On the binary origin of Be stars and the nature of exotic Be binary systems

<p>Despite the large impact of massive stars on their environment, important questions concerning their formation, evolution, and final explosions remain unanswered. One of them is the origin of the Be phenomenon, which occurs in ~20% of the early-type OB star. Observationally, Be stars are defined as B-type stars with Balmer line emission, indicative of a circumstellar decretion disk. While the presence of a disk is thought to strongly correlate with rapid rotation of the star, the processes that lead to such high rotation rates are still widely debated. One of the proposed mechanisms for the spin up of Be stars is mass and angular momentum transfer from an initially more massive companion in previous binary interactions. If merging is avoided, the mass donor is now a He-burning stripped star or a compact object. If this is the predominant channel for Be star formation, there are two important implications: first, there should be no Be + main sequence binaries on tight orbits; and second, Be stars should have a stripped star or a compact object as companion (unless the system has been disrupted). Here, we present observational evidence that the binary channel is predominant in Be star formation. We report on a lack of main-sequence companions to massive Be stars, and argue that the few known Be binaries are indeed exotic systems with stripped star or compact companions. We further present new analyses that shed light on the nature and evolutionary status of the two famous Be systems LB-1 and HR 6819 who have recently been reported to contain a black hole companion.</p>