Investigating Gaia EDR3 parallax systematics using asteroseismology of Cool Giant Stars observed by Kepler, K2, and TESS II. Deciphering Gaia parallax systematics using red clump stars

We analyse Gaia EDR3 parallax systematics as a function of magnitude and sky location using a recently published catalogue of 12,500 asteroseismic red-giant star distances. We selected ~ 3500 red clump (RC) stars of similar chemical composition as the optimal subsample for this purpose. We perform a detailed assessment of systematic uncertainties relevant for parallax offset estimation based on the asteroseismic distances. Following this assessment, we adopt for our baseline analysis the asteroseismic parameters measured as in Elsworth et al. (2020), spectroscopy from APOGEE (DR17), and we further restrict the sample to low-extinction RC stars with quality astrometric solutions from Gaia EDR3. We then investigated both the parallax offset relative to the published Gaia EDR3 parallaxes and the residual parallax offset after correcting Gaia EDR3 parallaxes following Lindegren et al. (2021). We find residual parallax offsets very close to zero (-1.6 +/- 0.5 (stat.) +/- 10 (syst.) muas) for stars fainter than G > 11 mag in the initial Kepler field. For 17 K2 campaigns in the same magnitude range, the residual parallax offset is +16.5 +/- 1.7 (stat.) +/- 10 (syst.) muas. At brighter magnitudes (G <= 11 mag), we find inconsistent residual parallax offsets between the Kepler field, 17 K2 campaigns, and the TESS southern continuous viewing zone, with differences of up to 60 muas. This suggests a significant dependence on sky location at bright magnitudes due to the lack of bright physical pairs available for determining the parallax offset corrections. Finally, we estimate the absolute magnitude of the RC and obtain M_Ks^RC = -1.650 +/- 0.025 mag in the 2MASS Ks-band and M_G^RC = (0.432 +/- 0.004) - (0.821 +/- 0.033) (Teff [K] - 4800K)/1000K [mag] in the Gaia G-band.