234 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&
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
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 circumstellar dust shell of the post-AGB star HD 161796
We have modeled the complete optical to millimeter spectrum of the Post-Asymptotic Giant Branch (Post-AGB) star HD 161796 and its circumstellar dust shell. A full 2–200 μm spectrum taken with the Infrared Space Observatory was used to constrain the dust properties. A good fit is achieved using only 4 dust components: amorphous silicates, the crystalline silicates forsterite and enstatite, and crystalline water ice, contributing respectively about 63, 4, 6 and 27% to the total dust mass. The different dust species were assumed to be co-spatial but distinct, resulting in different temperatures for the different grain populations. We find a temperature for the crystalline H2O ice of 70 K, which is higher than thermal equilibrium calculations of pure H2O ice would give. This implies that the ice must be formed as a mantle on top of an (amorphous) silicate core. In order to form H2O ice mantles the mass loss rate must exceed some yr-1. With a water-ice fraction of 27% a lower limit for the gas to dust mass ratio of 270 is found. At a distance of 1.2 kpc (Skinner et al. [CITE]) and adopting an outflow velocity of 15 km s-1 (Likkel et al. [CITE]) an AGB mass loss rate of ( yr-1) is found, which lasted 900 years and ended 430 years ago. During this phase a total of 0.46 was expelled. The mass loss rate was high enough to account for the presence of the H2O ice
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
Determining the forsterite abundance of the dust around Asymptotic Giant Branch stars
Aims. We present a diagnostic tool to determine the abundance of the
crystalline silicate forsterite in AGB stars surrounded by a thick shell of
silicate dust. Using six infrared spectra of high mass-loss oxygen rich AGB
stars we obtain the forsterite abundance of their dust shells.
Methods. We use a monte carlo radiative transfer code to calculate infrared
spectra of dust enshrouded AGB stars. We vary the dust composition, mass-loss
rate and outer radius. We focus on the strength of the 11.3 and the 33.6 \mu m
forsterite bands, that probe the most recent (11.3 \mu m) and older (33.6 \mu
m) mass-loss history of the star. Simple diagnostic diagrams are derived,
allowing direct comparison to observed band strengths.
Results. Our analysis shows that the 11.3 \mu m forsterite band is a robust
indicator for the forsterite abundance of the current mass-loss period for AGB
stars with an optically thick dust shell. The 33.6 \mu m band of forsterite is
sensitive to changes in the density and the geometry of the emitting dust
shell, and so a less robust indicator. Applying our method to six high
mass-loss rate AGB stars shows that AGB stars can have forsterite abundances of
12% by mass and higher, which is more than the previously found maximum
abundance of 5%.Comment: Accepted for publication in A&
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
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