A Radio Jet–H2O Maser System in W75N(B) at a 200 Au Scale: Exploring the Evolutionary Stages of Young Stellar Objects

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Abstract
We have observed simultaneously with the VLA in its A configuration the 1.3 cm continuum and H2O maser emission toward the star-forming region W75N(B) with 01 resolution using a powerful cross-calibration technique. Three continuum sources (VLA 1, VLA 2, and VLA 3) were detected in a region of 15. VLA 1 is elongated (043 × 012) approximately in the direction of the bipolar molecular outflow observed at scales 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 extended emission. We detected 29 H2O maser spots (spatial components) in a region of 13'' × 7'' around W75N(B). These masers are mainly distributed in two clusters, one associated with VLA 1 (11 maser spots), and the other one associated with VLA 2 (eight maser spots). One H2O maser spot is associated with VLA 3. The masers associated with VLA 1 are 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 the outflow at scales of 1''. On the other hand, the eight masers coincident with VLA 2 are distributed in a shell of 018 × 010, with a rough north-south velocity segregation that could indicate bound motions around this continuum source. From the comparison of H2O and OH maser 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 masers preferentially trace outflows.