| 1. |
- Andersson, Stefan, 1973, et al.
(författare)
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Mechanisms of SiO oxidation: Implications for dust formation
- 2023
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Ingår i: Frontiers in Astronomy and Space Sciences. - : Frontiers Media SA. - 2296-987X. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- Reactions of SiO molecules have been postulated to initiate efficient formation of silicate dust particles in outflows around dying (AGB) stars. Both OH radicals and H2O molecules can be present in these environments and their reactions with SiO and the smallest SiO cluster, Si2O2, affect the efficiency of eventual dust formation. Rate coefficients of gas-phase oxidation and clustering reactions of SiO, Si2O2 and Si2O3 have been calculated using master equation calculations based on density functional theory calculations. The calculations show that the reactions involving OH are fast. Reactions involving H2O are not efficient routes to oxidation but may under the right conditions lead to hydroxylated species. The reaction of Si2O2 with H2O, which has been suggested as efficient producing Si2O3, is therefore not as efficient as previously thought. If H2O molecules dissociate to form OH radicals, oxidation of SiO and dust formation could be accelerated. Kinetics simulations of oxygen-rich circumstellar environments using our proposed reaction scheme suggest that under typical conditions only small amounts of SiO2 and Si2O2 are formed and that most of the silicon remains as molecular SiO.
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| 2. |
- Baudry, A., et al.
(författare)
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ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H2O and OH lines
- 2023
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
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Tidskriftsartikel (refereegranskat)abstract
- Context. Water (H2O) and the hydroxyl radical (OH) are major constituents of the envelope of O-rich late-type stars. Transitions involving energy levels that are rotationally or vibrationally highly excited (energies & GSIM;4000 K) have been observed in both H2O and OH. These and more recently discovered transitions can now be observed at a high sensitivity and angular resolution in the inner wind close to the stellar photosphere with the Atacama Large Millimeter/submillimeter Array (ALMA). Aims. Our goals are: (1) to identify and map the emission and absorption of H2O in several vibrational states, and of OH in Lambda-doubling transitions with similar excitation energies; and (2) to determine the physical conditions and kinematics in gas layers close to the extended atmosphere in a sample of asymptotic giant branch stars (AGBs) and red supergiants (RSGs). Methods. Spectra and maps of H2O and OH lines observed in a 27 GHz aggregated bandwidth and with an angular resolution of similar to 0."02-1."0 were obtained at two epochs with the main ALMA array. Additional observations with the Atacama Compact Array (ACA) were used to check for time variability of water transitions. Radiative transfer models of H2O were revisited to characterize masing conditions. Up-to-date chemical models were used for comparison with the observed OH/H2O abundance ratio. Results. Ten rotational transitions of H2O with excitation energies similar to 4000-9000 K were observed in vibrational states up to (& upsilon;(1),& upsilon;(2),& upsilon;(3)) = (0,1,1). All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split Lambda-doubling transitions in & upsilon; = 0, J = 27/2 and 29/2 levels of the (2)& pi;(3/2) state, as well as J = 33/2 and 35/2 of the (2)& pi;(1/2) state of OH with excitation energies of similar to 4780-8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the & upsilon; = 0 and & upsilon; = 1 Lambda-doubling frequencies have been improved. Our H2O maps show compact emission toward the central star and extensions up to twelve stellar radii or more. The 268.149 GHz emission line of water in the & upsilon;(2) = 2 state is time variable, tends to be masing with dominant radiative pumping, and is widely excited in AGBs and RSGs. The widespread but weaker 262.898 GHz water line in the & upsilon;(2) = 1 state also shows signs of maser emission. The OH emission is weak and quasithermally excited. Emission and absorption features of H2O and OH reveal an infall of matter and complex kinematics influenced by binarity. From the OH and H2O column densities derived with nonmasing transitions in a few sources, we obtain OH/H2O abundance ratios of similar to(0.7-2.8) x 10(-2).
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| 3. |
- Danilovich, Taissa, 1987, et al.
(författare)
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Chemical tracers of a highly eccentric AGB–main-sequence star binary
- 2024
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Ingår i: Nature Astronomy. - 2397-3366.
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Tidskriftsartikel (refereegranskat)abstract
- Binary interactions have been proposed to explain a variety of circumstellar structures seen around evolved stars, including asymptotic giant branch (AGB) stars and planetary nebulae. Studies resolving the circumstellar envelopes of AGB stars have revealed spirals, disks and bipolar outflows, with shaping attributed to interactions with a companion. Here we use a combined chemical and dynamical analysis to reveal a highly eccentric and long-period orbit for W Aquilae, a binary system containing an AGB star and a main-sequence companion. Our results are based on anisotropic SiN emission, the detections of irregular NS and SiC emission towards the S-type star, and density structures observed in the CO emission. These features are all interpreted as having formed during periastron interactions. Our astrochemistry-based method can yield stringent constraints on the orbital parameters of long-period binaries containing AGB stars, and will be applicable to other systems.
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| 4. |
- Douglas, K. M., et al.
(författare)
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Experimental study of the removal of excited state phosphorus atoms by H2O and H-2: implications for the formation of PO in stellar winds
- 2022
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 515:1, s. 99-109
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Tidskriftsartikel (refereegranskat)abstract
- The reactions of the low-lying metastable states of atomic phosphorus, P(D-2) and P(P-2), with H2O and H-2 were studied by the pulsed laser photolysis at 248 nm of PCl3, combined with laser-induced fluorescence detection of P(D-2), P(P-2), and PO. Rate coefficients between 291 and 740 K were measured, along with a yield for the production of PO from P(D-2 or P-2) + H2O of (35 +/- 15) %. H-2 reacts with both excited P states relatively efficiently; physical (i.e. collisional) quenching, rather than chemical reaction to produced PH + H, is shown to be the more likely pathway. A comprehensive phosphorus chemistry network is then developed using a combination of electronic structure theory calculations and a Master Equation treatment of reactions taking place over complex potential energy surfaces. The resulting model shows that at the high temperatures within two stellar radii of a MIRA variable AGB star in oxygen-rich conditions, collisional excitation of ground-state P(S-4) to P(D-2), followed by reaction with H2O, is a significant pathway for producing PO (in addition to the reaction between P(S-4) and OH). The model also demonstrates that the PN fractional abundance in a steady (non-pulsating) outflow is underpredicted by about 2 orders of magnitude. However, under shocked conditions where sufficient thermal dissociation of N-2 occurs at temperatures above 4000 K, the resulting N atoms convert a substantial fraction of PO into PN.
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| 5. |
- Gobrecht, David, et al.
(författare)
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Bottom-up dust nucleation theory in oxygen-rich evolved stars: II. Magnesium and calcium aluminate clusters
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 680
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Tidskriftsartikel (refereegranskat)abstract
- Context. Spinel (MgAl2O4) and krotite (CaAl2O4) are alternative candidates to alumina (Al2O3) as primary dust condensates in the atmospheres of oxygen-rich evolved stars. Moreover, spinel was proposed as a potential carrier of the circumstellar 13 μm feature. However, the formation of nucleating spinel clusters is challenging; in particular, the inclusion of Mg constitutes a kinetic bottleneck. Aims. We aim to understand the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments using a quantum-chemical bottom-up approach. Methods. Starting with an elemental gas-phase composition, we constructed a detailed chemical-kinetic network that describes the formation and destruction of magnesium-, calcium-, and aluminium-bearing molecules as well as the smallest dust-forming (MgAl2O4)1 and (CaAl2O4)1 monomer clusters. Different formation scenarios with exothermic pathways were explored, including the alumina (Al2O3) cluster chemistry studied in Paper I of this series. The resulting extensive network was applied to two model stars, a semi-regular variable and a Mira-type star, and to different circumstellar gas trajectories, including a non-pulsating outflow and a pulsating model. We employed global optimisation techniques to find the most favourable (MgAl2O4)n, (CaAl2O4)n, and mixed (MgxCa(1-x)Al2O4)n isomers, with n = 1- 7 and x∈[0..1], and we used high level quantum-chemical methods to determine their potential energies. The growth of larger clusters with n = 2- 7 is described by the temperature-dependent Gibbs free energies. Results. In the considered stellar outflow models, spinel clusters do not form in significant amounts. However, we find that in the Mira-type non-pulsating model CaAl2O3(OH)2, a hydroxylated form of the calcium aluminate krotite monomer forms at abundances as large as 2 × 10-8 at 3 stellar radii, corresponding to a dust-to-gas mass ratio of 1.5 × 10-6. Moreover, we present global minimum (GM) candidates for (MgAl2O4)n and (CaAl2O4)n, where n = 1- 7. For cluster sizes n = 3- 7, we find new, hitherto unreported GM candidates. All spinel GM candidates found are energetically more favourable than their corresponding magnesium-rich silicate clusters with an olivine stoichiometry, namely (Mg2SiO4)n. Moreover, calcium aluminate clusters, (CaAl2O4)n, are more favourable than their Mg-rich counterparts; the latter show a gradual enhancement in stability when Mg atoms are substituted step by step with Ca. Conclusions. Alumina clusters with a dust-to-gas mass ratio of the order of 10-4 remain the favoured seed particle candidate in our physico-chemical models. However, CaAl2O4 could contribute to stellar dust formation and the mass-loss process. In contrast, the formation of MgAl2O4 is negligible due to the low reactivity of the Mg atom.
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| 6. |
- Gobrecht, David, et al.
(författare)
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The Corundum Conundrum
- 2023
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Ingår i: Astrophysics and Space Science Proceedings. - 1570-6591 .- 1570-6605. - 9783031290022
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Konferensbidrag (refereegranskat)abstract
- Corundum corresponds to solid, crystalline aluminium oxide (i.e. alumina) with a stoichiometric formula of Al 2 O 3. Alumina is likely to be the primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina dust grains can trigger mass loss released in stellar winds. The conundrum, or puzzling problem, consists in the formation of alumina. In particular, the nucleation of alumina, including the clustering of gas-phase molecules and the growth of nanometer-sized clusters, is challenging. We aim to address what Al:O stoichiometries are involved and what cluster size characterises the amorphous-to-crystalline transition. To tackle these questions, we employ a quantum-chemical bottom-up approach.
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| 7. |
- Kiefer, S., et al.
(författare)
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The effect of thermal non-equilibrium on kinetic nucleation
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 671
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Tidskriftsartikel (refereegranskat)abstract
- Context. Nucleation is considered to be the first step in dust and cloud formation in the atmospheres of asymptotic giant branch (AGB) stars, exoplanets, and brown dwarfs. In these environments dust and cloud particles grow to macroscopic sizes when gas phase species condense onto cloud condensation nuclei (CCNs). Understanding the formation processes of CCNs and dust in AGB stars is important because the species that formed in their outflows enrich the interstellar medium. Although widely used, the validity of chemical and thermal equilibrium conditions is debatable in some of these highly dynamical astrophysical environments.Aims. We aim to derive a kinetic nucleation model that includes the effects of thermal non-equilibrium by adopting different temperatures for nucleating species, and to quantify the impact of thermal non-equilibrium on kinetic nucleation.Methods. Forward and backward rate coefficients are derived as part of a collisional kinetic nucleation theory ansatz. The endother-mic backward rates are derived from the law of mass action in thermal non-equilibrium. We consider elastic collisions as thermal equilibrium drivers.Results. For homogeneous TiO2 nucleation and a gas temperature of 1250 K, we find that differences in the kinetic cluster temperatures as small as 20 K increase the formation of larger TiO2 clusters by over an order of magnitude. Conversely, an increase in cluster temperature of around 20 K at gas temperatures of 1000 K can reduce the formation of a larger TiO2 cluster by over an order of magnitude.Conclusions. Our results confirm and quantify the prediction of previous thermal non-equilibrium studies. Small thermal non-equilibria can cause a significant change in the synthesis of larger clusters. Therefore, it is important to use kinetic nucleation models that include thermal non-equilibrium to describe the formation of clusters in environments where even small thermal non-equilibria can be present.
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| 8. |
- Montargès, M., et al.
(författare)
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The VLT/SPHERE view of the A TOMIUM cool evolved star sample: I. Overview: Sample characterization through polarization analysis
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 671
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Tidskriftsartikel (refereegranskat)abstract
- Context. Low- and intermediate-mass asymptotic giant stars and massive red supergiant stars are important contributors to the chemical enrichment of the Universe. They are among the most efficient dust factories of the Galaxy, harboring chemically rich circumstellar environments. Yet, the processes that lead to dust formation or the large-scale shaping of the mass loss still escape attempts at modeling. Aims. Through the ATOMIUM project, we aim to present a consistent view of a sample of 17 nearby cool evolved stars. Our goals are to unveil the dust-nucleation sites and morphologies of the circumstellar envelope of such stars and to probe ambient environments with various conditions. This will further enhance our understanding of the roles of stellar convection and pulsations, and that of companions in shaping the dusty circumstellar medium. Methods. Here we present and analyze VLT/SPHERE-ZIMPOL polarimetric maps obtained in the visible (645- 820 nm) 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 of the central source, and in or near the plane of the sky. The polarized signal is mainly produced by structures with a total optical depth close to unity in the line of sight, and it represents 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 olivine, melilite, corundum, enstatite, or forsterite. The spatial structure of the DoLP shows a diverse set of shapes, including clumps, arcs, and full envelopes. Only for three sources do we note a correlation between the ALMA CO ν = 0, J = 2-1 and SiO ν = 0, J = 5-4 lines, which trace the gas density, and the DoLP, which traces the dust. Conclusions. The clumpiness of the DoLP and the lack of a consistent correlation between the gas and the dust location show that, in the inner environment, 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 circumstellar environment. Except for π1 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.
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| 9. |
- Sindel, J. P., et al.
(författare)
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Infrared spectra of TiO2 clusters for hot Jupiter atmospheres
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 680
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Tidskriftsartikel (refereegranskat)abstract
- Context. Clouds appear to be an unavoidable phenomenon in cool and dense environments. Hence, their inclusion is a necessary part of explaining observations of exoplanet atmospheres, most recently those of WASP 96b with the James Webb Space Telescope (JWST). Understanding the formation of cloud condensation nuclei in non-terrestrial environments is therefore crucial in developing accurate models to interpret current and future observations. Aims. The goal of the paper is to support observations with infrared spectra for (TiO2)N clusters to study cloud formation in exoplanet atmospheres. Methods. We derived vibrational frequencies from quantum-chemical calculations for 123 (TiO2)-clusters and their isomers and we evaluated their line-broadening mechanisms. Cluster spectra were 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. The rotational motion of clusters and the rotational transitions within them 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. The morning and evening terminator for the same planet can exhibit different total absorbances, due to the greater abundance of different cluster sizes. 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 do contribute to the local absorbance. Planets with a considerable day-night asymmetry will be most suitable in the search for (TiO2) cluster isomers with the goal of improving cloud formation modelling.
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| 10. |
- Sindel, J. P., et al.
(författare)
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Revisiting fundamental properties of TiO2nanoclusters as condensation seeds in astrophysical environments
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 668
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Tidskriftsartikel (refereegranskat)abstract
- Context. The formation of inorganic cloud particles takes place in several atmospheric environments, including those of warm, hot, rocky, and gaseous exoplanets, brown dwarfs, and asymptotic giant branch stars. The cloud particle formation needs to be triggered by the in situ formation of condensation seeds since it cannot be reasonably assumed that such condensation seeds preexist in these chemically complex gas-phase environments. Aims. We aim to develop a method for calculating the thermochemical properties of clusters as key inputs for modelling the formation of condensation nuclei in gases of changing chemical composition. TiO2 is used as benchmark species for cluster sizes N = 1- 15. Methods. We created a total of 90000 candidate (TiO2)N geometries for cluster sizes N = 3- 15. We employed a hierarchical optimisation approach, consisting of a force-field description, density-functional based tight-binding, and all-electron density-functional theory (DFT) to obtain accurate zero-point energies and thermochemical properties for the clusters. Results. In 129 combinations of functionals and basis sets, we find that B3LYP/cc-pVTZ, including Grimme' s empirical dispersion, performs most accurately with respect to experimentally derived thermochemical properties of the TiO2 molecule. We present a hitherto unreported global minimum candidate for size N = 13. The DFT-derived thermochemical cluster data are used to evaluate the nucleation rates for a given temperature-pressure profile of a model hot-Jupiter atmosphere. We find that with the updated and refined cluster data, nucleation becomes unfeasible at slightly lower temperatures, raising the lower boundary for seed formation in the atmosphere. Conclusions. The approach presented in this paper allows finding stable isomers for small (TiO2)N clusters. The choice of the functional and basis set for the all-electron DFT calculations has a measurable impact on the resulting surface tension and nucleation rate, and the updated thermochemical data are recommended for future considerations.
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