VLBI observations of circumstellar shells around two OH/IR stars

MERLIN and VLA maps of the OH masers around OH/IR stars show that their characteristic double-peaked spectrum is caused by an expanding shell of material ejected from the star. In order to investigate the compact structure of the OH masers within this shell we have observed two such OH/IR stars (OH 127.8 and 104.9) using the European VLBI network. The resulting VLBI aperture synthesis maps are the first to be made of OH/IR stars. In each case, the maps show compact sources on the nearside of the shell, but no corresponding source on the far side of the shell. This is interpreted in terms of amplification of the stellar thermal emission and a mechanism for this process is outlined. (author)