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This work presents a study of the milliarcsecond structure of the 6.7 GHzmethanol masers at high-velocity resolution (0.09 km s^(-1)) in four high-massstar-forming regions: G16.59-0.05, G23.01-0.41, IRAS20126+4104, and AFGL5142.We studied these sources by means of multi-epoch VLBI observations in the 22GHz water and 6.7 GHz methanol masers, to determine the 3-D gas kinematicswithin a few thousand AU from the (proto)star. The present work exploits the6.7 GHz maser data collected so far to investigate the milliarcsecond structureof this maser emission at high-velocity resolution. Most of the detected 6.7GHz maser features present an ordered (linear, or arc-like) distribution ofmaser spots on the plane of the sky, together with a regular variation in thespot LSR velocity (V_LSR) with position. Typical values for the amplitude ofthe V_LSR gradients (defined in terms of the derivative of the spot V_LSR withposition) are found to be 0.1-0.2 km s^(-1) mas^(-1). In each of the fourtarget sources, the orientation and the amplitude of most of the feature V_LSRgradients remain remarkably stable in time, on timescales of (at least) severalyears. We also find that the data are consistent with having the V_LSRgradients and proper motion vectors in the same direction on the sky,considered the measurement uncertainties. The time persistency, the orderedangular and spatial distribution, and the orientation generally similar to theproper motions, altogether suggest a kinematical interpretation for the originof the 6.7 GHz maser V_LSR gradients. This work shows that the organizedmotions (outflow, infall, and rotation) revealed by the (22 GHz water and 6.7GHz methanol) masers on large scales (~100-1000 AU) also persist to very small(~10 AU) scales.

Unveiling the gas kinematics at 10 AU scales in high-mass star-forming regions (Milliarcsecond structure of 6.7 GHz methanol masers)