630 research outputs found
A comprehensive GIS approach to dredging-related studies and datasets for the Belgian coastal waters and the Scheldt estuary
Attached intertidal diatoms have stronger photoprotective capacity compared to motile diatoms
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&
Photoprotection capacity differs among microphytobenthic diatoms inhabiting intertidal mudflats: Possible consequences on their spatial distribution related to the light environment
The Cosmic Crystallinity Conundrum: Clues from IRAS 17495-2534
Since their discovery, cosmic crystalline silicates have presented several
challenges to understanding dust formation and evolution. The mid-infrared
spectrum of IRAS 174952534, a highly obscured oxygen-rich asymptotic giant
branch (AGB) star, is the only source observed to date which exhibits a clear
crystalline silicate absorption feature. This provides an unprecedented
opportunity to test competing hypotheses for dust formation. Observed spectral
features suggest that both amorphous and crystalline dust is dominated by
forsterite (Mg\_2 SiO\_4) rather than enstatite (MgSiO\_3) or other silicate
compositions. We confirm that high mass-loss rates should produce more
crystalline material, and show why this should be dominated by forsterite. The
presence of Mg\_2 SiO\_4 glass suggests that another factor (possibly C/O) is
critical in determining astromineralogy. Correlation between crystallinity,
mass-loss rate and initial stellar mass suggests that only the most massive AGB
stars contribute significant quantities of crystalline material to the
interstellar medium, resolving the conundrum of its low crystallinity.Comment: 12 pages, 2 figure
Ecological niche predicts photoprotection capacity of microphytobenthic diatoms inhabiting intertidal mudflats
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