J/A+A/680/A18 (MgAl2O4)n and (CaAl2O4)n, n=1-7 clusters data (Gobrecht+, 2023)
Bottom-up dust nucleation theory in oxygen-rich evolved stars.
II. Magnesium and calcium aluminate clusters.
Gobrecht D., Hashemi S.R., Plane J.M.C., Bromley S.T., Nyman G., Decin L.
<Astron. Astrophys. 680, A18 (2023)>
=2023A&A...680A..18G 2023A&A...680A..18G (SIMBAD/NED BibCode)
ADC_Keywords: Stars, late-type ; Models, atmosphere ; Abundances ; Mass loss
Keywords: astrochemistry - molecular data - molecular processes -
stars: AGB and post-AGB - stars: abundances - stars: atmospheres
Abstract:
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 13um feature.
However, the formation of nucleating spinel clusters is challenging;
in particular, the inclusion of Mg constitutes a kinetic bottleneck.
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.
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 (Gobrecht et al.,
2022A&A...658A.167G 2022A&A...658A.167G, Cat. J/A+A/658/A167) 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 in [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.
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
2x10-8 at 3 stellar radii, corresponding to a dust-to-gas mass ratio
of 1.5x10-6. 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.
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.
Description:
Coordinates of the most favorable, B3LYP/cc-pVTZ optimised
(MgAl2O4)n, and (CaAl2O4)n, n=1-7, 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, 2B, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
coord.dat 47 777 (MgAl2O4)n and (CaAl2O4)n with n=1-7
cluster coordinates (table A2)
thermo.dat 83 1674 Thermo-chemical tables of the global minimum
(MgAl2O4)n and (CaAl2O4) with n=1-7 clusters
(table A3)
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See also:
J/A+A/658/A167 : (Al2O3)n, n=1-10, clusters data (Gobrecht+, 2022)
Byte-by-byte Description of file: coord.dat
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Bytes Format Units Label Explanations
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1- 2 A2 --- Cluster Cluster id
4- 15 A12 --- Mol Molecule name
18- 19 A2 --- El [Al/Mg/Ca/O ] Chemical element
21- 28 F8.5 0.1nm X x coordinate
30- 37 F8.5 0.1nm Y y coordinate
39- 47 F9.5 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- 2 A2 --- Cluster Cluster id
4- 15 A12 --- Mol Molecule name
18- 24 F7.2 K T Temperature
26- 33 F8.3 J/mol/K S Entropy, in J/mol.K
35- 42 F8.3 J/mol/K Cp Molar heat capacity, in J/mol.K
44- 51 F8.3 kJ/mol ddH Change of enthalpy w.r.t. to 0 K
53- 62 F10.3 kJ/mol dHf Enthalpy of formation
64- 72 F9.3 kJ/mol dGf Gibbs free energy of formation
74- 82 F9.3 [-] logKf ? Logarithm of equilibrium constant
83 A1 --- n_logKf [I] I for infinity
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Acknowledgements:
David Gobrecht, dave(at)gobrecht.ch
References:
Gobrecht et al., Paper I 2022A&A...658A.167G 2022A&A...658A.167G, Cat. J/A+A/658/A167
(End) D. Gobrecht [Gothenburg Univ., Sweden], P. Vannier [CDS] 06-Nov-2023