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 because (1) their similar luminosity allows for straightforward interpretation of trends with apparent magnitude; (2) RC stars are the most distant stars in our sample at a given apparent magnitude, so uncertainties related to asteroseismic radii and distances are the smallest; (3) and they provide the largest sample of intrinsically similar stars. We performed a detailed assessment of systematic uncertainties relevant for parallax offset estimation based on the asteroseismic distances. Specifically, we investigated (1) the impact of measuring the basic asteroseismic quantities νmax and 〈Δν〉 using different pipelines, (2) uncertainties related to extinction, (3) the impact of adopting spectroscopic information from different surveys, and (4) blending issues related to photometry. Following this assessment, we adopted for our baseline analysis the asteroseismic parameters measured in Elsworth et al. (2020, Res. Notes Am. Astron. Soc., 4, 177) and spectroscopy from the Apache Point Observatory Galactic Evolution Experiment (DR17), and we further restricted the sample to low-extinction (AV ≤ 0.5 mag) RC stars with quality astrometric solutions from Gaia EDR3, as indicated by RUWE < 1.4. 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, A&A, 649, A4). We found residual parallax offsets very close to zero (-1.6 ± 0.5 (stat.)±10 (syst.) µas) for stars fainter than G > 11 mag in the initial Kepler field, suggesting that the Lindegren parallax offset corrections are adequate in this magnitude range. For 17 K2 campaigns in the same magnitude range, the residual parallax offset is +16.5 ± 1.7 (stat.)±10 (syst.) µas. At brighter magnitudes (G ≤ 11 mag), we found inconsistent residual parallax offsets between the Kepler field, 17 K2 campaigns, and the TESS southern continuous viewing zone, with differences of up to 60 µas. This contradicts the studies that suggest a monotonic trend between magnitude and residual parallax offsets and instead suggests a significant dependence on sky location at bright magnitudes due to a lack of bright physical pairs being available to determine the parallax offset corrections. Inspection of the 17 K2 campaigns allowed for investigation of parallax offsets as a function of ecliptic longitude and revealed a possible signal. Finally, we estimated the absolute magnitude of the red clump and obtained MKsRC = -1.650 ± 0.025 mag in the 2MASS Ks band and MGRC = (0.432 ± 0.004) - (0.821 ± 0.033) · (Teff [K]-4800 K)/1000 K [mag] in the Gaia G-band.