138 research outputs found
Crystalline silicate dust around evolved stars II. The crystalline silicate complexes
This is the second paper in a series of three in which we present an
exhaustive inventory of the 49 solid state emission bands observed in a sample
of 17 oxygen-rich dust shells surrounding evolved stars. Most of these emission
bands are concentrated in well defined spectral regions (called complexes). We
define 7 of these complexes; the 10, 18, 23, 28, 33, 40 and 60 micron complex.
We derive average properties of the individual bands. Comparison with
laboratory data suggests that both olivines (Mg(2x)Fe(2-2x)SiO(4)) and
pyroxenes (Mg(x)Fe(1-x)SiO(3)) are present, with x close to 1, i.e. the
minerals are very Mg-rich and Fe-poor. This composition is similar to that seen
in disks surrounding young stars and in the solar system comet Hale-Bopp. A
significant fraction of the emission bands cannot be identified with either
olivines or pyroxenes. Possible other materials that may be the carriers of
these unidentified bands are briefly discussed. There is a natural division
into objects that show a disk-like geometry (strong crystalline silicate
bands), and objects whose dust shell is characteristic of an outflow (weak
crystalline silicate bands). In particular, stars with the 33.5 micron olivine
band stronger than about 20 percent over continuum are invariably disk sources.
Likewise, the 60 micron region is dominated by crystalline silicates in the
disk sources, while it is dominated by crystalline H(2)O ice in the outflow
sources. We show that the disk and outflow sources have significant differences
in the shape of the emission bands. This difference must be related to the
composition or grain shapes of the dust particles. The incredible richness of
the crystalline silicate spectra observed by ISO allows detailed studies of the
mineralogy of these dust shells, and is the origin and history of the dust.Comment: 20 pages, 21 figures, accepted by A&A, this paper and others (in this
serie) can also be found at http://zon.wins.uva.nl/~frankm/papers.htm
Crystalline silicate dust around evolved stars III. A correlations study of crystalline silicate features
We have carried out a quantitative trend analysis of the crystalline
silicates observed in the ISO spectra of a sample of 14 stars with different
evolutionary backgrounds. We have modeled the spectra using a simple dust
radiative transfer model and have correlated the results with other known
parameters. We confirm the abundance difference of the crystalline silicates in
disk and in outflow sources, as found by Molster et al. (1999, Nature 401,
563). We found some indication that the enstatite over forsterite abundance
ratio differs, it is slightly higher in the outflow sources with respect to the
disk sources. It is clear that more data is required to fully test this
hypothesis. We show that the 69.0 micron feature, attributed to forsterite, may
be a very suitable temperature indicator. We found that the enstatite is more
abundant than forsterite in almost all sources. The temperature of the
enstatite grains is about equal to that of the forsterite grains in the disk
sources but slightly lower in the outflow sources. Crystalline silicates are on
average colder than amorphous silicates. This may be due to the difference in
Fe content of both materials. Finally we find an indication that the ratio of
ortho to clino enstatite, which is about 1:1 in disk sources, shifts towards
ortho enstatite in the high luminosity (outflow) sources.Comment: 16 pages, 20 figures, accepted by A&A, this paper and others (in this
series) can also be found at http://zon.wins.uva.nl/~frankm/papers.htm
The mineralogy, geometry and mass-loss history of IRAS 16342-3814
We present the 2-200 um Infrared Space Observatory (ISO) spectrum and 3.8-20
um ISAAC and TIMMI2 images of the extreme OH/IR star IRAS 16342-3814. Amorphous
silicate absorption features are seen, together with crystalline silicate
absorption features up to almost 45 um. No other OH/IR star is known to have
crystalline silicate features in absorption up to these wavelengths. This
suggests that IRAS 16342-3814 must have, or recently had, an extremely high
mass-loss rate. Preliminary radiative transfer calculations suggest that the
mass-loss rate may be as large as 10^{-3} Msun/yr. The 3.8 um ISAAC image shows
a bipolar reflection nebula with a dark equatorial waist or torus, similar to
that seen in optical Hubble Space Telescope (HST) images. The position angle of
the nebula decreases significantly with increasing wavelength, suggesting that
the dominant source of emission changes from scattering to thermal emission.
Still, even up to 20 um the nebula is oriented approximately along the major
axis of the nebula seen in the HST and ISAAC images, suggesting that the torus
must be very cold, in agreement with the very red ISO spectrum. The 20 um image
shows a roughly spherically symmetric extended halo, approximately 6'' in
diameter, which is probably due to a previous phase of mass-loss on the AGB,
suggesting a transition from a (more) spherically symmetric to a (more) axial
symmetric form of mass-loss at the end of the AGB. We estimate the maximum dust
particle sizes in the torus and in the reflection nebula to be 1.3 and 0.09 um
respectively. The size of the particles in the torus is large compared to
typical ISM values, but in agreement with high mass-loss rate objects like AFGL
4106 and HD161796. We discuss the possible reason for the difference in
particle size between the torus and the reflection nebula.Comment: Accepted for publication by A&
The mineral composition and spatial distribution of the dust ejecta of NGC 6302
We have analysed the full ISO spectrum of the planetary nebula NGC 6302 in
order to derive the mineralogical composition of the dust in the nebula. We use
an optically thin dust model in combination with laboratory measurements of
cosmic dust analogues. We find two main temperature components at about 100 and
50 K respectively, with distinctly different dust compositions. The warm
component contains an important contribution from dust without strong infrared
resonances. In particular the presence of small warm amorphous silicate grains
can be excluded. The detection of weak PAH bands also points to a peculiar
chemical composition of the dust in this oxygen-rich nebula. The cool dust
component contains the bulk of the mass and shows strong emission from
crystalline silicates, which contain about 10 percent of the mass. In addition,
we identify the 92 micron band with the mineral calcite, and argue that the 60
micron band contains a contribution from the carbonate dolomite. We present the
mass absorption coefficients of six different carbonate minerals. The geometry
of the dust shell around NGC 6302 is studied with mid-infrared images obtained
with TIMMI2. We argue that the cool dust component is present in a
circumstellar dust torus, while the diffuse emission from the warm component
originates from the lobes.Comment: 13 pages, 10 figures, accepted for publication in A&
Location and sizes of forsterite grains in protoplanetary disks: interpretation from the Herschel DIGIT programme
The spectra of protoplanetary disks contain mid- and far- infrared emission
features produced by forsterite dust grains. The spectral features contain
information about the forsterite temperature, chemical composition and grain
size. We aim to characterize how the 23 and 69 micron features can be used to
constrain the physical locations of forsterite in disks. We check for
consistency between two independent forsterite temperature measurements: the
23/69 feature strength ratio and the shape of the 69 micron band. We performed
radiative transfer modeling to study the effect of disk properties to the
forsterite spectral features. Temperature-dependent forsterite opacities were
considered in self-consistent models to compute forsterite emission from
protoplanetary disks. Modelling grids are presented to study the effects of
grain size, disk gaps, radial mixing and optical depth to the forsterite
features. Independent temperature estimates derived from the 23/69 feature
strength ratio and the 69 micron band shape are most inconsistent for HD141569
and Oph IRS 48. A case study of the disk of HD141569 shows two solutions to fit
the forsterite spectrum. A model with T ~ 40 K, iron-rich (~0-1 % Fe) and 1
micron forsterite grains, and a model with warmer (T ~ 100 K), iron-free, and
larger (10 micron) grains. We find that for disks with low upper limits of the
69 micron feature (most notably in flat, self-shadowed disks), the forsterite
must be hot, and thus close to the star. We find no correlation between disk
gaps and the presence or absence of forsterite features. We argue that the 69
micron feature of the evolved transitional disks HD141569 and Oph IRS 48 is
most likely a tracer of larger (i.e. ~10 micron) forsterite grains.Comment: Accepted for publication in A&A. 14 pages, 9 figure
The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks
As stars evolve along the Asymptotic Giant Branch, strong winds are driven
from the outer envelope. These winds form a shell, which may ultimately become
a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at
the presence of a binary companion. Some post-Asymptotic Giant Branch objects
are surrounded by torii of crystalline dust, but there is no generally accepted
mechanism for annealing the amorphous grains in the wind to crystals. In this
Letter, we show that the shaping of the wind by a binary companion is likely to
lead to the formation of crystalline dust in the orbital plane of the binary.Comment: Submitted to ApJ
The composition and nature of the dust shell surrounding the binary AFGL 4106
We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron
ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K)
in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195
micron SWS and LWS spectra are dominated by strong emission from circumstellar
dust. We find that the dust consists of amorphous silicates, with a minor but
significant contribution from crystalline silicates. The amorphous silicates
consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be
excluded but if present they contain much less Fe than the amorphous olivines.
Comparison with laboratory data shows that the pure Mg-end members of the
crystalline olivine and pyroxene solid solution series are present. In
addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as
crystalline H2O ice. Several narrow emission features remain unidentified.
Modelling of the dust emission using a dust radiation transfer code shows that
large grains (~1 micron) must be present and that the abundance of the
crystalline silicates is between 7 and 15% of the total dust mass, depending on
the assumed enstatite to forsterite ratio, which is estimated to be between 1
and 3. The amorphous and crystalline dust components in the shell do not have
the same temperature, implying that the different dust species are not
thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a
period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106
binary is likely a post-red-supergiant in transition to a blue supergiant or WR
phase.Comment: 22 pages (including 12 figures), accepted by Astronomy and
Astrophysic
SPITZER survey of dust grain processing in stable discs around binary post-AGB stars
Aims: We investigate the mineralogy and dust processing in the circumbinary
discs of binary post-AGB stars using high-resolution TIMMI2 and SPITZER
infrared spectra. Methods: We perform a full spectral fitting to the infrared
spectra using the most recent opacities of amorphous and crystalline dust
species. This allows for the identification of the carriers of the different
emission bands. Our fits also constrain the physical properties of different
dust species and grain sizes responsible for the observed emission features.
Results: In all stars the dust is oxygen-rich: amorphous and crystalline
silicate dust species prevail and no features of a carbon-rich component can be
found, the exception being EPLyr, where a mixed chemistry of both oxygen- and
carbon-rich species is found. Our full spectral fitting indicates a high degree
of dust grain processing. The mineralogy of our sample stars shows that the
dust is constituted of irregularly shaped and relatively large grains, with
typical grain sizes larger than 2 micron. The spectra of nearly all stars show
a high degree of crystallinity, where magnesium-rich end members of olivine and
pyroxene silicates dominate. Other dust features of e.g. silica or alumina are
not present at detectable levels. Temperature estimates from our fitting
routine show that a significant fraction of grains must be cool, significantly
cooler than the glass temperature. This shows that radial mixing is very
efficient is these discs and/or indicates different thermal conditions at grain
formation. Our results show that strong grain processing is not limited to
young stellar objects and that the physical processes occurring in the discs
are very similar to those in protoplanetary discs.Comment: 22pages, 50 figures (in appendix), accepted for A&
The symmetric dust shell and the central star of the bipolar planetary nebula NGC 6537
We present high-resolution images of the strongly bipolar planetary nebula
NGC 6537, obtained with Hubble Space Telescope and with the infrared adaptive
optics system on the Very Large Telescope. The central star is detected for the
first time. Using the multi-band photometry and constraints from the dynamical
age of the nebula, we derive a temperature in the range 1.5-2.5 10^5 K, a
luminosity~10^3 L_sun and a core mass M_c~0.7-0.9 M_sun. The progenitor mass is
probably in the range M_i = 3-7 M_sun. The extinction map shows a largely
symmetric, and compact dust structure, which is most likely a shell, located at
the neck of the bipolar flow, only 4 arcsec from the star. The dust shell
traces a short-lived phase of very high mass loss at the end of the AGB. The
dynamical age of the shell and bipolar lobes are very similar but the
morphologies are very different. The data suggests that the mass loss during
the ejection of the compact shell was largely spherically symmetric, and the
pronounced bipolarity formed afterwards. The dynamical ages of the bipolar
lobes and dust shell are similar, which is consistent with suggestions that
bipolar structures form in a run-away event at the very last stages of the AGB
mass loss. The inner edge of the dust shell is ionized, and PAH emission is
seen just outside the ionized gas. We associate the PAH emission with the
photo-dissociation region of the molecular shell.Comment: 10 pages, accepted by MNRA
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