Improved VLBI astrometry of OH maser stars

TL;DR

This paper demonstrates that VLBI astrometry using in-beam phase referencing significantly improves distance measurements to OH maser stars, enabling accurate parallax and proper motion determinations for evolved stars obscured in optical wavelengths.

Contribution

The study introduces an improved VLBI astrometric technique with in-beam phase referencing, enhancing distance accuracy to OH maser stars compared to previous methods.

Findings

Significant improvement in parallax and proper motion measurements for U Her, S CrB, and RR Aql.

Distances to S CrB and RR Aql are consistent with P-L relations, U Her's distance is smaller.

In-beam phase referencing and low solar activity improve astrometric accuracy.

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.