Protoplanetary disk shadowing by gas infalling onto the young star AK Sco.

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 Ly alpha line (Ly alpha) 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 H-2 flux by up to 10% lasting 5.9 hr. We show that the decrease of the H-2 flux can be produced by the occultation of the stellar Ly alpha photons by a gas stream in free fall from 3. R*. Given the high optical depth of the Ly alpha line, a very low gas column of N-H 5 10 > 10(17) cm(-2) suffices to block the Ly alpha radiation without producing noticeable effects in the rest of the stellar spectral tracers