Molecular dust precursors in envelopes of oxygen-rich AGB stars and red supergiants

TL;DR

This paper reviews recent high-resolution observations of molecular clusters and metal oxides in oxygen-rich AGB stars and red supergiants, shedding light on the dust formation process close to stellar surfaces.

Contribution

It provides a comprehensive overview of observational evidence for molecular precursors to dust in evolved stars, highlighting recent ALMA studies and discussing interpretative challenges.

Findings

Identification of gas species involved in dust formation.

Observation of depletion of gas-phase molecules into dust.

Insights into non-equilibrium excitation effects.

Abstract

Condensation of circumstellar dust begins with formation of molecular clusters close to the stellar photosphere. These clusters are predicted to act as condensation cores at lower temperatures and allow efficient dust formation farther away from the star. Recent observations of metal oxides, such as AlO, AlOH, TiO, and TiO$_2$, whose emission can be traced at high angular resolutions with ALMA, have allowed first observational studies of the condensation process in oxygen-rich stars. We are now in the era when depletion of gas-phase species into dust can be observed directly. I review the most recent observations that allow us to identify gas species involved in the formation of inorganic dust of AGB stars and red supergiants. I also discuss challenges we face in interpreting the observations, especially those related to non-equilibrium gas excitation and the high complexity of stellar atmospheres in the dust-formation zone.