457 research outputs found
Optical properties of cosmic dust analogs: A review
Nanometer- and micrometer-sized solid particles play an important role in the
evolutionary cycle of stars and interstellar matter. The optical properties of
cosmic grains determine the interaction of the radiation field with the solids,
thereby regulating the temperature structure and spectral appearance of dusty
regions. Radiation pressure on dust grains and their collisions with the gas
atoms and molecules can drive powerful winds. The analysis of observed spectral
features, especially in the infrared wavelength range, provides important
information on grain size, composition and structure as well as temperature and
spatial distribution of the material.
The relevant optical data for interstellar, circumstellar, and protoplanetary
grains can be obtained by measurements on cosmic dust analogs in the laboratory
or can be calculated from grain models based on optical constants. Both
approaches have made progress in the last years, triggered by the need to
interpret increasingly detailed high-quality astronomical observations. The
statistical theoretical approach, spectroscopic experiments at variable
temperature and absorption spectroscopy of aerosol particulates play an
important role for the successful application of the data in dust astrophysics.Comment: 18 pages, 6 figures, invited review for Journal of Nanophotonics,
Special Section to honour C.F. Bohre
Optical constants of refractory oxides at high temperatures
Many cosmic dust species, among them refractory oxides, form at temperatures
higher than 300 K. Nevertheless, most astrophysical studies are based on the
room-temperature optical constants of solids, such as corundum and spinel. A
more realistic approach is needed for these materials, especially in the
context of modeling late-type stars. We aimed at deriving sets of optical
constants of selected, astrophysically relevant oxide dust species with high
melting points. A high-temperature-high-pressure-cell and a Fourier-transform
spectrometer were used to measure reflectance spectra of polished samples. For
corundum (alpha-AlO), spinel (MgAlO), and alpha-quartz
(SiO), temperature-dependent optical constants were measured from 300 K up
to more than 900 K. Small particle spectra were also calculated from these
data. All three examined oxides show a significant temperature dependence of
their mid-IR bands. For the case of corundum, we find that the 13m
emission feature - seen in the IR spectra of many AGB stars - can very well be
assigned to this mineral species. The best fit of the feature is achieved with
oblate corundum grains at mean temperatures around 550 K. Spinel remains a
viable carrier of the 13m feature as well, but only for T < 300 K and
nearly spherical grain shapes. Under such circumstances, spinel grains may also
account for the 31.8m band that is frequently seen in sources of the
13m feature and which has not yet been identified with certainty.Comment: Astronomy & Astrophysics, accepted, 26 February 2013. Article with 18
pages and 15 figure
How can heat maps of indexing vocabularies be utilized for information seeking purposes?
The ability to browse an information space in a structured way by exploiting
similarities and dissimilarities between information objects is crucial for
knowledge discovery. Knowledge maps use visualizations to gain insights into
the structure of large-scale information spaces, but are still far away from
being applicable for searching. The paper proposes a use case for enhancing
search term recommendations by heat map visualizations of co-word
relation-ships taken from indexing vocabulary. By contrasting areas of
different "heat" the user is enabled to indicate mainstream areas of the field
in question more easily.Comment: URL workshop proceedings: http://ceur-ws.org/Vol-1311
Reconsidering the origin of the 21 micron feature: Oxides in carbon-rich PPNe?
The origin of the so-called "21 micron" feature which is especially prominent
in the spectra of some carbon-rich protoplanetary nebulae (PPNe}) is the matter
of a lively debate. A large number of potential band carriers have been
presented and discarded within the past decade. The present paper gives an
overview of the problems related to the hitherto proposed feature
identifications, including the recently suggested candidate carrier silicon
carbide. We also discuss the case for spectroscopically promising oxides.
SiC is shown to produce a strong resonance band at 20-21 micron if coated by
a layer of silicon dioxide. At low temperatures, core-mantle particles composed
of SiC and amorphous SiO indeed have their strongest spectral signature at
a position of 20.1 micron, which coincides with the position of the "21 micron"
emission band.
The optical constants of another candidate carrier that has been relatively
neglected so far -- iron monoxide -- are proven to permit a fairly accurate
reproduction of the "21 micron" feature profile as well, especially when
low-temperature measurements of the infrared properties of FeO are taken into
account. As candidate carrier of the "21 micron" emission band, FeO has the
advantage of being stable against further oxidation and reduction only in a
narrow range of chemical and physical conditions, coinciding with the fact that
the feature, too, is detected in a small group of objects only. However, it is
unclear how FeO should form or survive particularly in carbon-rich PPNe.Comment: 28 pages, 15 figures, accepted for publication in ApJ (December
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
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