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

14:00 - 15:30: Red supergiant atmospheres and wind launching mechanisms

14:00 - 14:20 Invited  - Dr. Gemma González-Torà (Heidelberg University, Germany)

Resolving the atmospheres of cool evolved massive stars

Red supergiants (RSGs), the immediate progenitors of core-colapse Type II supernovae, possess extended, dynamic atmospheres shaped by poorly understood mass-loss processes. Spatially resolving these structures is key to uncovering the mechanisms driving their evolution and ultimate fate. In this review, I focus on observational techniques that probe RSG atmospheres at high angular resolution, particularly optical and infrared interferometry. I highlight recent advances in resolving surface inhomogeneities, mass loss events, and wind structures, and discuss how these observations inform models of massive star evolution and supernova progenitors.

14:20 - 14:40 Invited - Leen Decin (Katholieke Universiteit Leuven, Belgium)

The Enigma of Red Supergiant Mass Loss

Red supergiants are among the most important sources of mass return to the interstellar medium, yet the origin of their winds remains poorly understood. Over the years, many mechanisms have been proposed — pulsation, convection, turbulence, radiation pressure on dust, magnetic activity, shocks, or binary interaction — but none appears able, on its own, to explain the full phenomenology of red supergiant mass loss. Recent observations increasingly reveal these stars as highly dynamic systems in which atmosphere and circumstellar environment are intimately connected across many spatial and temporal scales. In this talk, I will review recent progress on red supergiant mass loss from both observational and theoretical perspectives, with emphasis on what spatially resolved observations are teaching us about the physical origin of the wind. I will discuss the idea that mass loss may not be driven by a single mechanism, but instead emerges naturally from the star’s attempt to maintain equilibrium near the limits imposed by gravity, radiation, and atmospheric instability.

14:40 - 14:55 Contributed - Susanne Höfner (Uppsala University, Sweden)

Dust-driven mass loss from RSG stars: global 3D RHD models

The “Great Dimming” of Betelgeuse in 2020 gave unexpected insights, indicating that radiation pressure on dust may play a role for wind formation in RSG stars. Using global 3D RHD models produced with the CO5BOLD code, we investigated causes of dimming events in RSG and AGB stars, and their possible connection with mass loss. We found different mechanisms that can lift dense clouds of material above the top of the convection zone, which then appear as local dark patches when seen against the stellar surface. Apart from periodic dimming due to large-amplitude radial pulsations, two other mechanisms that create obscuring clouds of cool gas in the lower atmosphere were identified: plumes due to Rayleigh-Taylor instabilities, or enhanced convective downdraft and rebound. The latter scenario is much rarer, but it can create a large-scale dark patch that is comparable to the dimming event in Betelgeuse. Our latest 3D RSG star simulations, combining non-equilibrium dust grain growth with an improved treatement of radiative transfer, demonstrate how a localized, sporadic dust-driven outflow can result from such an event. 

14:55 - 15:10 Contributed - Mark Siebert (Chalmers University of Technology, Sweden)

An ALMA view of the circumstellar environments of RSGs in galactic open clusters

The mass loss of red supergiants (RSGs) is critical to their evolution and the observed characteristics of Type II-p supernovae. However, the physical mechanisms driving this mass loss remain poorly understood. Contributing to this is a lack of gas phase constraints on RSG wind properties over a wide range of stellar parameters, as millimeter studies are often limited to a handful of nearby objects. To address this, we present an ALMA study of molecular line emission toward RSGs in the galactic clusters RSGC2 and RSGC1, utilizing data from new and archival programs. We analyze CO (J=2-1) emission toward 11 targets to derive updated constraints on their stellar positions, wind expansion velocities, envelope sizes, and mass loss rates. We find that the spatial extents of the outflows are quite compact, with one major exception being the star DFK 52, which is surrounded by an extremely large detached circumstellar envelope (up to 100,000 au in diameter). In this talk, I will review the mass-loss properties of RSGs in RSGC2 and RSGC1, compare our results with previous constraints from dust modeling, and discuss how unique objects such as DFK 52 challenge our current picture of evolution in these young massive clusters.

15:10 - 15:30: Moderated group discussion 

Possible topics: 

What are the missing ingredients in RSG (mass-loss) models? 

Which observational constraints can or should we aim to provide in the coming decade? 

How does binarity influence our understanding of the mass-loss processes?

15:30 - 16:00: Coffee break & poster viewing

16:00 - 17: 30: Red supergiants as SN progenitors: the effects of mass-loss and multiplicity

16:00 - 16:20 Invited - Takashi Moriya (NAOJ, Japan)

Observational signatures of RSG mass loss in supernovae and their remnants. 

Supernovae (SNe) occur within the circumstellar matter (CSM) formed by the mass loss of their progenitors. Thus, if we can identify the effects of CSM on Type II SN/SNR properties and observe them, we can extract the mass-loss history of RSGs from Type II SNe. In this talk, I summarize the expected effects of CSM on Type II SN properties. Then, I discuss the observed CSM effects in Type II SNe and the estimated RSG mass-loss properties based on them. While the estimated mass-loss rates until decades before RSG explosion roughly match to those observed in RSGs, the estimated mass-loss rates within decades before RSG explosion are much higher than those found in RSGs. I discuss the possible cause of this discrepancy such as mass-loss enhancement in RSGs in the last decades before explosion and other possible ways to form the dense CSM in the vicinity of RSGs.

16:20 - 16:35 Contributed - Daniel Jadlovsky (ESO & Masaryk University, Czech Republic)

Unveiling companions and binary interaction of red supergiants using VLTI-GRAVITY

The majority of massive stars are born in multiple systems, and their evolution is thus shaped by binary interaction. Red supergiants (RSGs) are the critical point of stellar evolution, as they may evolve toward a different spectral type before a supernova (SN) explosion. Apart from strong mass loss due to winds, one of the reasons may also be the binary interaction in the RSG phase, such as in the recent case of WOH G64. Recent surveys identified many new wide binary RSGs, but barely any of those have constrained properties.

We aim to constrain orbital parameters of known RSG systems and study their binary interaction. For our large observing campaign of single and binary RSG systems, we employ the unprecedented sensitivity and precision of VLTI-GRAVITY, allowing us to unveil massive companions with a separation down to a few au. For several interacting systems, where the companions are accreting from the RSG, we can trace the orbital motion of the companions using the Brackett γ line originating in the accretion disk, allowing us to uniquely constrain dynamical masses. 

Namely, we demonstrate the mass transfer mechanism for the KQ Pup RSG system (orbital period 26 yr). We discovered that its B-type main-sequence companion is actually an eclipsing inner binary (TESS, 17.3 d). The system shows signatures of accretion from the RSG to the hot companions near periastron. With the RSG filling its Roche lobe only by ∼70% at periastron, the mass transfer is instead driven by Wind Roche Lobe Overflow (RLOF) from the extended atmosphere of the RSG and may be aided by pulsations. The circumbinary accretion disk around the hot companions is formed already a few years before the periastron and dissipates by apastron. Signatures of occultations by the circumbinary accretion disk in the TESS data show that the hot companions are embedded in a similar environment as young massive stellar objects. Meanwhile, this is the first time that dynamical masses and orbital parallax could be determined for a RSG system.

Overall, this and our other recent results on single and RSGs demonstrate the necessity of including extended atmospheres when modelling SN progenitors and binary evolution, as the hydrostatic radii of red supergiants may be much larger than their photospheric radii. Consequently, this enables binary interaction for wide separations, up to tens and even hundreds of au, while the presence of extended atmosphere may also significantly affect the pre-SN structure of the RSG, providing flash-ionization features at early times (e.g., Type IIn), modifying the SN light curves.

16:35 - 16:50 Contributed - Das Sujit (Indian Institute of Astrophysics (IIA), Bangalore, India)

Countdown to Explosion: The Final Centuries of Dusty Red Supergiants

Red supergiants represent the final evolutionary stage of massive stars, which are known to end their lives as supernovae. However, their evolution in the final centuries remains mysterious and poorly constrained. Many supernovae show signatures of intense mass loss shortly before explosion, yet the underlying mechanism remains elusive. Red supergiants are understood to be embedded in dusty cocoons, and post-explosion supernovae often show evidence of dust formed prior to the explosion in stellar winds. In this study, we focus on: (a) how pulsation-driven mass loss and wind acceleration create phases of high-density circumstellar matter (CSM) surrounding red supergiant stars; (b) the mechanisms, timescales, and composition of dust formed in these stellar winds; and (c) how dusty winds from red supergiants impact the stellar spectrum. We develop a general theoretical framework connecting stellar evolution and dust formation, and apply our model to well-known supernovae such as SN 2023ixf, as well as Type IIn supernovae including SN 2005ip, SN 1998S, and SN 2020ywx.

16:50 - 16:05 Contributed - Meridith Joyce (University of Wyoming)

How to Find a Star by Accident, or How Not to Solve the "Great Dimming" of Betelgeuse

Alpha Orionis, popularly known as Betelgeuse, is a nearby red supergiant star visible to the naked eye. In light of the star's "Great Dimming"—a sudden, extreme drop in brightness that occurred in early 2020—a recent controversy surrounding Betelgeuse concerned whether it would explode as a supernova within the next few years, centuries, or millennia. Using a series of numerical techniques including one-dimensional stellar evolution models, hydrodynamic simulations, linear oscillation calculations, Fourier analysis, and the methods of a subfield of stellar astrophysics known as asteroseismology, my collaborators and I constrained the timeline for Betelgeuse's demise and revised many of the best estimates for its fundamental properties. In doing so, we discovered not only that Betelgeuse was not likely to undergo an imminent detonation, but that a pulsation signal unexplained by our models was, in fact, the signature of an as-yet-undiscovered binary companion. Its presence was confirmed earlier this year. What we never managed to do was explain the Great Dimming.

In this talk, I will use the story of the discovery of Betelgeuse’s hidden, low-mass binary companion, Alpha Orionis B—affectionately nicknamed “Betelbuddy”—both to highlight the computational and numerical techniques employed in modern stellar astrophysics and to illustrate how the most meaningful discoveries often arise not from confirming what we set out to find, but from venturing down the rabbit holes of unexpected problems that emerge along the way.

17:05 - 17:30 Moderated group discussion & closing remarks

Possible topics: 

How well do current RSG models and observations match what is seen in SNe/SNRs?

Which observational constraints can or should we aim to provide in the coming decade? 

How does multiplicity influence our understanding of the final stages of RSG evolution?