61 research outputs found
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
Seed particle formation for silicate dust condensation by SiO nucleation
Clustering of the abundant SiO molecules has been discussed as a possible
mechanism of seed particle formation for silicate dust in stellar outflows with
an oxygen rich element mixture. Previous results indicated that condensation
temperatures based on this mechanism are significant lower than what is really
observed. This negative result strongly rests on experimental data on vapour
pressure of SiO. New determinations show the older data to be seriously in
error. Here we aim to check with improved data the possibility that SiO
nucleation triggers the cosmic silicate dust formation. First we present
results of our measurements of vapour pressure of solid SiO. Second, we use the
improved vapour pressure data to re-calibrate existing experimental data on SiO
nucleation from the literature. Third, we use the re-calibrated data on SiO
nucleation in a simple model for dust-driven winds to determine the
condensation temperature of silicate in stellar outflows from AGB stars. We
show that onset of nucleation under circumstellar conditions commences at
higher temperature than was previously found. Calculated condensation
temperatures are still by about 100 K lower than observed ones, but this may be
due to the greenhouse effect of silicate dust temperatures. The assumption that
the onset of silicate dust formation in late-type M stars is triggered by
cluster formation of SiO is compatible with dust condensation temperatures
derived from IR observations.Comment: 11 pages, 11 figure
Transient dust in warm debris disks - Detection of Fe-rich olivine grains
(Abridged) Debris disks trace remnant reservoirs of leftover planetesimals in
planetary systems. A handful of "warm" debris disks have been discovered in the
last years, where emission in excess starts in the mid-infrared. An interesting
subset within these warm debris disks are those where emission features are
detected in mid-IR spectra, which points towards the presence of warm
micron-sized dust grains. Given the ages of the host stars, the presence of
these grains is puzzling, and questions their origin and survival in time. This
study focuses on determining the mineralogy of the dust around 7 debris disks
with evidence for warm dust, based on Spitzer/IRS spectroscopic data, in order
to provide new insights into the origin of the dust grains. We present a new
radiative transfer code dedicated to SED modeling of optically thin disks. We
make use of this code on the SEDs of seven warm debris disks, in combination
with recent laboratory experiments on dust optical properties. We find that
most, if not all, debris disks in our sample are experiencing a transient
phase, suggesting a production of small dust grains on relatively short
timescales. From a mineralogical point of view, we find that enstatite grains
have small abundances compared to crystalline olivine grains. The main result
of our study is that we find evidences for Fe-rich crystalline olivine grains
(Fe / [Mg + Fe] ~ 0.2) for several debris disks. This finding contrasts with
studies of gas-rich protoplanetary disks. The presence of Fe-rich olivine
grains, and the overall differences between the mineralogy of dust in Class II
disks compared to debris disks suggest that the transient crystalline dust is
of a new generation. We discuss possible crystallization routes to explain our
results, and comment on the mechanisms that may be responsible for the
production of small dust grains
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
Morphological effects on IR band profiles: Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars
To trace the source of the unique 13, 19.5, and 28 m emission features
in the spectra of oxygen-rich circumstellar shells around AGB stars, we have
compared dust extinction spectra obtained by aerosol measurements. We have
measured the extinction spectra for 19 oxide powder samples of eight different
types, such as Ti-compounds (TiO, TiO, TiO, TiO,
AlTiO, CaTiO), -, -,
---AlO, and MgAlO in the infrared region
(10 - 50 m) paying special attention to the morphological (size, shape,
and agglomeration) effects and the differences in crystal structure. Anatase
(TiO) particles with rounded edges are the possible 13, 19.5 and 28 m
band carriers as the main contributor in the spectra of AGB stars, and
spherically shaped nano-sized spinel and AlTiO dust grains are possibly
associated with the anatase, enhancing the prominence of the 13 m feature
and providing additional features at 28 m. The extinction data sets
obtained by the aerosol and CsI pellet measurements have been made available
for public use at http://elbe.astro.uni-jena.deComment: 17 pages, 8 figures, Accepted 24 March 2009 for publication in A&
Dust Processing and Grain Growth in Protoplanetary Disks in the Taurus-Auriga Star-Forming Region
Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using dust
at two temperatures to probe the radial variation in dust composition in the
uppermost layers of protoplanetary disks. Most spectra indicating crystalline
silicates require Mg-rich minerals and silica, but a few suggest otherwise.
Spectra indicating abundant enstatite at higher temperatures also require
crystalline silicates at temperatures lower than those required for spectra
showing high abundance of other crystalline silicates. A few spectra show 10
micron complexes of very small equivalent width. They are fit well using
abundant crystalline silicates but very few large grains, inconsistent with the
expectation that low peak-to-continuum ratio of the 10 micron complex always
indicates grain growth. Most spectra in our sample are fit well without using
the opacities of large crystalline silicate grains. If large grains grow by
agglomeration of submicron grains of all dust types, the amorphous silicate
components of these aggregates must typically be more abundant than the
crystalline silicate components. Crystalline silicate abundances correlate
positively with other such abundances, suggesting that crystalline silicates
are processed directly from amorphous silicates and that neither forsterite,
enstatite, nor silica are intermediate steps when producing either of the other
two. Disks with more dust settling typically have greater crystalline
abundances. Large-grain abundance is somewhat correlated with greater settling
of disks. The lack of strong correlation is interpreted to mean that settling
of large grains is sensitive to individual disk properties. Lower-mass stars
have higher abundances of large grains in their inner regions.Comment: 84 pages, 27 figures, submitted to the Astrophysical Journal on 7
November, 200
Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal
We present spectra of 1142 colour-selected stars in the direction of the
Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with
VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe
1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at
>99% confidence) for 592 stars on the basis of their radial velocity, and
spectral types are given. Very slow rotation is marginally detected around the
galaxy's major axis. We identify five S stars and 23 carbon stars, of which all
but four carbon stars are newly-determined and all but one (PQ Sgr) are likely
Sgr dSph members. We examine the onset of carbon-richness in this metal-poor
galaxy in the context of stellar models. We compare the stellar death rate (one
star per 1000-1700 years) to known planetary nebula dynamical ages and find
that the bulk population produce the observed (carbon-rich) planetary nebulae.
We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr,
compared to a total thermal-pulsing asymptotic giant branch lifetime of
530-1330 kyr. We conclude by discussing the return of carbon-rich material to
the ISM.Comment: 14 pages, 10 figures, accepted MNRA
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
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