A RADIO JET-H2O MASER SYSTEM IN W75N(B) AT A 200 AU SCALE - EXPLORINGTHE EVOLUTIONARY STAGES OF YOUNG STELLAR OBJECTS

We have observed simultaneously with the VLA in its A configuration th e 1.3 cm continuum and H2O maser emission toward the star-forming regi on W75N(B) with 0''11 resolution using a powerful cross-calibration te chnique. Three continuum sources (VLA 1, VLA 2, and VLA 3) were detect ed in a region of 1.''5. VLA 1 is elongated (0.''43 x 0.''12) approxim ately in the direction of the bipolar molecular outflow observed at sc ales of 2'. The frequency dependence of the flux density and size are consistent with an optically partially thick ionized thermal biconical jet. VLA 2 appears unresolved, while VLA 3 shows a bright core plus e xtended emission. We detected 29 H2O maser spots (spatial components) in a region of 13'' x 7'' around W75N(B). These masers are mainly dist ributed in two clusters, one associated with VLA 1 (11 maser spots), a nd the other one associated with VLA 2 (eight maser spots). One H2O ma ser spot is associated with VLA 3. The masers associated with VLA 1 ar e distributed along the major axis of the radio jet. We conclude that VLA 1 is the powering source of the extended bipolar molecular outflow and that the water masers along the radio jet axis are delineating th e outflow at scales of 1'' On the other hand, the eight masers coincid ent with VLA 2 are distributed in a shell of 0.''18 x 0.''10, with a r ough north-south velocity segregation that could indicate bound motion s around this continuum source. From the comparison of H2O and OH mase r distribution in the region with respect to the three radio sources, we consider an evolutionary scheme in which H2O masers are excited in gravitationally bound material (e.g., in circumstellar disks) in less evolved young stellar objects (YSOs), while in more evolved YSOs H2O m asers preferentially trace outflows.