The Gaia DR2 parallax zero point: Hierarchical modeling of red clump stars

TL;DR

This paper develops a hierarchical probabilistic model to accurately determine Gaia DR2 parallax zero point offset and calibrate red clump star luminosities, improving distance measurements and understanding spatial variations.

Contribution

It introduces a novel hierarchical modeling approach to simultaneously estimate Gaia parallax zero point offset and red clump luminosities using combined astrometric and photometric data.

Findings

Parallax zero point offset in Gaia DR2 is -48 ± 1 μas.

Red clump absolute magnitudes are precisely measured in multiple bands.

Zero point fluctuations across the sky are within a few tens of μas.

Abstract

The systematic offset of Gaia parallaxes has been widely reported with Gaia's second data release, and it is expected to persist in future Gaia data. In order to use Gaia parallaxes to infer distances to high precision, we develop a hierarchical probabilistic model to determine the Gaia parallax zero point offset along with the calibration of an empirical model for luminosity of red clump stars by combining astrometric and photometric measurements. Using a cross-matched sample of red clump stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Gaia Data Release 2 (DR2), we report the parallax zero point offset in DR2 to be $ \varpi_0 = -48 \pm 1~\mu\text{as} $. We infer the red clump absolute magnitude to be $ -1.622 \pm 0.004 $ in $ K_s $, $ 0.517 \pm 0.004 $ in $ G $, $ -1.019 \pm 0.004 $ in $ J $, and $ -1.516 \pm 0.004 $ in $ H $. The intrinsic scatter of the red clump is $ \sim 0.09 $ mag in $ J $, $ H $ and $ K_s $, or $ \sim 0.17 $ mag in $ G $. We tailor our models to accommodate more complex analyses such as investigating the variations of the parallax zero point with each source's observed magnitude, observed colour, and sky position. In particular, we find fluctuations of the zero point across the sky to be of order or less than a few 10s of $ \mu\text{as} $.