From massive stars through X-ray binaries to gravitational-wave sources

The last few years have seen an unprecedented growth in all the aspects of gravitational-wave (GW) astrophysics. At the frontier is population modeling of GW sources and their potential progenitors: massive stars in binary systems. With new studies appearing on the daily basis, it is good to take a step back and ask: how well do we really understand the ''basics'' of binary evolution and the ''classical'' formation channels? In my talk, I will summarize the findings of a series of papers that tackles this fundamental question through a combination of 1D stellar/binary models and observational constraints. I will argue against the formation of massive binary black holes through the common-envelope evolution, one of the most prominent channels discussed in the literature. Furthermore, I will show that the textbook picture of mass transfer evolution, where a giant star is stripped of its envelope by an accreting companion, may no longer hold in the case of massive stars at low metallicity. The discovery of partial envelope stripping, if confirmed in spectroscopic surveys of metal-poor star-forming galaxies, will have vast implications for high-mass X-ray binaries, supernova progenitors, stripped stars, and dormant BH binaries.