With initial masses greater than 8 M☉, massive stars are key players in the Universe due to their intense UV radiation field, strong stellar winds, and final supernova (SN) explosions. They played an important role in the reionisation of the early Universe  and enrich the interstellar medium with heavy elements, they shape the formation and evolution of galaxies , and they are the dominant source of momentum in the Universe . The violent deaths of massive stars can be observed over cosmic distances, and produce some of the most energetic phenomena known, such as pair-instability supernovae 4 and gamma-ray bursts . Metal poor, massive binary stars are the most likely progenitors of double black hole systems that could produce produce gravitational wave events within the age of the Universe .
In this context, it is crucial to understand the evolution of massive stars up to their eventual deaths as supernovae. Next to their initial mass, the domant factor determining this evolution is the amount of mass they lose during their lives. However, classical line-driven winds  have been proven to be insufficient to explain the masses of evolved stars on both theoretical  and observational grounds  , and additional mass-loss mechanisms in the form of binary interactions and episodic mass loss – e.g., Luminous Blue Variable (LBV) and pre-SN eruptions, extreme red supergiant (RSG) mass loss – are needed. Binary interactions are thought to occur in 70% of massive stars, stripping the envelopes of 30% of O-type stars (at least at solar metallicity) . The role and physics of episodic mass loss is however still largely unknown, and remains one of the major unsolved problems in astrophysics . Its importance has come to the forefront in both the massive star and supernova communities, as numerous circumstellar dust shells have been detected surrounding evolved massive stars , LBVs and extreme supergiants , and superluminous supernovae .
ASSESS (Episodic MAss LoSS in Evolved MaSsive Stars) is an ERC-funded project (2018-2023, PI Bonanos) investigating the role of episodic mass loss in the evolution of massive stars and its role in the early Universe. What is new about the proposed approach is the idea of conducting – for the first time – a systematic multi-wavelength survey of evolved massive stars in the local Universe, covering a large range of metallicities (1/15 ∼ 3 z). The project hinges on the fact that mass-losing stars form dust and are thus bright sources in the mid-IR. It targets 27 nearby galaxies for which Spitzer point-source catalogues are available [15,16,17]. The mid-IR photometry is supplemented by data from Pan-STARRS , the UKIRT Hemisphere Survey , the VISTA Hemisphere Survey , and Gaia .
Key scientific questions the ASSESS project aims to address are:
• What is the frequency of major mass ejection events – like the giant eruption of η Car – and how does this frequency depend on metallicity?
• Is the presence of significant mid-IR excess a hallmark of LBVs, and is mass loss from LBVs mostly episodic or in the form of a continuous wind?
• Do all massive stars undergo one or more mass ejection episodes in the months/years before they explode as supernova?
• What is the frequency and evolutionary status of B supergiant stars with circumstellar discs (sgB[e] stars), and is there a connection with LBVs?
• How long are the cool and hot hypergiant phases, and which parameters determine whether a massive star will go through these post-RSG stages?
Observations of the endpoints of massive stars are pointing to new evolutionary channels and the proposed research has the potential to dramatically transform this field by providing crucial data on the properties and statistics of systems mostly obscured by their own mass loss. We anticipate that our results will further have implications for supernovae, long GRBs, the evolution of the metal-poor early Universe and the chemical evolution of galaxies, as the first stars are expected to be massive  and to be significant contributors to the dust production in high-z galaxies .
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