Evolved stars at high angular resolution: present and future

TL;DR

This paper reviews recent advances in high angular resolution interferometry of evolved stars, highlighting how imaging and modeling of their complex atmospheres have improved our understanding of stellar processes and future observational prospects.

Contribution

It provides a comprehensive overview of interferometric techniques applied to evolved stars, emphasizing recent results, challenges, and future developments with new instruments.

Findings

Interferometry has evolved from diameter measurements to detailed stellar surface imaging.

Studies have constrained models of molecular stratification, dust formation, and stellar winds.

Upcoming instruments will enhance imaging capabilities and understanding of evolved star atmospheres.

Abstract

The late evolutionary stages of stellar evolution are a key ingredient for our understanding in many fields of astrophysics, including stellar evolution and the enrichment of the interstellar medium (ISM) via stellar yields. Already the first interferometric campaigns identified evolved stars as the primary targets because of their extended and partially optically thin atmospheres, and the brightness in the infrared. Interferometric studies spanning different wavelength ranges, from visual to mid-infrared, have greatly increased our knowledge of the complex atmospheres of these objects where different dynamic processes are at play. In less than two decades this technique went from measuring simple diameters to produce the first images of stellar surfaces. By scanning the extended atmospheres we constrained theoretical models, learnt about molecular stratification, dust formation, and stellar winds, and there is still a lot to be done. In this contribution I will review the recent results that optical/infrared interferometry has made on our current understanding of cool evolved stars. The presentation will focus on asymptotic giant branch stars, and red supergiants. I will discuss the challenges of image reconstruction, and highlight how this field of research will benefit from the synergy of the current interferometric instrument(s) with the second generation VLTI facilities GRAVITY and MATISSE. Finally I will conclude with a short introspection on applications of a visible interferometer and of the the Planet Formation Imager (PFI) to the field of evolved stars.