Testing the Radius Scaling Relation with Gaia DR2 in the Kepler Field

We compare radii based on Gaia parallaxes to radii based on asteroseismic scaling relations for ∼300 dwarfs and subgiants and ∼3600 first-ascent giants from the Kepler mission. Systematics due to temperature, bolometric correction, extinction, asteroseismic radius, and the spatially correlated Gaia parallax zero-point contribute to a 2% systematic uncertainty on the agreement in Gaia–asteroseismic radius. We find that dwarf and giant scaling radii are on a parallactic scale at the level of −2.1% ± 0.5% (rand.) ± 2.0% (syst.) (dwarfs) and +1.7% ± 0.3% (rand.) ± 2.0% (syst.) (giants), supporting the accuracy and precision of scaling relations. In total, the 2% agreement that we find holds for stars spanning radii between 0.8 R ⊙ and 30 R ⊙ . We do, however, see evidence for relative errors in scaling radii between dwarfs and giants at the level of 4% ± 0.6%, and find evidence of departures from simple scaling relations for radii above 30 R ⊙ . Asteroseismic masses for very metal-poor stars are still overestimated relative to astrophysical priors, but at a reduced level. We see no trend with metallicity in radius agreement for stars with −0.5 < [Fe/H] < +0.5. We quantify the spatially correlated parallax errors in the Kepler field, which globally agree with the Gaia team’s published covariance model. We provide Gaia radii, corrected for extinction and the Gaia parallax zero-point, for our full sample of ∼3900 stars, including dwarfs, subgiants, and first-ascent giants.