  Young solar-type stars grow through the accretion of material from the
circumstellar disk during pre-main sequence (PMS) evolution. The ultraviolet
radiation generated in this process plays a key role in the chemistry and
evolution of young planetary disks. In particular, the hydrogen Lyman-alpha
line (Lya) etches the disk surface by driving photoevaporative flows that
control disk evolution. Using the Hubble Space Telescope, we have monitored the
PMS binary star AK Sco during the periastron passage and have detected a drop
of the H2 flux by up to 10% lasting 5.9 hours. We show that the decrease of the
H2 flux can be produced by the occultation of the stellar Lya photons by a gas
stream in free fall from 3 R*. Given the high optical depth of the Lya line, a
very low gas column of NH > 5e17 cm-2 suffices to block the Lya radiation
without producing noticeable effects in the rest of the stellar spectral
tracers.
