出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Methanol masers are closely associated with the earliest stages of massive star formation as they
form in close (100−1000AU) surrounding of protostars. Their high brightness and compactness
offer a unique opportunity to study these dense environments at the milliarcsecond scale.
We present recent EVN observations of five methanol masers at the 6.7-GHz transition. Two
sources, G23.707−00.198 and G24.541+00.312, show arc-like structures with overall velocity
gradients. They fit well to the model of bipolar outflow. Emission from G23.207−00.377 and
G27.221+00.136 appears as coming from randomly distributed maser components. Such complex
structures likely arise as a result of propagation of a few shock fronts driven by close protostars
seen in infrared. G23.657−00.127 shows a spectacular, nearly circular ring which is related
to either a bubble driven by a central protostar or a circumstellar disc. New VLBA observations
reveal that the 12.2-GHz methanol maser emission follows exactly the same ring. This enables us
to investigate the nature of the unique structure by means of proper motion studies over a period
of one year and to measure the parallax.
