Statistic of Stars with Poor Astrometric Solutions of Gaia DR3 in Open Star Clusters

We performed counts of stars with poor astrometric solutions of Gaia DR3 in the regions of open star clusters: NGC 188, NGC 1039, NGC 2287, NGC 2301, NGC 2360, NGC 2420, NGC 2527, NGC 2548, NGC 2682 (M 67), NGC 3114, NGC 3766, NGC 5460, NGC 6649. The selection of the possible cluster members is based on the Gaia photometry using the Hess diagram. We look for stars that fall within the region of the Hess diagram plotted from probable cluster members based on Hunt and Reffert data. We take stars with two-parameter solutions, with the parameter RUWE > 1. 4, as well as with large relative parallax errors that fall within the Hess diagram region for probable cluster members. The radii of clusters based on stars with poor astrometric solutions and the number of such possible cluster members were estimated. The number of stars with poor astrometric solutions relative to the number of stars from the Hunt and Reffert sample N{₁₀₃}/N varies very widely with a median average of approximately 30%. This means that when one selects probable cluster members based on precise astrometric data from Gaia DR3, an average of about 23% of cluster members may be lost. Among the lost stars there may be a significant number of unresolved binary and multiple systems. We investigated the dependence of the relative number of stars with poor astrometric solutions on the galactic latitude and on the average number density of stars. The brightness functions with and without stars with poor solutions differ significantly in the region of faint stars, G 14ᵐ, 18ᵐ, for clusters with a relative number of possible cluster members with poor astrometric solutions of N{₁₀₃}/N 0. 15. The work is partially based on a talk presented at the Modern Stellar Astronomy 2025 conference.