2016A&A...587A.104B

Methanol masers at 6.7GHz are well known tracers of high-mass star-forming regions. However, their origin is still not clearly understood. We aimed to determine the morphology and velocity structure for a large sample of the maser emission with generally lower peak flux densities than those in previous surveys. Using the European VLBI Network (EVN) we imaged the remaining sources from a sample of sources that were selected from the unbiased survey using the Torun 32m dish. In this paper we report the results for 17 targets. Together they form a database of a total of 63 source images with high sensitivity (3σ_rms=15-30mJy/beam), milliarcsecond angular resolution (6-10mas) and very good spectral resolution (0.09km/s or 0.18km/s) for detailed studies. We studied in detail the properties of the maser clouds and calculated the mean and median values of the projected size (17.4±1.2au and 5.5au, respectively) as well as the FWHM of the line (0.373±0.011km/s and 0.315km/s for the mean and median values, respectively), testing whether it was consistent with Gaussian profile. We also found maser clouds with velocity gradients (71%) that ranged from 0.005km/s/au to 0.210km/s/au. We tested the kinematic models to explain the observed structures of the 6.7GHz emission. There were targets where the morphology supported the scenario of a rotating and expanding disk or a bipolar outflow. Comparing the interferometric and single-dish spectra we found that, typically, 50-70% of the flux was missing. This phenomena is not strongly related to the distance of the source. The EVN imaging reveals that in the complete sample of 63 sources the ring-like morphology appeared in 17% of sources, arcs were seen in a further 8%, and the structures were complex in 46% cases. The ultra-compact (UC) HII regions coincide in position in the sky for 13% of the sources. They are related both to extremely high and low luminosity masers from the sample.