Confirmation of the ${\rm \it Gaia}$ DR2 parallax zero-point offset using asteroseismology and spectroscopy in the ${\rm \it Kepler}$ field

TL;DR

This study confirms that Gaia DR2 parallaxes are systematically underestimated by approximately 50 microarcseconds, using asteroseismic data of evolved stars in the Kepler field, accounting for color and magnitude dependencies.

Contribution

It provides an independent, asteroseismology-based validation of Gaia DR2 parallax zero-point offset, including detailed dependence on color and magnitude, specific to the Kepler field.

Findings

Gaia parallaxes are too small by about 50 microarcseconds.

The offset depends on Gaia's pseudo-color and G-band magnitude.

Results are consistent with previous independent studies.

Abstract

We present an independent confirmation of the zero-point offset of ${\rm \it Gaia}$ Data Release 2 (DR2) parallaxes using asteroseismic data of evolved stars in the ${\rm \it Kepler}$ field. Using well-characterized red giant branch (RGB) stars from the APOKASC-2 catalogue we identify a ${\rm \it Gaia}$ astrometric pseudo-color ($\nu_{\rm eff}$)- and ${\rm \it Gaia}$ $G$-band magnitude-dependent zero-point offset of $\varpi_{\rm seis} - \varpi_{Gaia} = 52.8 \pm 2.4 {\rm\ (rand.)} \pm 8.6 {\rm\ (syst.)} - (150.7 \pm 22.7)(\nu_{\rm eff} - 1.5) - (4.21 \pm 0.77)(G - 12.2) \mu{\rm as}$, in the sense that ${\rm \it Gaia}$ parallaxes are too small. The offset is found in high and low-extinction samples, as well as among both shell H-burning red giant stars and core He-burning red clump stars. We show that errors in the asteroseismic radius and temperature scales may be distinguished from errors in the ${\rm \it Gaia}$ parallax scale. We estimate systematic effects on the inferred global ${\rm \it Gaia}$ parallax offset, $c$, due to radius and temperature systematics, as well as choices in bolometric correction and the adopted form for ${\rm \it Gaia}$ parallax spatial correlations. Because of possible spatially-correlated parallax errors, as discussed by the ${\rm \it Gaia}$ team, our ${\rm \it Gaia}$ parallax offset model is specific to the ${\rm \it Kepler}$ field, but broadly compatible with the magnitude- and color-dependent offset inferred by the ${\rm \it Gaia}$ team and several subsequent investigations using independent methods.