1997A&A...320..478B

The young star TY CrA was known as an eclipsing binary, which surprisingly appeared circularized but not synchronized. Recently, a third companion was detected, orbiting at roughly 1A.U. from the central binary, on a significantly eccentric and inclined orbit. The dynamics of this triple system is here investigated, on the basis of the tentative fits of the tertiary orbit. We show that according to pure 3-body dynamics, the TY CrA central binary should be subject to periodic changes related to the so-called Kozai mechanism in cometary dynamics, which should lead to a rapid collapse. Adding tidal effects within that binary actually prevents it from collapsing. We also show that thanks to the combination of tidal effects and 3-body dynamics, the rotation axes of the components of the central binary may by locked in a particular position (in the orbital plane) which might explain the apparent non-synchronism of the binary. Such a situation would not be stable if the central binary was alone. The calculations are performed using interior models of the stars depending on their age. We show that the particular position is only stable if the system is less than a few 10⁶yrs old, i.e. if the secondary is a pre-main sequence star, otherwise the binary synchronizes. Hence we constrain the age of this system, which is in agreement with the conclusion reached from the comparison with evolutionary tracks.