Be stars in binaries

In recent years, multiple lines of evidence have emerged supporting the idea that Be stars are mass gainers in interacting binaries. Together, these findings suggest a compelling scenario in which all Be stars are formed through binary interactions. Nevertheless, the broader implications of this scenario must be carefully examined within the context of massive binary evolution. We must not forget substantial complementary data from studies of Be stars in open clusters, as well as comparative analyses of Be star populations in the Milky Way and the Small Magellanic Cloud (SMC) with their markedly different metallicities. Several key observations, with overwhelming empirical support, challenge the binary interaction hypothesis. Among them, (1) Be stars in open clusters are seldom blue stragglers; (2) blue stragglers in open clusters rarely exhibit Be characteristics; and (3) Oe stars in the SMC are far more common than in the Milky Way, yet the primaries of Be/X-ray binaries in the SMC exhibit the same spectral-type distribution as those in the Milky Way, indicating that they have lower masses. To form a coherent model of Be star formation through binary interactions, these discrepancies must be addressed, likely requiring a much deeper understanding of how metallicity influences binary formation and evolution.