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

Abstract

This work presents a study of the milliarcsecond structure of the 6.7 GHz methanol masers at high-velocity resolution (0.09 km s^(-1)) in four high-mass star-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 22 GHz water and 6.7 GHz methanol masers, to determine the 3-D gas kinematics within a few thousand AU from the (proto)star. The present work exploits the 6.7 GHz maser data collected so far to investigate the milliarcsecond structure of this maser emission at high-velocity resolution. Most of the detected 6.7 GHz 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 0.1-0.2 km s^(-1) mas^(-1). 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. 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 (22 GHz water and 6.7 GHz methanol) masers on large scales (~100-1000 AU) also persist to very small (~10 AU) scales.

14 pages, 7 figures, accepted for publication in Astronomy and Astrophysics Journal

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• Velocity and magnetic fields within 1000 AU from a massive YSO
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• A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59-0.05 high-mass star-forming region
• Multi-frequency VLBI observations of maser lines during the 6.7~GHz maser flare in the high-mass young stellar object G24.33$+$0.14}
• Modelling CH${}_3$OH masers: Sobolev approximation and accelerated lambda iteration method
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• A Face-on Accretion System in High-Mass Star-Formation: Possible Dusty Infall Streams within 100 AU
• A Herschel and BIMA study of the sequential star formation near the W48A HII region
• Long-term Variability of H${}_2$CO Masers in Star-forming Regions
• Milliarcsecond structure of water maser emission in two young high-mass stellar objects associated with methanol masers
• Proper motion study of the 6.7 GHz methanol maser rings. I. A sample of sources with little variation
• Milliarcsecond structure and variability of methanol maser emission in three high-mass protostars
• Periodic 6.7 GHz $\mathrm{C}{\mathrm{H}}_3\mathrm{OH}$ maser emission in G353.273+0.641: First candidate for a pulsating high-mass protostar