Multi-epoch VLBA H$_2$O maser observations toward the massive YSOs AFGL
  2591 VLA 2 and VLA 3
  We present multi-epoch Very Long Baseline Array (VLBA) H$_2$O maser
observations toward the massive young stellar objects (YSOs) VLA 2 and VLA 3 in
the star-forming region AFGL 2591. Through these observations, we have extended
the study of the evolution of the masers towards these objects up to a time
span of $\sim$ 10 yrs, measuring their radial velocities and proper motions.
The H$_2$O masers in VLA 3, the most massive YSO in AFGL 2591 ($\sim$
30--40~M$_{\odot}$), are grouped within projected distances of $\lesssim$ 40
mas ($\lesssim$ 130 AU) from VLA 3. In contrast to other H$_2$O masers in AFGL
2591, the masers associated with VLA 3 are significantly blueshifted (up to
$\sim$ 30 km s$^{-1}$) with respect to the velocity of the ambient molecular
cloud. We find that the H$_2$O maser cluster as a whole, has moved westwards of
VLA~3 between the 2001 and 2009 observations, with a proper motion of $\sim$
1.2 mas yr$^{-1}$ ($\sim$ 20 km s$^{-1}$). We conclude that these masers are
tracing blueshifted outflowing material, shock excited at the inner parts of a
cavity seen previously in ammonia molecular lines and infrared images, and
proposed to be evacuated by the outflow associated with the massive VLA 3
source. The masers in the region of VLA 2 are located at projected distances of
$\sim$ 0.7$''$ ($\sim$ 2300 AU) north from this source, with their kinematics
suggesting that they are excited by a YSO other than VLA 2. This driving source
has not yet been identified.
