Distance Dependent Offsets between Parallaxes for Nearby Stars and Gaia DR1 Parallaxes

TL;DR

This study evaluates Gaia DR1 parallaxes for nearby stars within 25 pc, revealing a distance-dependent offset that varies with position relative to the ecliptic, with no correlation to stellar brightness or color.

Contribution

It provides the first detailed analysis of Gaia DR1 parallax offsets for nearby stars, highlighting a distance and position-dependent systematic bias.

Findings

Gaia parallaxes are on average 0.24 mas smaller than ground-truth measurements.

The parallax offset increases with distance up to 100 pc.

Offset is approximately twice as large south of the ecliptic.

Abstract

We use 612 single stars with previously published trigonometric parallaxes placing them within 25 pc to evaluate parallaxes released in Gaia's first data release (DR1). We find that the Gaia parallaxes are, on average, $0.24 \pm 0.02$ mas smaller than the weighted mean trigonometric parallax values for these stars in the solar neighborhood. We also find that the offset changes with distance out to 100 pc, in the sense that the closer the star, the larger the offset. We find no systematic trends in the parallax offsets with stellar $V$ magnitude, $V-K$ color, or proper motion. We do find that the offset is roughly twice as large for stars south of the ecliptic compared to those that are north.