The visual binary AG Tri in β\beta Pictoris Association: can a debris disc cause very different rotation periods of its components?

We measure the photometric rotation periods of the components of multiple systems in young stellar associations to investigate the causes of the observed rotation period dispersion. We present the case of the wide binary AG Tri in the 23-Myr young beta Pictoris Association consisting of K4 + M1 dwarfs. Our multi-band, multi-season photometric monitoring allowed us to measure the rotation periods of both components P_A = 12.4d and P_B = 4.66d, to detect a prominent magnetic activity in the photosphere, likely responsible for the measured radial velocity variations, and for the first time, a flare event on the M1 component AG Tri B. We investigate either the possibility that the faster rotating component may have suffered an enhanced primordial disc dispersal, starting its PMS spin-up earlier than the slower rotating component, or the possibility that the formation of a debris disc may have prevented AG Tri A from gaining part of the angular momentum from the accreting disc.Comment: 28 pages, 7 figures, accepted for publication in Astrophysics and Space Science 2015, (ASTR-D-15-00445R2