# Parameters of the Link between the Optical and Radio Frames from Gaia DR2 Data and VLBI Measurements

## TL;DR

This study compares Gaia DR2 optical data with VLBI radio measurements for 88 radio stars, estimating the systematic offset between the frames and confirming the stability of the proper motion reference frame within 3 kpc.

## Contribution

It provides a new estimate of the optical-radio frame offset and confirms the near-identity of the parallax scale and the absence of significant rotation in the proper motion frame.

## Key findings

- Gaia DR2 parallaxes should be increased by 0.038 mas.
- Parallax scale factor is approximately 1.002 within 3 kpc.
- No significant rotation detected in the proper motion differences.

## Abstract

Based on published data, we have assembled a sample of 88 radio stars for which there are both trigonometric parallax and proper motion measurements in the Gaia DR2 catalogue and VLBI measurements. A new estimate of the systematic offset between the optical and radio frames has been obtained by analyzing the GaiaDR2-VLBI trigonometric parallax differences: $\Delta\pi=-0.038\pm0.046$ mas (with a dispersion of 0.156 mas). This means that the Gaia DR2 parallaxes should be increased by this correction. The parallax scale factor is shown to be always very close to unity within $\sim$3 kpc of the Sun: $b=1.002\pm0.007.$ Our analysis of the proper motion differences for the radio stars based on the model of solid-body mutual rotation has revealed no rotation components differing significantly from zero: $(\omega_x,\omega_y,\omega_z)=(-0.14,0.03,-0.33)\pm(0.15,0.22,0.16)$ mas yr$^{-1}.$