Bottom-up dust nucleation theory in oxygen-rich evolved stars II. Magnesium and calcium aluminate clusters

David Gobrecht; S. Rasoul Hashemi; John M. C. Plane; Stefan T. Bromley; Gunnar Nyman; Leen Decin

doi:10.1051/0004-6361/202347546

paper

Bottom-up dust nucleation theory in oxygen-rich evolved stars II. Magnesium and calcium aluminate clusters

arXiv:2310.08521 · doi:10.1051/0004-6361/202347546

Abstract

Spinel (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 13 $μ$ m 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 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 ...

20 pages, 25 figures, accepted for publication in A&A

Bottom-up dust nucleation theory in oxygen-rich evolved stars II. Magnesium and calcium aluminate clusters

Abstract

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