SIMBAD references

We have used the Australian Long Baseline Array (LBA) to produce milliarcsecond images of five sites of methanol (CH₃OH) maser emission at 6.7 GHz. These are all sites that have linear morphologies at arcsecond resolutions, which have been hypothesized to be due to the masers forming in edge-on circumstellar discs. We find that a simple disc model cannot explain the observations. We discuss various alternatives, and suggest a new model that explains how linear velocity gradients can be produced in methanol masers that arise in planar shocks propagating nearly perpendicular to the line of sight.