tokens: estimate of 5.36 ± 0.30 kpc was obtained by Thompson et al. ( 2001 ) from the detached eclipsing binary OGLEGC 17 (Kaluzny et al. 1996 ). A once challenging aspect of using Gaia parallaxes from the earlier DR2 release (Gaia Collaboration et al. 2018a ) to directly measure the parallax of ω Cen was the systematic uncertainty related to the parallax offset found by Lindegren et al. ( 2018 ), and which has been demonstrated to vary with source color, magnitude, and position (e.g., Arenou et al. 2018 ; Riess et al. 2018 ; Zinn et al. 2019 ). The DR2 offset was calibrated from quasars, which are bluer than the bulk of ω Cen stars and only cover a small fraction of their magnitude range. The DR2 parallax estimates for ω Cen by Gaia Collaboration et al. ( 2018b ) and by Shao Li ( 2019 ) are 0.1237 ± 0.0011 and 0.1368 ± 0.0015 mas, respectively, and are substantially smaller than other determinations, presumably because the parallax offset estimated by Lindegren et al. ( 2018 ) on the basis of quasar data is much smaller than the values found for brighter stars (Arenou et al. 2018 ; Riess et al. 2018 ; Zinn et al. 2019 ). Our own analysis of the DR2 parallax data for stars in ω Cen revealed a large trend of parallax with star magnitude and color, leaving us to conclude that a parallax from DR2 was not reliable without further characterization of its parallax offset. Thanks to the great improvement realized in EDR3, such a measurement is now feasible, and we exclude the DR2 parallax-based estimates from further consideration. Both ground-based and HST observations have shown that ω Cen has a complex structure, with many separate color–magnitude sequences (Bellini et al. 2009 , 2010 , 2017 ) that can have different structural (Bellini et al. 2017 , 2018 ; Calamida et al. 2017 ) and metallicity (Milone et al. 2017 ; Zennaro et al. 2019 ) properties, including an unusually broad range in helium abundance for a globular cluster. We would not expect this large range to influence the peak of the TRGB in the I -band as the mean helium abundance in ω Cen is not much affected by the high and low sub populations, and the TRGB is known to be quite insensitive to star formation and chemical abundance in general (McQuinn et al. 2019 ). The complex kinematic structure of ω Cen does not affect its parallax measurement, but needs to be taken into account in kinematics-based distance measurements (Watkins et al. 2015 ; Baumgardt et al. 2019 ). It is also worth recognizing that the distribution of abundances in the halos of galaxies where TRGB is often measured is largely unknown making comparisons difficult; it is not unlikely that the multiple populations of ω Cen could more closely resemble the complexity of a galaxy’s stellar halo than a simpler, single-population system. 5.2. Comparison with the TRGB Calibration Anchored to 47 Tuc The determination of the parallax to a globular cluster as massive as ω Cen represents a milestone of distance measurements and
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