The parallax zero point offset from Gaia EDR3 data

Abstract

The second data release of Gaia revealed a parallax zero point offset of −0.029 mas based on quasars. The value depended on the position on the sky, and also likely on magnitude and colour. The offset and its dependence on other parameters inhibited an improvement in the local distance scale using e.g. the Cepheid and RR Lyrae period-luminosity relations. Analysis of the recent Gaia Early Data Release 3 (EDR3) reveals a mean parallax zero point offset of −0.021 mas based on quasars. The Gaia team addresses the parallax zero point offset in detail and proposes a recipe to correct for it, based on ecliptic latitude, G-band magnitude, and colour information. This paper is a completely independent investigation into this issue focussing on the spatial dependence of the correction based on quasars and the magnitude dependence based on wide binaries. The spatial and magnitude corrections are connected to each other in the overlap region between 17 < G < 19. The spatial correction is presented at several spatial resolutions based on the HEALPix formalism. The colour dependence of the parallax offset is unclear and in any case secondary to the spatial and magnitude dependence. The spatial and magnitude corrections are applied to two samples of brighter sources, namely a sample of ∼100 stars with independent trigonometric parallax measurements from HST data, and a sample of 75 classical cepheids using photometric parallaxes. The mean offset between the observed GEDR3 parallax and the independent trigonometric parallax (excluding outliers) is about −39 µas, and after applying the correction it is consistent with being zero. For the classical cepheid sample it is suggested that the photometric parallaxes may be underestimated by about 5%.

Substructure in the stellar halo near the Sun. II. Characterisation of independent structures

“…Although it is possible to improve the characterisation of this parallax offset in Gaia EDR3 (see e.g. Lindegren et al 2021;Ren et al 2021;Groenewegen 2021), this turns out to have a negligible effect on our analyses. We also correct for the effect of reddening on the magnitudes of individual stars using the dust map from Lallement et al (2018) and the recipes to transform to Gaia magnitudes provided in Gaia Collaboration et al (2018a) 5 .…”

Cepheid Metallicity in the Leavitt Law (C-MetaLL) survey: I. HARPS-N@TNG spectroscopy of 47 Classical Cepheid and 1 BL Her variables

“…The fact that the ZPO correction and the metallicity terms appear degenerate means that it is important to have independent estimates of the parallax ZPO. In this sense we will test the recipe to calculate individual parallax ZPOs recently published in pre-print form by Groenewegen (2021). In any case, it seems that we are still not close to have a precision of 3-4 𝜇as on the parallax ZPO that is mandatory to derive the H 0 constant with 1% precision.…”

The Photo-Astrometric Vertical Tracer Density of the Milky Way II: Results from Gaia

“…We apply the zero-point correction recommended in Lindegren et al 2021b for sources with 5 and 6 parameter astrometric solutions. Many other groups have attempted to measure the zero point parallax offset from Cepheid variables (Riess et al 2021), Red Clump stars (Huang et al 2021), eclipsing binaries (Stassun & Torres 2021;Ren et al 2021) and quasars (Groenewegen 2021) (although the Lindegren et al 2021b model was constructed using quasars so it is unsurprising that these results match well). The conclusions are that for the majority of sources, the parallax offset is reduced to under 10𝜇as.…”