Massive stars in their late stages of evolution as Red Supergiants experience mass loss. The resulting winds show various degrees of dynamical and chemical complexity and produce molecules and dust grains. This review summarises our knowledge of the molecular and dust components of the wind of Red Supergiants, including VY CMa and Betelgeuse. We discuss the synthesis of dust as a non equilibrium process in stellar winds, and present the current knowledge of the chemistry involved in the formation of oxygen-rich dust such as silicates and metal oxides.

The complex circumstellar environments associated with several of the most luminous cool supergiants provide evidence for episodic high mass loss events. The origin of the high mass loss is not understood but circumstantial evidence strongly suggests that large scale surface activity and magnetic fields are responsible. I briefly review the observational evidence from multi-epoch imaging and spectroscopy for the high mass loss events and instabilities in these stars. New results from high-resolution AO near and mid-IR imaging of VY CMa, IRC +10420 and μ Cep is also presented.

The formation of inorganic dust in circumstellar environments of evolved stars is poorly understood. Spectra of molecules thought to be most important for the nucleation, i.e. AlO, TiO, and TiO2, have been recently detected in the red supergiant VY CMa. These molecules are effectively formed in VY CMa and the observations suggest that non-equilibrium chemistry must be involved in their formation and nucleation into dust. In addition to exploring the recent observations of VY CMa, we briefly discuss the possibility of detecting these molecules in the “dust-poor” circumstellar environment of Betelgeuse.

The violent convective motions, low surface gravity, and high brightness of red supergiants combine to trigger an intense stellar wind. As the distance from the star increases, the standard scenario is that the ejected material forms molecules, then dust particles. But this general picture is still fragmentary. Our goal is to assemble a better understanding of mass loss in Betelgeuse, considered as a prototype for its class, from its photosphere to the interface of its wind with the interstellar medium. Thanks to its proximity ( ≈ 197 pc), it is ideally suited for such a detailed study. Over the past few years, our team obtained an extensive set of observations of Betelgeuse from high angular resolution instruments, probing a broad range of spatial scales: 1) interferometric imaging of its photosphere and close envelope in the near- and thermal-IR domains (IOTA/IONIC), 2) adaptive optics “lucky imaging” of its compact molecular envelope (VLT/NACO, 1.0–2.2 μm), and 3) diffraction-limited imaging of its dusty envelope (VLT/VISIR, 8–20 μm). From our interferometric data, we detect the presence of spots at the surface of the star, as well as CO and H2O molecules, and dust particles close to the star. Within 6 R⋆, the flux distribution of the envelope is compatible with the presence of the CN molecule. At a few arcseconds from the central star, we observe a complex dusty envelope probably containing O-rich dust (e.g. silicates, alumina). We present an overview of these recent observational results and ongoing work. They provide new hints on the physical and chemical mechanisms through which Betelgeuse interacts with its environment.

Betelgeuse is just starting to produce a wind which is thick enough to form dust. However, the grains seem to coalesce at much greater distances than those in “dust-driven” winds from later-stage AGB and RSG stars. Is the mass loss mechanism different, and how will it evolve? We know a great deal about the kinematics of the more evolved winds, thanks to sub-au imaging using masers, and easily-resolved CO shells, but Betelgeuse is a much fainter target, only resolved with great difficulty (although ALMA will change that). On the other hand, Betelgeuse was the first star other than the Sun to be imaged in detail in the radio as well as optical. Radio studies from the photosphere to the astropause will reveal: How is mass lost from the stellar surface? In what form is this returned to the ISM? These results could even help to answer: How will Betelgeuse evolve in the next few millennia?

The observations of mass losing stellar sources in the H i line at 21 cm allow us to study in detail the kinematics in their large size circumstellar shells. We report on the results that have been obtained on Betelgeuse with the Nançay Radiotelescope (NRT) and with the Very Large Array (VLA). On the stellar position, we find a double-horn line profile characteristic of a freely expanding wind at a velocity of  ~ 14 km s-1. We find also that the stellar outflow is slowed down by the pressure from the ambient medium, and forms a quasi-stationary detached shell of  ~ 4′ in diameter (0.24 pc at a distance of 200 pc). The H i line profile from this detached shell has a width of 3 km s-1, and is centered at a velocity close to the star radial velocity (Vlsr = +3 km s-1). The bulk of the material detected in H i ( ~ 0.05 M⊙) has been heated at a temperature  ~ 6000 K, and is cooling down to  ~ 200 K. Furthermore, due to the motion relative to the local interstellar medium, the detached shell is distorted and elongated in a direction close to the space motion. Finally, we have detected H i emission associated with the 6′ radius far-infrared arc seen by IRAS, and with a counterpart to this arc that we have recently discovered in the far-ultraviolet. This H i emission is found in a velocity range (from +6 to +10 km s-1) which matches that of the interstellar medium observed on the same line-of-sight.

The interaction between stellar winds and the interstellar medium (ISM) can create complex bow shocks. We have studied the bow shock region around Betelgeuse using Herschel PACS images at 70, 100, and 160 μm and SPIRE images at 250, 350, and 500 μm. These data were complemented with ultraviolet GALEX data, near-infrared WISE data, and radio 21 cm GALFA-HI data.

The infrared Herschel images of the environment around Betelgeuse are spectacular, showing the occurrence of multiple arcs at  ~6–7′ from the central target and the presence of a linear bar at  ~9′. Remarkably, no large-scale instabilities are seen in the outer arcs and linear bar. The dust temperature in the outer arcs varies between 40 and 140 K, with the linear bar having the same colour temperature as the arcs. The inner envelope shows clear evidence of a non-homogeneous clumpy structure (beyond 15′′). The non-homogeneous distribution of the material even persists until the collision with the ISM. A strong variation in brightness of the inner clumps at a radius of  ~2′ suggests a drastic change in mean gas and dust density  ~32 000 yr ago. Using hydrodynamical simulations (see van Marle & Decin, these proceedings), we try to explain the observed morphology of the bow shock around Betelgeuse.

Different hypotheses, based on observational and theoretical constraints, are formulated to explain the origin of the multiple arcs and the linear bar and the fact that no large-scale instabilities are visible in the bow shock region. We infer that the two main ingredients for explaining these phenomena are a non-homogeneous mass-loss process and the influence of the Galactic magnetic field. The linear bar is probably an interstellar structure illuminated by Betelgeuse itself.

We present hydrodynamical simulations of the interaction between the stellar wind of α-Orionis and the interstellar gas. In particular we focus our efforts on reproducing the multiple arc structure of the bow-shock that was observed with Herschel. We do this by varying the mass loss rate of the star over a period of 10 000 years, representing the effect of thermal pulses.

We find that the effect of repeated collisions between high density shells and the wind termination shock can create multiple arc-like structures in the inner part of the bow-shock region. This may explain the arcs seen in the observations.

Unlike the observed bow-shock of α-Orionis, our simulations show strong instabilities at the contact discontinuity between the shocked wind and the shocked interstellar medium. This can be an indication of the age of the bow-shock, which may be to young to have formed the instabilities.

Betelgeuse, the bright red supergiant (RSG) in Orion, is a runaway star. Its supersonic motion through the interstellar medium has resulted in the formation of a bow shock, a cometary structure pointing in the direction of motion. We present the first 3D hydrodynamic simulations of the formation and evolution of Betelgeuse’s bow shock. We show that the bow shock morphology depends substantially on the growth timescale for Rayleigh-Taylor versus Kelvin-Helmholtz instabilities. We discuss our models in light of the recent Herschel, GALEX and VLA observations. If the mass in the bow shock shell is low (~few  × 10-3M⊙), as seems to be implied by the AKARI and Herschel observations, then Betelgeuse’s bow shock is very young and is unlikely to have reached a steady state. The circular, smooth bow shock shell is consistent with this conclusion. We further discuss the implications of our results, in particular, the possibility that Betelgeuse may have only recently entered the RSG phase.

The nearby red supergiant (RSG) Betelgeuse has a complex circumstellar medium out to at least 0.5 parsecs from its surface, shaped by its mass-loss history within the past  ≈ 0.1 Myr, its environment, and its motion through the interstellar medium (ISM). In principle its mass-loss history can be constrained by comparing hydrodynamic models with observations. Observations and numerical simulations indicate that Betelgeuse has a very young bow shock, hence the star may have only recently become a RSG. To test this possibility we calculated a stellar evolution model for a single star with properties consistent with Betelgeuse. We incorporated the resulting evolving stellar wind into 2D hydrodynamic simulations to model a runaway blue supergiant (BSG) undergoing the transition to a RSG near the end of its life. The collapsing BSG wind bubble induces a bow shock-shaped inner shell which at least superficially resembles Betelgeuse’s bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG wind’s interaction with the ISM. We investigate whether this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuse’s bow shock.

SPHERE, the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument for the VLT is optimized towards reaching the highest contrast in a limited field of view and at short distances from the central star, thanks to an extreme AO system. SPHERE is very well suited to study the close environment of Betelgeuse, and has a strong potential for detecting the ejection activity around this key red supergiant.

Galaxies in the Local Group span a factor of 15 in metallicity, ranging from the super-solar M 31 to the Wolf-Lundmark-Melotte (WLM) galaxy, which is the lowest-metallicity (0.1 Z⊙) Local Group galaxy currently forming stars. Studies of massive star populations across this broad range of environments have revealed important metal-licity-dependent evolutionary trends, allowing us to test the accuracy of stellar evolutionary tracks at these metallicities for the first time. The RSG population is particularly valuable as a key mass-losing phase of moderately massive stars and a source of core-collapse supernova progenitors. By reviewing recent work on the RSG populations in the Local Group, we are able to quantify limits on these stars’ effective temperatures and masses and probe the relationship between RSG mass loss behaviors and host environments. Extragalactic surveys of RSGs have also revealed several unusual RSGs that display signs of unusual spectral variability and dust production, traits that may potentially also correlate with the stars’ host environments. I will present some of the latest work that has advanced our understanding of RSGs in the Local Group, and consider the many new questions posed by our ever-evolving picture of these stars.

The open cluster Stephenson 2 contains the largest collection of red supergiants known in the Galaxy, and at present is the second most massive young cluster known in the Milky Way. We have obtained multi-epoch, intermediate-resolution spectra around the Ca ii triplet for more than 30 red supergiants in Stephenson 2 and its surroundings. We find a clear separation between a majority of RSGs having spectral types M0-M2 and the brightest members in the NIR, which have very late spectral types and show strong evidence for heavy mass loss. The distribution of spectral types is similar to that of RSGs in other clusters, such as NGC 7419, or associations, like Per OB1. The cluster data strongly support the idea that heavy mass loss and maser emission is preferentially associated with late-M spectral types, suggesting that they represent an evolutionary phase.

The chemical abundances of galaxies provide key constraints to models of their formation and evolution. Unfortunately, the standard method of determining abundances, from the strong emission lines of Hii-regions, is well-known to be subject to large systematic errors, particularly at high metallicities. To address this problem we are currently working on a project to measure a galaxy’s chemical abundances from spectral analysis of individual RSGs. By focussing on a spectral window in the J-band which contains few molecular lines, we are able to derive accurate chemical abundances from relatively low resolution data. In fact, we show that this technique can operate at distances of 3–4 Mpc using the VLT, and a factor of 10 further if we target massive RSG-dominated star clusters. Furthermore, in the ELT-era, we show that we can perform abundance analysis on individual stars out to 70 Mpc, and redshifts of 0.1 for star clusters.

We present near-infrared integral field observations of the super star cluster in the amorphous galaxy NGC1705. Data have been collected with SINFONI mounted on the VLT. Adaptive optics was used under good seeing conditions. Mosaics of the cluster and its immediate surrounding have been constructed. The cluster is not spatially resolved. Its radius is smaller than 2.85 ± 0.50pc. The K-band spectrum of the cluster is dominated by strong CO absorption bandheads. It is typical of a Galactic K 4–5 supergiant. Its age is estimated to be 12 ± 6Myr. The large error bar is rooted in the uncertainties of the input physics and ingredients of different evolutionary models.

We are using a sample of more than 50 standard very late-type stars, taken from Isaac Newton Telescope at R  ~ 9000 in the 8100–9100 Å range, in order to develop criteria for spectral and luminosity classification of very late-type stars in the Gaia region. In this article, we brief in a few pages the main challenges on this issue, as well as the literature about it, and we present some preliminary results.

Betelgeuse is one of the most magnificent stars in the sky, and one of the nearest red supergiants. Astronomers gathered in Paris in the Autumn of 2012 to decide what we know about its structure, behaviour, and past and future evolution, and how to place this in the general context of the class of red supergiants. Here I reflect on the discussions and propose a synthesis of the presented evidence. I believe that, in those four days, we have achieved to solve a few riddles.