8,815 research outputs found
Laboratory mid-IR spectra of equilibrated and igneous meteorites. Searching for observables of planetesimal debris
Meteorites contain minerals from Solar System asteroids with different
properties (like size, presence of water, core formation). We provide new
mid-IR transmission spectra of powdered meteorites to obtain templates of how
mid-IR spectra of asteroidal debris would look like. This is essential for
interpreting mid-IR spectra of past and future space observatories, like the
James Webb Space Telescope. We show that the transmission spectra of wet and
dry chondrites, carbonaceous and ordinary chondrites and achondrite and
chondrite meteorites are distinctly different in a way one can distinguish in
astronomical mid-IR spectra. The two observables that spectroscopically
separate the different meteorites groups (and thus the different types of
parent bodies) are the pyroxene-olivine feature strength ratio and the peak
shift of the olivine spectral features due to an increase in the iron
concentration of the olivine
The lunar phases of dust grains orbiting Fomalhaut
Optical images of the nearby star Fomalhaut show a ring of dust orbiting the
central star. This dust is in many respects expected to be similar to the
zodiacal dust in the solar system. The ring displays a clear brightness
asymmetry, attributed to asymmetric scattering of the central starlight by the
circumstellar dust grains. Recent measurements show that the bright side of the
Fomalhaut ring is oriented away from us. This implies that the grains in this
system scatter most of the light in the backward direction, in sharp contrast
to the forward-scattering nature of the grains in the solar system. In this
letter, we show that grains considerably larger than those dominating the solar
system zodiacal dust cloud provide a natural explanation for the apparent
backward scattering behavior. In fact, we see the phases of the dust grains in
the same way as we can observe the phases of the Moon and other large solar
system bodies. We outline how the theory of the scattering behavior of
planetesimals can be used to explain the Fomalhaut dust properties. This
indicates that the Fomalhaut dust ring is dominated by very large grains. The
material orbiting Fomalhaut, which is at the transition between dust and
planetesimals, can, with respect to their optical behavior, best be described
as micro-asteroids.Comment: Accepted for publication in A&
The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia
We have computed radiative equilibrium models for the gas in the
circumstellar envelope surrounding the hot, classical Be star Cassiopeia. This calculation is performed using a code that incorporates a
number of improvements over previous treatments of the disk's thermal structure
by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates
are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their
relevant ions. Thus, for the first time, the thermal structure of a Be disk is
computed for a gas with a solar chemical composition as opposed to assuming a
pure hydrogen envelope. We compare the predicted average disk temperature, the
total energy loss in H, and the near-IR excess with observations and
find that all can be accounted for by a disk that is in vertical hydrostatic
equilibrium with a density in the equatorial plane of to
. We also discuss the changes in
the disk's thermal structure that result from the additional heating and
cooling processes available to a gas with a solar chemical composition over
those available to a pure hydrogen plasma.Comment: 11 pages, 8 figures high resolution figures available at
http://inverse.astro.uwo.ca/sig_jon07.htm
Micron-sized forsterite grains in the pre-planetary nebula of IRAS 17150-3224 - Searching for clues on the mysterious evolution of massive AGB stars
We study the grain properties and location of the forsterite crystals in the
circumstellar environment of the pre-planetary nebula (PPN) IRAS 17150-3224 in
order to learn more about the as yet poorly understood evolutionary phase prior
to the PPN. We use the best-fit model for IRAS 17150-3224 of Meixner et al.
(2002) and add forsterite to this model. We investigate different spatial
distributions and grain sizes of the forsterite crystals in the circumstellar
environment. We compare the spectral bands of forsterite in the mid-infrared
and at 69 micrometre in radiative transport models to those in ISO-SWS and
Herschel/PACS observations. We can reproduce the non-detection of the
mid-infrared bands and the detection of the 69 micrometre feature with models
where the forsterite is distributed in the whole outflow, in the superwind
region, or in the AGB-wind region emitted previous to the superwind, but we
cannot discriminate between these three models. To reproduce the observed
spectral bands with these three models, the forsterite crystals need to be
dominated by a grain size population of 2 micrometre up to 6 micrometre. We
hypothesise that the large forsterite crystals were formed after the superwind
phase of IRAS 17150-3224, where the star developed an as yet unknown hyperwind
with an extremely high mass-loss rate (10^-3 Msol/yr). The high densities of
such a hyperwind could be responsible for the efficient grain growth of both
amorphous and crystalline dust in the outflow. Several mechanisms are discussed
that might explain the lower-limit of 2 micrometre found for the forsterite
grains, but none are satisfactory. Among the mechanisms explored is a possible
selection effect due to radiation pressure based on photon scattering on
micron-sized grains.Comment: Accepted by A&
Variability and nature of the binary in the Red Rectangle Nebula
We present new observations of the central binary inside the Red Rectangle
nebula. The detection of zinc in the optical spectrum confirms that the
peculiar photospheric abundances are due to accretion of circumstellar gas.
Grey brightness variations with the orbital period are observed. They are
interpreted as being due to the variation of the scattering angle with orbital
phase. The small orbital separation of the system is not compatible with
previous normal evolution of the primary on the AGB. We point out the
similarity of the orbital history of this and other similar systems with those
of some close Barium stars and suggest that the nonzero eccentricity of the
orbit is the result of tidal interaction with the circumbinary disk.Comment: 4 pages, 3 figures, A&A Letters accepte
The absence of the 10 um silicate feature in the isolated Herbig Ae star HD 100453
We analyse the optical and IR spectra, as well as the spectral energy
distribution (UV to mm) of the candidate Herbig Ae star HD100453. This star is
particular, as it shows an energy distribution similar to that of other
isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have
a silicate emission feature at 10 um, as is shown in Meeus (2001). We confirm
the HAEBE nature of HD100453 through an analysis of its optical spectrum and
derived location in the H-R diagram. The IR spectrum of HD100453 is modelled by
an optically thin radiative transfer code, from which we derive constraints on
the composition, grain-size and temperature distribution of the circumstellar
dust. We show that it is both possible to explain the lack of the silicate
feature as (1) a grain-size effect - lack of small silicate grains, and (2) a
temperature effect - lack of small, hot silicates, as proposed by Dullemond
(2001), and discuss both possibilities.Comment: 9 pages, 7 figures; accepted by A&
The problematically short superwind of OH/IR stars - Probing the outflow with the 69 {\mu}m spectral band of forsterite
Spectra of OH/IR stars show prominent spectral bands of crystalline olivine
(MgFeSiO). To learn more about the timescale of the
outflows of OH/IR stars, we study the spectral band of crystalline olivine at
69 {\mu}m. The 69 {\mu}m band is of interest because its width and peak
wavelength position are sensitive to the grain temperature and to the exact
composition of the crystalline olivine. With Herschel/PACS, we observed the 69
{\mu}m band in the outflow of 14 OH/IR stars. By comparing the crystalline
olivine features of our sample with those of model spectra, we determined the
size of the outflow and its crystalline olivine abundance.
The temperature indicated by the observed 69 {\mu}m bands can only be
reproduced by models with a geometrically compact superwind
( 2500 AU = 1400 R).This means that the superwind
started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The
small amount of mass lost in one superwind and the high progenitor mass of the
OH/IR stars introduce a mass loss and thus evolutionary problem for these
objects, which has not yet been understood.Comment: Accepted by A&
Dust-grain processing in circumbinary discs around evolved binaries. The RV Tauri spectral twins RU Cen and AC Her
Context: We study the structure and evolution of circumstellar discs around
evolved binaries and their impact on the evolution of the central system. Aims:
To study in detail the binary nature of RUCen and ACHer, as well as the
structure and mineralogy of the circumstellar environment. Methods: We combine
multi-wavelength observations with a 2D radiative transfer study. Our radial
velocity program studies the central stars, while our Spitzer spectra and
broad-band SEDs are used to constrain mineralogy, grain sizes and physical
structure of the circumstellar environment. Results: We determine the orbital
elements of RUCen showing that the orbit is highly eccentric with a rather long
period of 1500 days. The infrared spectra of both objects are very similar and
the spectral dust features are dominated by Mg-rich crystalline silicates. The
small peak-to-continuum ratios are interpreted as being due to large grains.
Our model contains two components with a cold midplain dominated by large
grains, and the near- and mid-IR which is dominated by the emission of smaller
silicates. The infrared excess is well modelled assuming a hydrostatic passive
irradiated disc. The profile-fitting of the dust resonances shows that the
grains must be very irregular. Conclusions: These two prototypical RVTauri
pulsators with circumstellar dust are binaries where the dust is trapped in a
stable disc. The mineralogy and grain sizes show that the dust is highly
processed, both in crystallinity and grain size. The cool crystals show that
either radial mixing is very efficient and/or that the thermal history at grain
formation has been very different from that in outflows. The physical processes
governing the structure of these discs are similar to those observed in
protoplanetary discs around young stellar objects.Comment: 11 pages, 12 figures, accepted for publication by A&
Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss
Low- and intermediate-mass stars go through a period of intense mass-loss at
the end of their lives in a phase known as the asymptotic giant branch (AGB).
During the AGB a significant fraction of their initial mass is expelled in a
stellar wind. This process controls the final stages of their evolution and
contributes to the chemical evolution of galaxies. However, the wind-driving
mechanism of AGB stars is not yet well understood, especially so for
oxygen-rich sources. Characterizing both the present-day mass-loss and wind
structure and the evolution of the mass-loss rate of such stars is paramount to
advancing our understanding of this processes. We modelled the dust envelope of
W Hya using an advanced radiative transfer code. The dust model was analysed in
the light of a previously calculated gas-phase wind model and compared to
measurements available in the literature, such as infrared spectra, infrared
images, and optical scattered light fractions. We find that the dust spectrum
of W Hya can partly be explained by a gravitationally bound dust shell that
probably is responsible for most of the amorphous AlO emission. The
composition of the large (\,0.3\,m) grains needed to explain the
scattered light cannot be constrained, but probably is dominated by silicates.
Silicate emission in the thermal infrared was found to originate from beyond 40
AU from the star and we find that they need to have substantial near-infrared
opacities to be visible at such large distances. The increase in near-infrared
opacity of the dust at these distances roughly coincides with a sudden increase
in expansion velocity as deduced from the gas-phase CO lines. Finally, the
recent mass loss of W Hya is confirmed to be highly variable and we identify a
strong peak in the mass-loss rate that occurred about 3500 years ago and lasted
for a few hundred years.Comment: 15 pages, 13 figure
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