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Astron. Astrophys. 357, 945-950 (2000)

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VLBI astrometry of the stellar image of U Herculis, amplified by the 1667 MHz OH maser

H.J. van Langevelde 1, W. Vlemmings 2, P.J. Diamond 3, A. Baudry 4 and A.J. Beasley 5

1 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
2 Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
3 MERLIN/VLBI National Facility, Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL, UK
4 Observatoire de Bordeaux, B.P. 89, 33270 Floirac, France
5 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

Received 22 December 1999 / Accepted 16 March 2000


The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Her has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax ([FORMULA] mas) were measured.

Key words: masers – techniques: interferometric – astrometry – stars: circumstellar matter – stars: individual: U Her – stars: AGB and post-AGB

Send offprint requests to: W. Vlemmings (vlemming@strw.leidenuniv.nl)

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000