J/A+A/658/A167 (Al2O3)n, n=1-10, clusters data (Gobrecht+, 2022)
Bottom-up dust nucleation theory in oxygen-rich evolved stars
I. Aluminium oxide clusters.
Gobrecht D., Plane J.M.C., Bromley S.T., Decin L., Cristallo S., Sekaran S.
<Astron. Astrophys. 658, A167 (2022)>
=2022A&A...658A.167G 2022A&A...658A.167G (SIMBAD/NED BibCode)
ADC_Keywords: Stars, late-type ; Models, atmosphere ; Abundances ; Mass loss
Keywords: astrochemistry - molecular processes - stars: AGB and post-AGB -
molecular data - stars: atmospheres - dust, extinction
Abstract:
Aluminium oxide (alumina; Al2O3) is a promising candidate as a
primary dust condensate in the atmospheres of oxygen-rich evolved
stars. Therefore, alumina 'seed' particles might trigger the onset of
stellar dust formation and of stellar mass loss in the wind. However,
the formation of alumina dust grains is not well understood. Aims. We
aim to shed light on the initial steps of cosmic dust formation (i.e.
nucleation) in oxygen-rich environments via a quantum- chemical
bottom-up approach.
Starting with an elemental gas-phase composition, we construct a
detailed chemical-kinetic network that describes the formation and
destruction of aluminium-bearing molecules and dust- forming
(Al2O3)n clusters up to the size of dimers (n=2) coagulating to
tetramers (n=4). Intermediary species include the prevalent gas- phase
molecules AlO and AlOH as well as AlxOy clusters with x=1-5,
y=1-6. The resulting extensive network is applied to two model stars,
which represent a semi-regular variable and a Mira type, and to
different circumstellar gas trajectories, including a non-pulsating
outflow and a pulsating model. The growth of larger-sized
(Al2O3)n clusters with n=4-10 is described by the
temperature-dependent Gibbs free energies of the most favourable
structures (i.e. the global minima clusters) as derived from global
optimisation techniques and calculated via density functional theory.
We provide energies, bond characteristics, electrostatic properties,
and vibrational spectra of the clusters as a function of size, n, and
compare these to corundum, which corresponds to the crystalline bulk
limit (n to infinity).
The circumstellar aluminium gas-phase chemistry in oxygen- rich giants
is primarily controlled by AlOH and AlO, which are tightly coupled by
the reactions AlO+H2, AlO+H2O, and their reverse. Models of
semi-regular variables show comparatively higher AlO abundances, as
well as a later onset and a lower efficiency of alumina cluster
formation when compared to Mira-like models. The Mira-like models
exhibit an efficient cluster production that accounts for more than
90% of the available aluminium content, which is in agreement with the
most recent ALMA observations. Chemical equilibrium calculations fail
to predict both the alumina cluster formation and the abundance trends
of AlO and AlOH in the asymptotic giant branch dust formation zone.
Furthermore, we report the discovery of hitherto unreported global
minimum candidates and low-energy isomers for cluster sizes n=7, 9,
and 10. A homogeneous nucleation scenario, where Al2O3 monomers are
successively added, is energetically viable. However, the formation of
the Al2O3 monomer itself represents an energetic bottleneck.
Therefore, we provide a bottom-up interpolation of the cluster
characteristics towards the bulk limit by excluding the monomer,
approximately following an n(-1/3) dependence.
Description:
Coordinates of the most favorable, B3LYP/6-311+G(d) optimised
(Al2O3)n, n=1-10, cluster structures and thermo-chemical tables
of the global minima cluster candidates. The cluster structure
designations are annotated in the main paper. The following clusters
are included: 1A, 1B, 2A, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 7C,
7D, 7E, 7F, 7G, 7H, 7I, 7J, 8A. 8B, 9A, 9B, 9C, 9D, 9E, 9F, 9G, 10A,
10B, 10C, 10D, 10E 10F, and 10G.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
coord.dat 53 1295 (Al2O3)n, n=1-10, cluster coordinates (table B2)
thermo.dat 105 620 Thermo-chemical tables of the global minima
(Al2O3)n, n=1-10, clusters (table C1)
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Byte-by-byte Description of file: coord.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Cluster Cluster id (G1)
6- 7 A2 --- El [Al/O ] Chemical element
16- 23 F8.5 0.1nm X x coordinate
31- 38 F8.5 0.1nm Y y coordinate
46- 53 F8.5 0.1nm Z z coordinate
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Byte-by-byte Description of file: thermo.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Cluster Cluster id (G1)
6- 12 F7.2 K T Temperature
18- 25 F8.3 J/mol/K S Entropy, in J/mol.K
34- 41 F8.3 J/mol/K cp Molar heat capacity, in J/mol.K
50- 57 F8.3 kJ/mol ddH Change of enthalpy w.r.t. to 0 K
64- 73 F10.3 kJ/mol dHf Enthalpy of formation
80- 89 F10.3 kJ/mol dGf Gibbs free energy of formation
97-104 F8.3 [-] logKf ? Logarithm of equilibrium constant
105 A1 --- n_logKf [I] I for infinity
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Global notes:
Note (G1): Clusters are 1A, 1B, 2A, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B,
7A, 7B, 7C, 7D, 7E, 7F, 7G, 7H, 7I, 7J, 8A. 8B, 9A, 9B, 9C, 9D, 9E, 9F, 9G,
10A, 10B, 10C, 10D, 10E 10F, and 10G.
1A = (Al2O3)1, ..., 10A = (Al2O3)10
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Acknowledgements:
David Gobrecht, dave(at)gobrecht.ch
(End) Patricia Vannier [CDS] 03-Feb-2022