All the heavy elements and much of the helium present in the Universe today were produced by stellar nucleosynthesis. These elements are deposited in the interstellar medium (ISM) by stellar winds and supernovae. The prime birthplaces for the interstellar gas and dust species are the stellar winds of evolved (super)giant stars. Crucial to the understanding of the chemical life cycle of the ISM is hence a profound insight in the chemical and physical structure governing the stellar winds of these (super)giant stars.
As of today, more than 80 molecules and 15 different dust species are detected. However, a comprehensive understanding of the chemical yields and dominant chemical routes in stellar winds is still lacking. The combination of new high spatial and high spectral resolution observations, novel theoretical wind models, dedicated laboratory experiments and quantum-chemical calculations has the power to make significant headway in this field.
In this presentation, I will summarize the current state-of-the-art of circumstellar gas-phase and solid-state species, including their potential formation mechanism. With the formation of inorganic dust grains in the winds of evolved stars being the least understood, I will elaborate on one example.
Specifically, aluminium oxides and iron-free silicates are often put forward as promising candidates for the first little dust seeds in oxygen-rich AGB stars. I will illustrate that ALMA high-spatial resolution observations can constrain the amount of aluminium locked up by simple gas-phase molecules. I will show that both for a high and for a low mass-loss rate O-rich AGB star, only ~2% of aluminium is locked up in AlO, AlOH, and AlCl and that each of these species is detected well beyond the main dust condensation region. This proves that the aluminium dust condensation is not 100% efficient.
I will discuss how the current proposed scenario of aluminium dust condensation for low mass-loss rate AGB stars poses a challenge if one wishes to explain both the dust spectral features in the spectral energy distribution (SED), in interferometric data, and in polarized light signal. We postulate that large gas-phase (Al2O3)n-clusters (n>34) can be the potential agents of the broad 11 micron feature in the SED and interferometric data and we explain how these large clusters can be formed.
Finally, using density functional theory, we calculate the stability and emissivity of small (Al2O3)n clusters (n£4). The spectral resolution of current infrared observations is insufficient to prove the presence of these small clusters. But ELT/METIS will be able to unequivocally answer the question on the presence and vibrational excitation of these small aluminium oxide clusters. Hence, the ELT will be of paramount importance to pinpoint the chemical building blocks and to unravel the chemical pathways leading to extraterrestrial solid-state material.