Orbital parallax of binary systems compared to Gaia DR3 and the parallax zero-point offset at bright magnitudes

Abstract

Multiple systems for which the astrometric and spectroscopic orbit are known offer the unique possibility of determining the distance to these systems directly without any assumptions. They are therefore ideal objects for a comparison of Gaia data release 3 (GDR3) parallax data, especially since GDR3 presents the results of the non-single star (NSS) analysis that potentially results in improved parallaxes. This analysis is relevant in studying the parallax zero-point offset (PZPO) that is crucial in improving upon the distance scale. An sample of 192 orbital parallax determinations for 186 systems is compiled from the literature. The stars are also potentially in wide binary systems (WBS). A search was performed and 37 WBS (candidates) were found. Only for 21 objects does the NSS analysis provide information, including 8 from the astrometric binary pipeline, for which the parallaxes do improve significantly compared to those in the main catalogue with significant lower goodness-of-fit (GOF) parameters. It appears that most of the objects in the sample are eliminated in the pre-filtering stage of the NSS analysis. The difference between the orbital parallax and the (best) Gaia parallax was finally obtained for 170 objects. A raw comparison is meaningless, however, due to limitations in accuracy both in the orbital and in Gaia data. As many systems have been eliminated in the pre-filtering stage of the astrometric NSS pipeline, they remain in GDR3 with values for the GOF parameter in the range from several tens to several hundreds. When objects with large parallax errors or unrealistically large differences between the orbital and Gaia parallaxes are eliminated, and objects with a GOF <100 or < 8 are selected (the latter also with G < 10.5 mag selected), samples of 68 and 20 stars remain. Parallax differences in magnitude bins and for the sample are presented. Three recipes from the literature that calculate the PZPO are tested. After these corrections are applied the remaining parallax differences are formally consistent with zero within the error bar for all three recipes. In all cases, an uncertainty in these averages of about 10–15 µas remains for these samples due to the small number statistics. The proof of concept of using orbital parallaxes is shown to work, but the full potential is not reached as an improved parallax from the NSS analysis is available for only for eight systems. In the final selection, the orbital parallax of 18 of 20 stars is known to better than 5%, and the parallax determination for 6 stars is better than from Gaia. In the full sample, 148 objects reach this precision in orbital parallax and therefore the full potential of using orbital parallaxes may hopefully be reached with GDR4.