Confirmation of the zero-point offset in Gaia Data Release 2 parallaxes using asteroseismology and APOGEE spectroscopy in the Kepler field

Joel C. Zinn, Marc H. Pinsonneault, Daniel Huber, Dennis Stello

Published 2018-05-07Version 1

We present an independent confirmation of the zero-point offset of Gaia Data Release 2 (DR2) parallaxes using asteroseismic data of evolved stars in the Kepler field. Using well-characterized red giant branch (RGB) stars from the APOKASC-2 catalogue we identify a color- and magnitude-dependent zero-point offset of $52.8 \pm 2.4 (stat.) \pm 1 (syst.) \muas$, in the sense that Gaia parallaxes are too small. The offset is present at nearly the same level in high and low extinction samples, and remains whether using Gaia DR2 or APOKASC-2 extinction estimates derived from SED fitting. The same exercise, performed with core He-burning red clump stars, yields an offset in the same sense of $50.2 \pm 2.5 (stat.) \pm 1 (syst.) \muas$. We argue that the difference between these two measures likely reflects known systematic differences in the APOKASC-2 radius scale of RC and RGB stars at the $1\%$ level. We recommend adopting the RGB scale because the APOKASC-2 sample was calibrated on RGB stars. Because of possible spatially-correlated parallax errors, as discussed by the Gaia team, our zero-point solution is specific to the Kepler field, but broadly compatible with the global zero-point uncertainty inferred by the Gaia team and independent results using Cepheids. Additionally, there is evidence for a larger absolute offset error for smaller parallaxes, which we will explore in future work.