Over the past two decades, our understanding of massive star evolution has undergone a paradigm shift. For nearly 40 years, theoretical and observational efforts focused on refining a framework in which massive stars were assumed to evolve predominantly as single, with four key factor playing a dominant role: mass, rotation, stellar winds, and metallicity. However, this framework is being significantly revised due to the realization that a large fraction of massive stars are born in close-by binary systems, where strong interactions between components during their evolution are inevitable. This shift has become even more critical following the discovery of gravitational wave (GW) events associated with the merger of black hole or neutron star binaries, one of the natural outcomes of massive binary evolution. Consequently, the study of massive binaries has emerged as a priority within the stellar astrophysics community, integrating observational studies with advanced theoretical modeling.
Binary interactions drastically change the evolutionary tracks of the individual components. Processes such as mass transfer, common envelope evolution, tidal effects, and mergers redistribute mass and angular momentum, modify internal structures, and alter key properties like luminosity and mass-loss rates. As a result, binary evolution not only adds on the complexity of massive star physics, but also yields a rich set of post-interaction products such as, e.g. stripped stars, rapidly rotating accretors, mergers, exotic supernovae, X-ray binaries, and double-degenerate binaries. Moreover, the occurrence of binary interactions opens the door to alternative pathways to phenomena such as e.g. WRs and helium stars, and complicates even further the unique interpretation of observables. In this context, even studies focusing on apparently single stars must account for the possibility of past binary interactions to avoid misattributing observed properties to single-star evolution.
This coordinated project builds on decades of experience in investigating massive stars from an observational perspective. The primary goal is to continue our long-term committment to provide robust empirical constraints to inform and refine theoretical models of single and binary massive star evolution, this time putting special emphasis in gaining a deeper understanding of how binarity and multiplicity influence the interpretation of the empirical observables used to constrain massive star evolution. The Tenerife and Alicante teams will work in parallel, employing a methodology that combines three complementary approaches: (1) comprehensive studies of massive star populations across the Hertzsprung-Russell diagram (HRD), (2) in-depth investigations of controlled populations, and (3) detailed studies of individual objects. In this endeavour, we will benefit from our complementary expertise and privileged access to world-class spectroscopic datasets, including IACOB, DYNOSTAR, and WEAVE-SCIP, as well as high-quality data from the Gaia and TESS missions. These datasets will enable a new level of statistical significance in studying Galactic massive star populations, encompassing a broad range of masses, evolutionary stages, and environments. Eventually, this project aims to advance our understanding of massive star physics and its profound implications for the interpretation of stellar populations in galaxies across space and cosmictime, supernova progenitors, and GW sources.
