The coolest massive stars: winds, companions, and the aftermath

Red supergiants (RSGs), the coolest massive stars, remain poorly understood. While many of them experience quiescent mass loss, others exhibit more extreme behaviour with mass-loss rates which are several orders of magnitude larger. This begs the question of how much of the hydrogen envelope can be stripped before the star undergoes core collapse.  The appearance of core-collapse supernovae (SNe) is increasingly being linked to highly dense, asymmetric circumstellar matter ejected shortly before core collapse, with SN2023ixf as a recent, nearby, and most captivating example. The discovery of the “Green Monster” in the Cas A supernova remnant (SNR) has also been attributed to localised, clumpy structures in the progenitor RSG’s circumstellar environment. And recently, a possibly carbon-rich RSG was discovered as the progenitor to SN2025pht. Meanwhile, the recently recognised class of Red Stragglers, RSGs which appear unusually massive or young for their environments, underscores the role of binary interaction and merger processes in shaping the final stages of massive stars. Yet, the connection between these various phenomena remains elusive.

RSGs are the largest known stars. Technological advances with, for example, VLTI and ALMA, make it possible to resolve the extended atmospheres and close-in environments of nearby RSGs and even the circumstellar environment of WOH G64, the largest known star in the Magellanic Clouds (MCs). Future facilities like the ngVLA and further upgrades to ALMA and VLTI will deliver higher sensitivity and higher angular resolution, enabling us to characterise the morphology, clumpiness, and temporal variability of RSG mass loss across the Galaxy and the MCs. Ultimately, these major advances will constrain the dominant wind-launching mechanisms, whether they be pulsation, convection, magnetic fields, dust-driven outflows, or a combination of these, across a much larger population of RSGs than ever before possible. All of these aspects of mass loss are essential to understand the role of RSGs in the cosmic matter cycle. 

The amount of mass loss determines the level of envelope stripping before the stars go supernova, and hence the appearance of the SN and the stellar yields. While first-principle predictions are still exceedingly difficult to make, empirical constraints on RSG mass loss point towards a striking diversity across the RSG population, from chromospherically active objects with moderate, quiescent mass loss, to those producing enormous amounts of dust in highly non-spherical outflows. The smoothness of RSG outflows sets the initial conditions for SN shock expansion and the morphology of SNRs. And constraining the wind-launching mechanisms, ultimately, lets us hone in on predicting the mass-loss properties of RSGs from first principles. 

Despite the prevalence of binarity among massive stars, only very few RSG binaries have been identified. The effect of companions on the structure and evolution of RSGs, including the shaping of circumstellar environments and possible common-envelope phases, remains largely unexplored. The growing interest in binary RSG systems and the emergence of Red Stragglers as a likely product of mass transfer or merger events highlight the need to integrate binarity into our understanding of RSG evolution and pre-supernova mass loss.

This splinter session aims to bring together observers and theoreticians working on RSGs, their environments, and their progeny, spanning scales in both space and time. 

On spatial scales, this ranges from the atmospheres and wind-launching zones (smallest), over the outflows as a whole (intermediate), to RSGs as actors in clusters and galaxies (largest). On time scales, this ranges from atmospheric motions (smallest), over mass-loss history (intermediate), to galaxy evolution (largest). By fostering dialogue across these fronts, the session aims to unify observational and theoretical efforts toward a coherent picture of RSG evolution and their final fates.

This splinter session will address multiple hot topics. 

The intricacies and diversity of mass loss across the class of RSGs form a long-standing problem, which has recently seen both great progress and surprises. The recent Great Dimming of Betelgeuse highlighted that much is still to be learnt about RSG mass loss, including dust formation, mass-loss variability, and intrinsic stellar variability. Even more striking, real-time stellar changes, from an RSG to a Yellow Hypergiant, have been seen for WOH G64, one of the largest known stars in the LMC. Moreover, it is now possible to even image this star’s close-in environment.

Recent studies of Betelgeuse, one of the very closest RSGs, have tentatively revealed a long-sought-after binary companion. Observed circumstellar characteristics often require the likely presence of binary companions, like the dramatic mass-loss episode that created an unexpectedly large circumstellar environment and disk-like structure surrounding DFK 52, a Galactic cluster RSG.  Population-wide statistics on RSGs in binary systems can now be measured, mostly in extragalactic environments, enabling quantitative studies of their multiplicity properties. However, the effects of binary interaction, including clear signatures of mass transfer or mergers, remain elusive. Accounting for the contributions of mass transfer and mergers is a hot topic in understanding the physical properties of RSGs and is strongly linked to the recent advances in stellar models.

The recent SN2023ixf is the closest Type IIL SN and shows strong signatures of dense asymmetric circumstellar material, linked to asymmetric, extreme, and possibly eruptive pre-SN mass loss and maybe binarity in the progenitor system. It is currently unclear whether this SN is anecdotal or a representative object to help constrain the latest phases of RSG evolution. The first JWST detection of a possibly carbon-rich RSG as an SN progenitor in the case of SN2025pht, raises questions about circumstellar chemistry, but also internal nuclear processing and ultimately elemental and dust yields from RSGs.

The splinter session will bring together communities connected to 4 out of the 5 main topics of Cool Stars 23: 1. Fundamental properties and processes of Cool Star Interiors, 2. Advances in Models and Observations of Atmospheres in Cool Objects, 3. Magnetic Activity and Large-scale Events, and 5. Cool Stars in the Context of Cosmic Evolution. 

The splinter session will actively connect across these 4 main topics to enable a holistic view of the evolution of the largest, and maybe most extreme, of all cool stars.