Grain Formation in the Expanding Envelopes of T Tauri Stars

The possibility of the nucleation in the expanding envelopes of T Tauri stars is investigated by using the nucleation theory by Becker and Doring. The nucleation time scale r,. in .the gas is investigated assuming that only graphite is condensable. The expansion time scale roxo of the gas ejected from a T Tauri star is calculated assuming that the gas flow is steady and spherically symmetric. The time scale r,. is compared with roxp for Xo=l0-2 (Xo is the concentration of carbori by mass) assuming that the gas expands adiabatically, and the massloss rate needed for nucleation is found to be larger than those observed in T Tauri stars. Thi~ suggests that nucleation around T Tauri stars is rather difficult.