vicinity of the contact points between
their spherical constituents. By applying a recently developed method to
separate the material optical constants from the geometrical parameters in the
DDA approach, we are able to demonstrate that the most critical material
properties are those where the real part of the refractive index is much
smaller than unity.Comment: Accepted for publication in the Journal of Quantitative Spectroscopy
& Radiative Transfer (JQSRT
Low Temperature Opacities
Previous computations of low temperature Rosseland and Planck mean opacities
from Alexander & Ferguson (1994) are updated and expanded. The new computations
include a more complete equation of state with more grain species and updated
optical constants. Grains are now explicitly included in thermal equilibrium in
the equation of state calculation, which allows for a much wider range of grain
compositions to be accurately included than was previously the case. The
inclusion of high temperature condensates such as AlO and CaTiO
significantly affects the total opacity over a narrow range of temperatures
before the appearance of the first silicate grains.
The new opacity tables are tabulated for temperatures ranging from 30000 K to
500 K with gas densities from 10 g cm to 10 g cm.
Comparisons with previous Rosseland mean opacity calculations are discussed. At
high temperatures, the agreement with OPAL and Opacity Project is quite good.
Comparisons at lower temperatures are more divergent as a result of differences
in molecular and grain physics included in different calculations. The
computation of Planck mean opacities performed with the opacity sampling method
are shown to require a very large number of opacity sampling wavelength points;
previously published results obtained with fewer wavelength points are shown to
be significantly in error. Methods for requesting or obtaining the new tables
are provided.Comment: 39 pages with 12 figures. To be published in ApJ, April 200
The Limiting Effects of Dust in Brown Dwarf Model Atmospheres
We present opacity sampling model atmospheres, synthetic spectra and colors
for brown dwarfs and very low mass stars in two limiting case of dust grain
formation: 1) inefficient gravitational settling i.e. the dust is distributed
according to the chemical equilibrium predictions, 2) efficient gravitational
settling i.e. the dust forms and depletes refractory elements from the gas, but
their opacity does not affect the thermal structure. The models include the
formation of over 600 gas phase species, and 1000 liquids and crystals, and the
opacities of 30 different types of grains including corundum (AlO), the
magnesium aluminum spinel MgAlO, iron, enstatite (MgSiO),
forsterite (MgSiO), amorphous carbon, SiC, and a number of calcium
silicates. The models extend from the beginning of the grain formation regime
well into the condensation regime of water ice (\teff= 3000 - 100 K) and
encompasses the range of at solar metallicity.
We find that silicate dust grains can form abundantly in the outer
atmospheric layers of red and brown dwarfs with spectral type later than M8.
The greenhouse effects of dust opacities provide a natural explanation for the
peculiarly red spectroscopic distribution of the latest M dwarfs and young
brown dwarfs. The grainless (Cond) models on the other hand, correspond closely
to methane brown dwarfs such as Gliese 229B. We also recover that the
5891,5897\AA Na I D and 7687,7701\AA K I resonance doublets
plays a critical role in T dwarfs where their red wing define the
pseudo-continuum from the to the bandpass.Comment: 49 pages, ApJ, in press. 22 figures (included). Corrected nasty
typos. Also available at http:/phoenix.physast.uga.ed