A new VLBI observation of an OH maser in a post-asymptotic giant branch star using the European VLBI Network

OH 17.7 - 2.0 is a post-asymptotic giant branch star that is of great interest. The 1612-MHz OH emission from OH 17.7 - 2.0 is characterized by a double-peaked spectrum. Such a line profile has been assumed to represent maser emission from an expanding circumstellar shell. A new VLBI observation of the OH maser in OH 17.7 - 2.0 has been made using the European VLBI Network, and a relative position map of the eight OH maser spots has been obtained. Using the relative position map, it is found that the eight OH maser spots are distributed on an expanding circumstellar shell. The parameters of the expanding circumstellar shell have been obtained.