Multi-epoch VLBA H${}_2$O maser observations toward the massive YSOs AFGL 2591 VLA 2 and VLA 3

Abstract

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${}^{\prime \prime }$ ($\sim$ 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.