2011A&A...536A..38M

High-mass stars play a prominent role in Galactic evolution, but their formation mechanism is still poorly understood. This lack of knowledge reflects the observational limitations of present instruments, whose angular resolution (at the typical distances of massive protostars) precludes probing circumstellar gas on scales of 1-100 AU, relevant for a detailed investigation of accretion structures and launch/collimation mechanims of outflows in high-mass star formation. This work presents a study of the milliarcsecond structure of the 6.7GHz methanol masers at high-velocity resolution (0.09km/s) in four high-mass star-forming regions: G16.59-0.05, G23.01-0.41, IRAS 20126+4104, and AFGL 5142. We studied these sources by means of multi-epoch VLBI observations in the 22GHz water and 6.7GHz methanol masers, to determine the 3-D gas kinematics within a few thousand AU from the (proto)star. Our results demonstrate the ability of maser emission to trace kinematic structures close to the (proto)star, revealing the presence of fast wide-angle and/or collimated outflows (traced by the H₂O masers), and of rotation and infall (indicated by the CH₃OH masers). The present work exploits the 6.7GHz maser data collected so far to investigate the milliarcsecond structure of this maser emission at high-velocity resolution. Most of the detected 6.7GHz maser features present an ordered (linear, or arc-like) distribution of maser spots on the plane of the sky, together with a regular variation in the spot LSR velocity (V_LSR) with position. Typical values for the amplitude of the V_LSR gradients (defined in terms of the derivative of the spot V_LSR with position) are found to be 0.1-0.2km/s/mas. In each of the four target sources, the orientation and the amplitude of most of the feature V_LSR gradients remain remarkably stable in time, on timescales of (at least) several years. We also find that the data are consistent with having the V_LSR gradients and proper motion vectors in the same direction on the sky, considered the measurement uncertainties. In three (G16.59-0.05, G23.01-0.41, and IRAS 20126+4104) of the four sources under examination, feature gradients with the best determined (sky-projected) orientation divide into two groups directed approximately perpendicular to each other. The time persistency, the ordered angular and spatial distribution, and the orientation generally similar to the proper motions, altogether suggest a kinematical interpretation for the origin of the 6.7 GHz maser V_LSR gradients. This work shows that the organized motions (outflow, infall, and rotation) revealed by the (22GHz water and 6.7GHz methanol) masers on large scales (∼100-1000AU) also persist to very small (∼10AU) scales. In this context, the present study demonstrates the potentiality of the mas-scale 6.7GHz maser gradients as a unique tool for investigating the gas kinematics on the smallest accessible scales in proximity to massive (proto)stars.