The Role of Binary-Stripped Stars: from Atomic Scales to Cosmic Dawn

A third of all massive stars are predicted to lose their hydrogen-rich envelopes through interaction with a binary companion star, leaving the helium core exposed. These stripped stars are thought to be the main progenitors of hydrogen-poor core collapse supernovae, to constitute two necessary steps in the formation of merging neutron stars, and to provide so copious amounts of hard ionizing radiation that they contributed to cosmic reionization. Despite their importance, it took over half a century until we last year confirmed their predicted existence with observations. This means that theoretical models that previously had few to no observational benchmarks now face reality. 2SStars is designed to challenge theory with observation, while adressing the questions (1) What is the role of stripped stars in binary star evolution? and (2) How can stripped stars be used to understand different parts of astrophysics? We will study the stripped star binaries in detail to obtain measures for example for stellar winds, mass transfer efficiency and X-ray emission. We will also venture beyond binary evolution and use stripped stars as laboratories to investigate both how magnetism and rotation operate in stellar interiors and what the source of the enigmatic He II emission observed in galaxies across redshifts is. To fully exploit the new discovery, 2SStars is constructed as a symbiosis of observations and theory, for which we will use and develop modern codes of binary stellar evolution, hot star atmospheres, nebular spectra, and stellar populations. With upcoming space missions in both the ultraviolet (UVEX) and milli-Hz gravitational wave regime (LISA), and new survey telescopes (VRO/LSST, WFIRST, 4MOST) and ongoing gravitational wave detections (LIGO/Virgo/KAGRA), a large fraction of the future science hinges on our understanding of binary evolution. 2SStars will provide the long-needed constraints and new theoretical models by making use of our newest type of star.