Milliarcsecond structure and variability of methanol maser emission in three high-mass protostars

Abstract

{The variability study of 6.7\,GHz methanol masers has become a useful way to improve our understanding of the physical conditions in high-mass star-forming regions.} {Based on the single-dish monitoring using the Irbene telescopes, we selected three sources with close sky positions.} {We imaged them using the European Very Long Baseline Interferometer Network and searched available data on VLBI archives to follow detailed changes in their structures and single maser spot variability.} {All three targets show a few groups of maser cloudlets of a typical size of 3.5\,mas and the majority of them show linear or arched structures with velocity gradients of order 0.22\kms\,mas$^{-1}$. The cloudlets and overall source morphologies are remarkably stable on time scales of 7-15\,yr supporting a scenario of variability due to changes in the maser pumping rate.}