Volume 393, Number 2, October II 2002
|Page(s)||L33 - L36|
|Published online||23 September 2002|
Letter to the Editor
Astrometry of the stellar image of U Her amplified by the circumstellar 22 GHz water masers
Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
2 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
3 Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire, SK11 9DL, England, UK
Corresponding author: W. H. T. Vlemmings, firstname.lastname@example.org
Accepted: 9 August 2002
The 22 GHz H2O 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 H2O 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 H2O 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.
Key words: masers / stars: circumstellar matter / stars: individual: U Her / stars: AGB and post-AGB / techniques: interferometric / astrometry
© ESO, 2002
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