  AK Sco is a unique source: a ~10 Myrs old pre-main sequence spectroscopic
binary composed of two nearly equal F5 stars that at periastron are separated
by barely eleven stellar radii so, the stellar magnetospheres fill the Roche
lobe at periastron. The orbit is not yet circularized (e=0.47) and very strong
tides are expected. This makes of AK Sco, the ideal laboratory to study the
effect of gravitational tides in the stellar magnetic field building up during
pre-main sequence (PMS) evolution. In this letter, the detection of a highly
disturbed (sigma ~ 100 km/s) and very dense atmosphere (ne = 1.6e10cm-3) is
reported. Significant line broadening blurs any signs of ion belts or bow
shocks in the spectrum of the atmospheric plasma. The radiative loses cannot be
accounted solely by the dissipation of energy from the tidal wave propagating
in the stellar atmosphere; neither by the accreting material. The release of
internal energy from the star seems to be the most likely source of the plasma
heating. This is the first clear indication of a highly disturbed atmosphere
surrounding a pre-main sequence close binary.
