48 research outputs found
Spectroscopic diagnostic for the mineralogy of large dust grains
We examine the thermal infrared spectra of large dust grains of different
chemical composition and mineralogy. Strong resonances in the optical
properties result in detectable spectral structure even when the grain is much
larger than the wavelength at which it radiates. We apply this to the thermal
infrared spectra of compact amorphous and crystalline silicates. The weak
resonances of amorphous silicates at 9.7 and 18 micron virtually disappear for
grains larger than about 10 micron. In contrast, the strong resonances of
crystalline silicates produce emission dips in the infrared spectra of large
grains; these emission dips are shifted in wavelength compared to the emission
peaks commonly seen in small crystalline silicate grains. We discuss the effect
of a fluffy or compact grain structure on the infrared emission spectra of
large grains, and apply our theory to the dust shell surrounding Vega.Comment: Submitted to A&A Letter
FU Orionis - The MIDI/VLTI Perspective
We present the first mid-infrared interferometric measurements of FU Orionis.
We clearly resolve structures that are best explained with an optically thick
accretion disk. A simple accretion disk model fits the observed SED and
visibilities reasonably well and does not require the presence of any
additional structure such as a dusty envelope. The inclination and also the
position angle of the disk can be constrained from the multibaseline
interferometric observations. Our disk model is in general agreement with most
published near-infrared interferometric measurements. From the shape and
strength of the 8-13 micrometer spectrum the dust composition of the accretion
disk is derived for the first time. We conclude that most dust particles are
amorphous and already much larger than those typically observed in the ISM.
Although the high accretion rate of the system provides both, high temperatures
out to large radii and an effective transport mechanism to distribute
crystalline grains, we do not see any evidence for crystalline silicates
neither in the total spectrum nor in the correlated flux spectra from the inner
disk regions. Possible reasons for this non-detection are mentioned. All
results are discussed in context with other high-spatial resolution
observations of FU Ori and other FU Ori objects. We also address the question
whether FU Ori is in a younger evolutionary stage than a classical TTauri star.Comment: 41 pages (aastex style), 11 figures, 8 tables, accepted by Ap
C2D Spitzer-IRS spectra of disks around T Tauri stars V. Spectral decomposition
(Abridged) Dust particles evolve in size and lattice structure in
protoplanetary disks, due to coagulation, fragmentation and crystallization,
and are radially and vertically mixed in disks. This paper aims at determining
the mineralogical composition and size distribution of the dust grains in disks
around 58 T Tauri stars observed with Spitzer/IRS. We present a spectral
decomposition model that reproduces the IRS spectra over the full spectral
range. The model assumes two dust populations: a warm component responsible for
the 10\mu m emission arising from the disk inner regions and a colder component
responsible for the 20-30\mu m emission, arising from more distant regions. We
show evidence for a significant size distribution flattening compared to the
typical MRN distribution, providing an explanation for the usual boxy 10\mu m
feature profile generally observed. We reexamine the crystallinity paradox,
observationally identified by Olofsson et al. (2009), and we find a
simultaneous enrichment of the crystallinity in both the warm and cold regions,
while grain sizes in both components are uncorrelated. Our modeling results do
not show evidence for any correlations between the crystallinity and either the
star spectral type, or the X-ray luminosity (for a subset of the sample). The
size distribution flattening may suggests that grain coagulation is a slightly
more effective process than fragmentation in disk atmospheres, and that this
imbalance may last over most of the T Tauri phase. This result may also point
toward small grain depletion via strong stellar winds or radiation pressure in
the upper layers of disk. The non negligible cold crystallinity fractions
suggests efficient radial mixing processes in order to distribute crystalline
grains at large distances from the central object, along with possible nebular
shocks in outer regions of disks that can thermally anneal amorphous grains
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 shape and composition of interstellar silicate grains
We investigate the composition and shape distribution of silicate dust grains
in the interstellar medium. The effect of the amount of magnesium in the
silicate lattice is studied. We fit the spectral shape of the interstellar 10
mu extinction feature as observed towards the galactic center. We use very
irregularly shaped coated and non-coated porous Gaussian Random Field particles
as well as a statistical approach to model shape effects. For the dust
materials we use amorphous and crystalline silicates with various composition
and SiC. The results of our analysis of the 10 mu feature are used to compute
the shape of the 20 mu silicate feature and to compare this with observations.
By using realistic particle shapes we are, for the first time, able to derive
the magnesium fraction in interstellar silicates. We find that the interstellar
silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry
lies between pyroxene and olivine type silicates. This composition is not
consistent with that of the glassy material found in GEMS in interplanetary
dust particles indicating that these are, in general, not unprocessed remnants
from the interstellar medium. Also, we find a significant fraction of SiC
(~3%). We discuss the implications of our results for the formation and
evolutionary history of cometary and circumstellar dust. We argue that the fact
that crystalline silicates in cometary and circumstellar grains are almost
purely magnesium silicates is a natural consequence of our findings that the
amorphous silicates from which they were formed were already magnesium rich.Comment: Accepted for publication in A&
Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
We present the first mid-IR long baseline interferometric observations of the
circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD
52961, were observed using the VLTI/MIDI instrument during Science
Demonstration Time. Both objects are known binaries for which a stable
circumbinary disc is proposed to explain the SED characteristics. This is
corroborated by our N-band spectrum showing a crystallinity fraction of more
than 50 % for both objects, pointing to a stable environment where dust
processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved
in the interferometric observations providing an upper limit of 11 mas (or 18
AU at the distance of this object) on the diameter of the dust emission. This
confirms the very compact nature of its circumstellar environment. The dust
emission around HD 52961 originates from a very small but resolved region,
estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results
confirm the disc interpretation of the SED of both stars. In HD 52961, the dust
is not homogeneous in its chemical composition: the crystallinity is clearly
concentrated in the hotter inner region. Whether this is a result of the
formation process of the disc, or due to annealing during the long storage time
in the disc is not clear.Comment: 12 pages, 10 figures, accepted for publication in A &
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
Analysis of the dust evolution in the circumstellar disks of TTauri stars
We present a compositional analysis of 8-13um spectra of 32 young stellar
objects (YSOs). Our sample consists of 5 intermediate-mass stars and 27
low-mass stars. While the spectra and first scientific results have already
been published by Przygodda et al. (2003) and Kessler-Silacci et al. (2004) we
perform a more detailed analysis of the 10um silicate feature. In our analysis
we assume that this emission feature can be represented by a linear
superposition of the wavelength-dependent opacity
describing the optical properties of silicate grains with different chemical
composition, structure, and grain size. The determination of an adequate
fitting equation is another goal of this study. Using a restricted number of
fitting parameters we investigate which silicate species are necessary for the
compositional fitting. Particles with radii of 0.1um- and 1.5um consisting of
amorphous olivine and pyroxene, forsterite, enstatite, and quartz have been
considered. Only compact, homogeneous dust grains have been used in the
presented fitting procedures. In this context we show that acceptable fitting
results can also be achieved if emission properties of porous silicate grains
are considered instead. Although some previous studies give reasons for the
similarity between the dust in circumstellar disks of TTauri stars and Herbig
Ae/Be stars, a quantitative comparison has been missing, so far. Therefore, we
conclude with a discussion of the results of a 10um spectroscopic survey of van
Boekel et al. (2005) who focus on Herbig Ae/Be stars, the higher mass
counterparts of T Tauri stars and draw comparisons to this and other studies.
We find that the results of our study of T Tauri systems partly agree with
previous studies of Herbig Ae/Be stars.Comment: 17 pages, 6 figure
C2D Spitzer-IRS spectra of disks around T Tauri stars: I. Silicate emission and grain growth
Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7
intermediate-mass Herbig Ae stars with circumstellar disks were obtained using
the Spitzer Space Telescope as part of the c2d IRS survey. This work
complements prior spectroscopic studies of silicate infrared emission from
disks, which were focused on intermediate-mass stars, with observations of
solar-mass stars limited primarily to the 10 um region. The observed 10 and 20
um silicate feature strengths/shapes are consistent with source-to-source
variations in grain size. A large fraction of the features are weak and flat,
consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um
in radius). In addition, approximately half of the T Tauri star spectra show
crystalline silicate features near 28 and 33 um indicating significant
processing when compared to interstellar grains. A few sources show large
10-to-20 um ratios and require even larger grains emitting at 20 um than at 10
um. This size difference may arise from the difference in the depth into the
disk probed by the two silicate emission bands in disks where dust settling has
occurred. The 10 um feature strength vs. shape trend is not correlated with age
or Halpha equivalent width, suggesting that some amount of turbulent mixing and
regeneration of small grains is occurring. The strength vs. shape trend is
related to spectral type, however, with M stars showing significantly flatter
10 um features (larger grain sizes) than A/B stars. The connection between
spectral type and grain size is interpreted in terms of the variation in the
silicate emission radius as a function of stellar luminosity, but could also be
indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation,
stellar/disk winds, etc.).Comment: 17 pages, 13 figures, accepted for publication by ApJ, formatted with
emulateapj using revtex4 v4.
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&