Astrometry of the stellar image of U Her amplified by the circumstellar 22 GHz water masers

Abstract

The 22 GHz H_2O masers in the circumstellar envelope of the Mira variable star U Her have been observed with MERLIN using a phase referencing technique to determine accurate astrometric positions. The positions were compared with the optical positions obtained with the Hipparcos satellite to an accuracy of 18 mas. The absolute radio position of the brightest H_2O maser spot is found to match the optical position, indicating that this spot is the stellar image amplified by the maser screen in front of it. The occurrence of an amplified image in the 22 GHz maser can be used to accurately determine the positions of the H_2O with respect to the star as well as with respect to the SiO and OH masers. Our observations seem to indicate that the star is not in the centre of the distribution of maser spots, which has been interpreted as a ring.

4 pages, 1 figure; accepted for publication in A&A letters

Cited by in corpus (8)

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• JVN observations of H2O masers around the evolved star IRAS 22480+6002
• Stellar masers, circumstellar envelopes, and supernova remnants
• Water vapour masers in long-period variable stars III. Mira variables U Her and RR Aql