Improved VLBI astrometry of OH maser stars

Abstract

Aims: Accurate distances to evolved stars with high mass loss rates are needed for studies of many of their fundamental properties. However, as these stars are heavily obscured and variable, optical and infrared astrometry is unable to provide enough accuracy. Methods: Astrometry using masers in the circumstellar envelopes can be used to overcome this problem. We have observed the OH masers of a number of Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long Baseline Array (VLBA). We have used the technique of phase referencing with in-beam calibrators to test the improvements this technique can provide to Very Long Baseline Interferometry (VLBI) OH maser astrometric observations. Results: We have significantly improved the parallax and proper motion measurements of the Mira variable stars U Her, S CrB and RR Aql. Conclusions: It is shown that both in-beam phase-referencing and a decrease in solar activity during the observations significantly improves the accuracy of the astrometric observations. The improved distances to S CrB (418 +21 -18 pc) and RR Aql (633 +214 -128 pc) are fully consistent with published P-L relations, but the distance to U Her (266 +32 -28 pc) is significantly smaller. We conclude that for sources that are bright and have a nearby in-beam calibrator, VLBI OH maser astrometry can be used to determine distances to OH masing stars of up to ~2 kpc.

arXiv comment15 pages, 10 figures; accepted for publication in A&A; for a version with high-resolution figures see http://www.astro.uni-bonn.de/~wouter/papers/astrom/astrom.shtml

References in corpus (10)

• The Distance to the Perseus Spiral Arm in the Milky Way
• VLBA Measurement of Nine Pulsar Parallaxes
• Water maser motions in W3(OH) and a determination of its distance
• Getting Its Kicks: A VLBA Parallax for the Hyperfast Pulsar B1508+55
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• VLBI astrometry of circumstellar OH masers; proper motions and parallaxes of four AGB stars
• Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars
• First Fringe Detection with VERA's Dual-Beam System and its Phase Referencing Capability
• Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA
• Astrometry of the stellar image of U Her amplified by the circumstellar 22 GHz water masers

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• Water vapour masers in long-period variable stars III. Mira variables U Her and RR Aql
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