# Astrometry of OH/IR stars using 1612 MHz hydroxyl masers. I. Annual parallaxes of WX Psc and OH138.0+7.2

## TL;DR

This study measures the distances to two evolved stars using hydroxyl maser parallaxes with VLBA, introducing a new error analysis method and confirming the accuracy of trigonometric distances against phase-lag estimates.

## Contribution

It provides the first hydroxyl maser parallax below one milliarcsecond and presents a novel error analysis technique for astrometric measurements.

## Key findings

- Parallax of WX Psc is <=5.3 mas, distance >190 pc.
- Parallax of OH138.0+7.2 is 0.52±0.09 mas, distance ~1.9 kpc.
- Good agreement between trigonometric and phase-lag distances.

## Abstract

We report on the measurement of the trigonometric parallaxes of 1612 MHz hydroxyl masers around two asymptotic giant branch stars, WX Psc and OH138.0+7.2, using the NRAO Very Long Baseline Array with in-beam phase referencing calibration. We obtained a 3-sigma upper limit of <=5.3 mas on the parallax of WX Psc, corresponding to a lower limit distance estimate of >~190 pc. The obtained parallax of OH138.0+7.2 is 0.52+/-0.09 mas (+/-18%), corresponding to a distance of 1.9(+0.4,-0.3) kpc, making this the first hydroxyl maser parallax below one milliarcsecond. We also introduce a new method of error analysis for detecting systematic errors in the astrometry. Finally, we compare our trigonometric distances to published phase-lag distances toward these stars and find a good agreement between the two methods.