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Be stars make up almost 20% of the B star population, and are rapidlyrotating stars surrounded by a disc; however the origin of this rotationremains unclear. Mass transfer within close binaries provides the leadinghypothesis, with previous detections of stripped companions to Be starssupporting this. Here, we exploit the exquisite astrometric precision of Gaiato carry out the largest to date comparative study into the binarity of matchedsamples of nearby B and Be stars from the Bright Star Catalogue. By utilisingnew "proper motion anomaly" values, derived from Gaia DR2 and DR3 astrometricdata alongside previous values calculated using Hipparcos and Gaia data, andthe Gaia provided RUWE, we demonstrate that we can identify unresolved binariesdown to separations of 0.02". Using these measures, we find that the binaryfractions of B and Be stars are similar between 0.04 - 10", but the Be binaryfraction is significantly lower than that of the B stars for separations below0.04". As the separation range of these "missing" binaries is too large formass transfer, and stripped companions are not retrieved by these measures, wesuggest the companions migrate inwards via binary hardening within a triplesystem. This confirms statistically for the first time the hypothesis thatbinary interaction causes the Be phenomenon, with migration causing the dearthof Be binaries between 0.02 - 0.04". Furthermore, we suggest that triplicityplays a vital role in this migration, and thus in the formation of Be stars asa whole.

Gaia uncovers difference in B and Be star binarity at small scales: evidence for mass transfer causing the Be phenomenon