Condensation sequence of circumstellar cluster seeds (CSCCS)

David Gobrecht

doi:10.3389/fspas.2025.1632593

paper

Condensation sequence of circumstellar cluster seeds (CSCCS)

arXiv:2510.13657 · doi:10.3389/fspas.2025.1632593

Abstract

Traditionally, the condensation sequence of circumstellar dust is predicted based on the thermodynamic stabilities of specific condensates in the macroscopic bulk phase. However, at the (sub-)nanometer scale clusters with non-crystalline structures and significantly different properties are energetically favoured. For this reason, we study the thermodynamic stabilities of metal oxide clusters with generic stoichiometries of M $_{2}$ O $_{3}$ and M $_{3}$ O $_{4}$ , where M represents a metal atom. With an upper size limit of 50 atoms, we consider clusters with sizes n=1 $-$ 10 for (M $_{2}$ O $_{3}$ ) $_{n}$ , and n=1 $-$ 7 for (M $_{3}$ O $_{4}$ ) $_{n}$ . The M $_{2}$ O $_{3}$ clusters comprise alumina (Al $_{2}$ O $_{3}$ ), Mg-rich pyroxene (MgSiO $_{3}$ ) and a size-limited sample of titanates (CaTiO $_{3}$ ), whereas the M $_{3}$ O $_{4}$ clusters include spinel (MgAl $_{2}$ O $_{4}$ ), Mg-rich olivine (Mg $_{2}$ SiO $_{4}$ ) and calcium aluminates (CaAl $_{2}$ O $_{4}$ ). We find that, apart from the alumina monomer, the aluminum-bearing clusters (Al $_{2}$ O $_{3}$ ) $_{n}$ , n=1 $-$ 10, and (MgAl $_{2}$ O $_{4}$ ) $_{n}$ , n=1 $-$ 7, are favoured over their silicate counterparts (MgSiO $_{3}$ ) $_{n}$ , n=1 $-$ 10 and (Mg $_{2}$ SiO $_{4}$ ) $_{n}$ , n=1 $-$ 7. Also, we find that calcium aluminate clusters, CaAl $_{2}$ O $_{4}$ , are energetically more favourable than magnesium aluminate clusters, MgAl $_{2}$ O $_{4}$ . Furthermore, for a limited data set of (CaTiO $_{3}$ ) $_{n}$ , n=1 $-$ 2, clusters we find significantly larger stabilities than for the other considered (M $_{2}$ O $_{3}$ ) $_{n}$ clusters, namely Al $_{2}$ O $_{3}$ and MgSiO $_{3}$ . Future investigations, in particular on titanates and on Ca-rich silicates, are required to draw a more thorough and complete picture of the condensation sequence at the (sub-)nanoscale.

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