J/A+A/680/A18ivo://CDS.VizieR/J/A+A/680/A18doi:10.26093/cds/vizier/bc-n8/lbiCDSGobrecht D.Hashemi S.R.Plane J.M.C.Bromley S.T.Nyman G.Decin L.2026-07-01T14:01:25Z2023-12-05T10:02:55ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frlate-type-starsastronomical-modelsstellar-atmosphereschemical-abundancesstellar-mass-lossSpinel (MgAl_2_O_4_) and krotite (CaAl_2_O_4_) are alternative candidates to alumina (Al_2_O_3_) 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 (MgAl_2_O_4_)_1_ and (CaAl_2_O_4_)_1_ monomer clusters. Different formation scenarios with exothermic pathways were explored, including the alumina (Al_2_O_3_) cluster chemistry studied in Paper I (Gobrecht et al., 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 (MgAl_2_O_4_)_n_, (CaAl_2_O_4_)_n_, and mixed (Mg_x_Ca_(1-x)_Al_2_O_4_)_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 CaAl_2_O_3_(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 (MgAl_2_O_4_)_n_ and (CaAl_2_O_4_)_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 (Mg_2_SiO_4_)_n_. Moreover, calcium aluminate clusters, (CaAl_2_O_4_)_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, CaAl_2_O_4_ could contribute to stellar dust formation and the mass-loss process. In contrast, the formation of MgAl_2_O_4_ is negligible due to the low reactivity of the Mg atom.2023A&A...680A..18Ghttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/680/A18CatalogResearchIsServedByTAP VizieR generic servicerelated-toJ/A+A/658/A167 : (Al2O3)n, n=1-10, clusters data (Gobrecht+, 2022)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/680/A18https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/680/A18http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/680/A18https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/680/A18https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/680/A18http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/680/A18GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapdefaultJ/A+A/680/A18/coord(MgAl2O4)n and (CaAl2O4)n with n=1-7 cluster coordinates (table A2)recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintClusterCluster idmeta.idcharMolMolecule namemeta.id;phys.molcharEl[Al/Mg/Ca/O ] Chemical elementphys.atmol.elementcharXx coordinate0.1nmphys.atmoldoubleYy coordinate0.1nmphys.atmoldoubleZz coordinate0.1nmphys.atmoldoubleJ/A+A/680/A18/thermoThermo-chemical tables of the global minimum (MgAl2O4)n and (CaAl2O4) with n=1-7 clusters (table A3)MolMolecule namemeta.id;phys.molcharTTemperatureKphys.temperaturefloatrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintClusterCluster idmeta.idcharSEntropy, in J/mol.KJ.mol**-1.K**-1phys.atmoldoubleCpMolar heat capacity, in J/mol.KJ.mol**-1.K**-1phys.mol.formationHeatdoubleddHChange of enthalpy w.r.t. to 0 KkJ/molphys.atmoldoubledHfEnthalpy of formationkJ/molphys.SFRdoubledGfGibbs free energy of formationkJ/molphys.SFRdoublelogKf? Logarithm of equilibrium constantarith.zpdoublenullablen_logKf[I] I for infinitymeta.notechar