VLBI study of maser kinematics in high-mass SFRs. I. G16.59-0.05
  The present paper focuses on the high-mass star-forming region G16.59-0.05.
Methods: Using the VLBA and the EVN arrays, we conducted phase-referenced
observations of the three most powerful maser species in G16.59-0.05: H2O at
22.2 GHz (4 epochs), CH3OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1
epoch). In addition, we performed high-resolution (> 0".1), high-sensitivity (<
0.1 mJy) VLA observations of the radio continuum emission from the star-forming
region at 1.3 and 3.6 cm. Results: This is the first work to report accurate
measurements of the "relative" proper motions of the 6.7 GHz CH3OH masers. The
different spatial and 3-D velocity distribution clearly indicate that the 22
GHz water and 6.7 GHz methanol masers are tracing different kinematic
environments. The bipolar distribution of 6.7 GHz maser l.o.s. velocities and
the regular pattern of observed proper motions suggest that these masers are
tracing rotation around a central mass of about 35 solar masses. The flattened
spatial distribution of the 6.7 GHz masers, oriented NW-SE, suggests that they
can originate in a disk/toroid rotating around the massive YSO which drives the
12CO(2-1) outflow, oriented NE-SW, observed on arcsec scale. The extended,
radio continuum source observed close to the 6.7 GHz masers could be excited by
a wide-angle wind emitted from the YSO associated with the methanol masers, and
such a wind is proven to be sufficiently energetic to drive the NE-SW 12CO(2-1)
outflow. The H2O masers distribute across a region offset about 0".5 to the NW
of the CH3OH masers, in the same area where emission of high-density molecular
tracers, typical of HMCs, was detected. We postulate that a distinct YSO,
possibly in an earlier evolutionary phase than that exciting the methanol
masers, is responsible for the excitation of the water masers and the HMC
molecular lines. (Abridged)
