The maser source S140-H2O as a protoplanetary disk

We present the observations of the H₂O maser in S140 with the 22-m radio telescope of the Pushchino Radio Astronomy Observatory in 1992–1999. The H₂O maser emission is mainly concentrated in three symmetrically placed narrow radial-velocity intervals. In contrast to our earlier observations (1981–1991), we did not detect emission simultaneously in all three intervals; instead, the emission appeared successively in each. We discuss possible origins for this behavior. To explain the flux variability and radial-velocity drift of the main components, we propose a model with Keplerian orbital motion of clumps (protoplanets) and calculate their orbital parameters.