Report on the outcomes of a Short-Term Scientific Mission

Action number: CA21101

Applicant name: David Gobrecht

Details of the STSM

Title: Systematic study of stardust seeds

Start and end date: 09/10/2023 to 30/10/2023

Description of the work carried out during the STSM

We performed global optimisation searches for MxOy clusters, with M=Al,Ca, Mg, Si, Ti, x=1-4 and y=1-6 by using several seed structures for each compound. The seed structures were chosen to correspond to the most common geometries for known binary oxide clusters. For a few of the stoichiometries the optimisation did not converge or lead to transition states and higher order saddle points. In these cases we were able to achieve convergence with slightly distored geometries and open shell spin multiplicities. For the set of studied global minimum MxOy clusters we derived the correponding Gibbs free energies of dissociation that can be readlily used in parametrised form in chemcial equilibrium modelling programmes. Moreover, the resulting equilibrium abundances were combined with the unique vibrational fingerprints of the individual clusters to obatin a model spectra for different temperatures and gas densities. The intensities of the computed harmonic spectra of the MxOy global minima candidates were converted in wavelength-dependent absorption cross sections. These synthetic spectra will be compared to observations of dust-forming astronomical environments including circumstellar environments and exoplanetary atmospheres. The investiagtion of the different M4O5 and M4O6 cluster isomers are not yet fully completed, but are currently running. We aim to finalize these investigations by end of this year.

Description of the STSM main achievements and planned follow-up activities

As potential dust precursors the studied MxOy clusters bridge the molecular gas phase with the solid (or condensed) phase. Therefore, we believe to have contributed to the COSY research objective of WG5 in characterising the thermochemical and spectral properties of molecular systems with astrochemical relevance. In a broader sense our work also contributes to WG3 as we explicitely address and study metal oxide clusters with sizes of a few atoms. Despite the approximative character of our approach that include the harmonic oscillaor approximation and chemical equilibrium we are confident in having established a first step in a systematic understanding of the chemical nature of oxygen-rich stardust seeds. We aim to report our findings that include some apriori unexpected results in a forthcoming publication, once the remaining calculations (M4O5 and M4O6) have terminated and have been analysed. Furthermore, we plan to provide the astrochemical community with thermochemical tables and vibrational frequencies of the studied MxOy clusters.

Our systematic high level electronic structure calculations of MxOy clusters will provide a suitable and extensive training set for machine learning approaches, which is one of the COSY general research objectives. Moreover, we will be happy to supply our resulting cluster data to the COSY-ASTRO and COSY-NANO datasets. We gratefully acknowledge support from CA21101 that enabled us to carry out our study and stengthened our collaboration.

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