241 research outputs found
Developing a self-consistent AGB wind model - II. Non-classical, non-equilibrium polymer nucleation in a chemical mixture
Unravelling the composition and characteristics of gas and dust lost by asymptotic giant branch
(AGB) stars is important as these stars play a vital role in the chemical life cycle of galaxies.
The general hypothesis of their mass-loss mechanism is a combination of stellar pulsations
and radiative pressure on dust grains. However, current models simplify dust formation,
which starts as a microscopic phase transition called nucleation. Various nucleation theories
exist, yet all assume chemical equilibrium, growth restricted by monomers, and commonly
use macroscopic properties for a microscopic process. Such simplifications for initial dust
formation can have large repercussions on the type, amount, and formation time of dust.
By abandoning equilibrium assumptions, discarding growth restrictions, and using quantum
mechanical properties, we have constructed and investigated an improved nucleation theory
in AGB wind conditions for four dust candidates, TiO2, MgO, SiO, and Al2O3. This paper
reports the viability of these candidates as first dust precursors and reveals implications of
simplified nucleation theories. Monomer restricted growth underpredicts large clusters at low
temperatures and overpredicts formation times. Assuming the candidates are present, Al2O3
is the favoured precursor due to its rapid growth at the highest considered temperatures.
However, when considering an initially atomic chemical mixture, only TiO2-clusters form.
Still, we believe Al2O3 to be the prime candidate due to substantial physical evidence in
presolar grains, observations of dust around AGB stars at high temperatures, and its ability to
form at high temperatures and expect the missing link to be insufficient quantitative data of
Al-reaction
Infrared spectra of TiO2 clusters for hot Jupiter atmospheres
Context. Clouds seem unavoidable in cool and dense environments, and hence,
are necessary to explain observations of exoplanet atmospheres, most recently
of WASP 96b with JWST. Understanding the formation of cloud condensation nuclei
in non-terrestrial environments is therefore crucial to develop accurate models
to interpret present and future observations. Aims. The goal of the paper is to
support observations with infrared spectra for (TiO2)N clusters in order to
study cloud formation in exoplanet atmospheres. Methods. Vibrational
frequencies are derived from quantum-chemical calculations for 123
(TiO2)-clusters and their isomers, and line-broadening mechanisms are
evaluated. Cluster spectra are calculated for several atmospheric levels for
two example exoplanet atmospheres (WASP 121b-like and WASP 96b-like) to
identify possible spectral fingerprints for cloud formation. Results.
Rotational motion of and transitions in the clusters cause significant line
broadening, so that individual vibrational lines are broadened beyond the
spectral resolution of the medium resolution mode of the JWST mid-infrared
instrument MIRI at R = 3000. However, each individual cluster isomer exhibits a
"fingerprint" IR spectrum. In particular, larger (TiO2)-clusters have
distinctly different spectra from smaller clusters. Morning and evening
terminator for the same planet can exhibit different total absorbances due to
different cluster sizes being more abundant. Conclusions. The largest
(TiO2)-clusters are not necessarily the most abundant (TiO2)-clusters in the
high-altitude regions of ultra-hot Jupiters, and the different cluster isomers
will contribute to the local absorbance. Planets with a considerable day-night
asymmetry will be most suitable to search for (TiO2)-cluster isomers in order
to improve cloud formation modelling.Comment: 8 pages, 8 figures, 1 table, accepted for publication in A&
Vanadium oxide clusters in substellar atmospheres: A quantum chemical study
We aim to understand the formation of cloud condensation nuclei in
oxygen-rich substellar atmospheres by calculating fundamental properties of the
energetically most favorable vanadium oxide molecules and clusters. A
hierarchical optimization approach is applied in order to find the most
favorable structures for clusters of (VO) and (VO) for N=1-10,
and (VO) for N=1-4 and to calculate their thermodynamical
potentials. The candidate geometries are initially optimized applying classical
interatomic potentials and then refined at the B3LYP/cc-pVTZ level of theory to
obtain accurate zero-point energies and thermochemical quantities. We present
previously unreported vanadium oxide cluster structures as lowest-energy
isomers. We report revised cluster energies and their thermochemical
properties. Chemical equilibrium calculations are used to asses the impact of
the updated and newly derived thermodynamic potentials on the gas-phase
abundances of vanadium-bearing species. In chemical equilibrium, larger
clusters from different stoichiometric families are found to be the most
abundant vanadium-bearing species for temperatures below ~1000 K, while
molecular VO is the most abundant between ~1000 K and ~2000 K. We determine the
nucleation rates of each stoichiometric family for a given (T,
p) profile of a brown dwarf using classical and non-classical
nucleation theory. Small differences in the revised Gibbs free energies of the
clusters have a large impact on the abundances of vanadium bearing species in
chemical equilibrium at temperatures below ~1000 K, which subsequently has an
impact on the nucleation rates of each stoichiometric family. We find that with
the revised and more accurate cluster data non-classical nucleation rates are
up to 15 orders of magnitude higher than classical nucleation rates.Comment: accepted to A&A, 16 pages, 10 figure
Protective effect of ultrathin alumina film against diffusion of iron into carbon fiber during growth of carbon nanotubes for hierarchical composites investigated by ptychographic X-ray computed tomography
Composite materials based on carbon fiber (CF) are prone to failure at the fiber-matrix interface upon compression or stress transverse to the fiber axis. The direct growth of carbon nanotubes on CF constitutes a novel approach to enhance the mechanical properties of the interface. However, the challenge is that, during the growth, tensile properties of the fiber are altered due to the diffusion effect of iron nanoparticles used in the process, leading to CF surface defect formation. In this work, we deliver and discuss an analysis methodology on ptychographic X-ray computed tomography (PXCT) images in order to assess the iron nanoparticle abundance within CFs. PXCT provides 50 nm - resolved 3D electron density maps of the CFs. We evidence the protective effect of an ultrathin alumina film against iron infiltration into CF during the CNT growth. This method potentially allows to evaluate the efficiency of other diffusion-minimizing approaches. The conclusions of the PXCT examination are validated by energy-dispersive X-ray spectroscopy and scanning transmission electron microscopy carried out on thin sample slices cut with a focused ion beam. The results provide a new insight into the mechanical performance of CFs and therefore constitute valuable knowledge for the development of hierarchical composites
Surface micromachining of UV transparent materials
Abstract A method which utilizes XeCl excimer laser and an absorbing liquid in contact with the material for precise structuring of UV transparent materials is presented. This one step micromachining process enables the fabrication of micro-optical elements with continuous profiles such as Fresnel micro-lenses in CaF and quartz with fluences well below the damage threshold of these 2 materials. The roughness of the etched features varies from 10 nm to 3 mm depending on the laser fluence and material. The etch rates of different UV transparent materials (such as CaF , BaF , sapphire and quartz) at various laser fluences suggest that 2 2 several different parameters influence the etching process.
ATOMIUM: A high-resolution view on the highly asymmetric wind of the AGB star pi(1)Gruis: I. First detection of a new companion and its effect on the inner wind
The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch (AGB) star π1Gruis, which has a known companion at ∼440 au and is thought to harbour a second, closer-by (< 10 au) companion, was observed with the Atacama Large Millimeter/submillimeter Array as part of the ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCN molecular line transitions are analysed. The continuum map shows two maxima, separated by 0.04″ (6 au). The CO data unambiguously reveal that π1Gru’s circumstellar environment harbours an inclined, radially outflowing, equatorial density enhancement. It contains a spiral structure at an angle of ∼38 ± 3° with the line-of-sight. The HCN emission in the inner wind reveals a clockwise spiral, with a dynamical crossing time of the spiral arms consistent with a companion at a distance of 0.04″ from the AGB star, which is in agreement with the position of the secondary continuum peak. The inner wind dynamics imply a large acceleration region, consistent with a beta-law power of ∼6. The CO emission suggests that the spiral is approximately Archimedean within 5″, beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.5″ exhibits signatures of gas in rotation, which is found to fit the expected behaviour of gas in the wind-companion interaction zone. An investigation of SiO maser emission reveals what could be a stream of gas accelerating from the surface of the AGB star to the companion. Using these dynamics, we have tentatively derived an upper limit on the companion mass to be ∼1.1 M⊙
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview:Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM sources. They were obtained
contemporaneously with the ALMA high spatial resolution data. To help interpret
the polarized signal, we produced synthetic maps of light scattering by dust,
through 3D radiative transfer simulations with the RADMC3D code.
Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads
across several optical filters. We infer that it primarily probes dust located
just outside of the point spread function, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
only a fraction of the total circumstellar dust. The maximum DoLP ranges from
0.03-0.38 depending on the source, fractions that can be reproduced by our 3D
pilot models for grains composed of common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), dust formation occurs at very specific sites.
This has potential consequences for the derived mass-loss rates and dust-to-gas
ratio in the inner region of the CSE. Except for ~Gru and perhaps GY
Aql, we do not detect interactions between the circumstellar wind and the
hypothesized companions that shape the wind at larger scales. This suggests
that the orbits of any other companions are tilted out of the plane of the sky.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
The VLT/SPHERE view of the ATOMIUM cool evolved star sample. I. Overview: Sample characterization through polarization analysis
Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to
present a consistent view of a sample of 17 nearby cool evolved stars
(Aymptotic Giant Branch and red supergiant stars).
Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the
visible of 14 out of the 17 ATOMIUM sources. They were obtained
contemporaneously with the ALMA high spatial resolution data. To help interpret
the polarized signal, we produced synthetic maps of light scattering by dust,
through 3D radiative transfer simulations with the RADMC3D code.
Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads
across several optical filters. We infer that it primarily probes dust located
just outside of the point spread function, and in or near the plane of the sky,
with a total optical depth close to unity in the line of sight, representing
only a fraction of the total circumstellar dust. The maximum DoLP ranges from
0.03-0.38 depending on the source, fractions that can be reproduced by our 3D
pilot models for grains composed of common dust species. The spatial structure
of the DoLP shows a diverse set of shapes. Only for three sources do we note a
correlation between the ALMA CO and SiO lines, which trace the gas density, and
the DoLP, which traces the dust.
Conclusion. The clumpiness of the DoLP and the lack of a consistent
correlation between the gas and the dust location show that, in the inner
circumstellar environment (CSE), dust formation occurs at very specific sites.
This has potential consequences for the derived mass-loss rates and dust-to-gas
ratio in the inner region of the CSE. Except for ~Gru and perhaps GY
Aql, we do not detect interactions between the circumstellar wind and the
hypothesized companions that shape the wind at larger scales. This suggests
that the orbits of any other companions are tilted out of the plane of the sky.Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15
figures, 5 table
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