PROTOPLANETARY DISK SHADOWING BY GAS INFALLING ONTO THE YOUNG STAR AK Sco

Abstract

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 α line (Ly α ) 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 α photons by a gas stream in free fall from 3  R . Given the high optical depth of the Ly α line, a very low gas column of cm −2 suffices to block the Ly α radiation without producing noticeable effects in the rest of the stellar spectral tracers.