2002A&A...383..614M

We consider the milliarsecond scale structures of three representative methanol maser sites observed at high resolution with the EVN at 6.7GHz and with the VLBA at 12.2GHz. These observations suggest that the majority of the individual masing regions within a maser site consist of a compact core surrounded by extended emission, i.e. a core/halo structure. In many cases, the extended emission is resolved by the shortest baselines of the VLBA. We conclude that these regions of extended emission are responsible for the missing flux density when comparing cross-power spectra on the shortest VLBI baselines to single dish spectra. The halos of diffuse emission have diameters ranging between 12 and 290AU and their brightness temperatures depend on their diameters as T_b∝d^–2.2. The core diameters vary from 2 to 20AU. Three possible phenomena could explain the presence of large halos around bright cores. The core/halo pattern could be the result of saturation effects in a uniform spherical maser cloud. The compact and diffuse masers could also originate in physically distinct regions: the maser core could form within highly dense gas while the maser halo could be the result of weak masers in diffuse gas. Finally, turbulence could cause the appearance of core/halo structures in a disrupted homogeneous medium. Scattering effects do not seem to play a role in the broadening of maser images since masers have similar sizes at 6.7 and 12.2GHz.