# Comparison of Gaia DR2 Parallaxes of Stars with VLBI Astrometry

## TL;DR

This study compares Gaia DR2 star parallaxes with VLBI measurements, revealing a systematic offset of about -75 microarcseconds after excluding certain star types, aiding calibration of Gaia data.

## Contribution

It provides a detailed comparison between Gaia DR2 and VLBI parallaxes, quantifying the zero-point offset and identifying star types affecting accuracy.

## Key findings

- Gaia DR2 parallaxes are systematically offset by about -75 microarcseconds.
- Excluding AGB stars and binaries reduces discrepancies.
- The parallax zero-point is estimated between -100 and 0 microarcseconds.

## Abstract

We compare the parallaxes of stars from VLBI astrometry in the literature to those in the Gaia DR2 catalog. Our full sample contains young stellar objects, evolved AGB stars, pulsars and other radio stars. Excluding AGB stars, which show significant discrepancies between Gaia and VLBI parallaxes, and stars in binary systems, we obtain an average, systematic, parallax offset of $-75 \pm 29~\mu$as for Gaia DR2, consistent with their estimate of a parallax zero-point between $-100$ and 0 $\mu$as.