The rapid rotation of these Be stars has always been a subject of intrigue within the scientific community, with the root cause remaining elusive. A leading theory proposes that this rotation is instigated by mass transfer from a companion star. By employing astrometric data from the Gaia satellite, our team identified binary systems that were spatially unresolved with separations as minuscule as 0.02 arc seconds.
Our investigation revealed that the fractions of B and Be stars forming part of binary systems were strikingly similar for separations between 0.04 and 10 arc seconds. However, when examining binary systems with smaller separations, the prevalence of Be stars noticeably dwindled for separations less than 0.04 arc seconds. This discrepancy begs the question - what could account for this significant difference?
One explanation aligns with the theory of triple star systems. The conspicuous absence of close binary stars with a primary Be star corresponds with the existence of stripped companions, which become undetectable due to their proximity, diminished mass, and dimness. This phenomenon is a consequence of the aforementioned mass transfer.
Furthermore, our research found that Be stars are frequently part of higher-order triple and multiple systems, far exceeding the expectations for B stars overall. This finding suggests that the third companion's actions in the system contribute to the migration of one star to a distance low enough from the primary for a mass and associated angular momentum transfer to occur. This mechanism could elucidate both the formation of the characteristic accretion disk and the high-speed rotation of Be stars.
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