SIMBAD references

Aims. In order to check the astrometric solution quality, dependences of parallaxes and proper motions on precision, reliability, and consistency of sample solutions are studied for the quasars in the celestial reference frame of the second release of Gaia data (Gaia-CRF2).
Methods. Astrometric statistics (the number of visibility periods, the semi-major axis of the astrometric five-dimensional error ellipse σ_5d,max, the unit weight error u, the correlation coefficients ρ_µα*,ϖ and ρ_µδ,ϖ) were selected to serve as indicators of the solution qualities of quasars. The dependences of the astrometric parameters, parallaxes and proper motions, on these indicators are evaluated. We also investigated mean values of astrometric statistics in equal-area spherical cells to study the impact of the scanning law.
Results. The astrometric parameters of quasars with fewer than 9 or more than 18 visibility periods show a departure from the global average. Moreover, the mean values of astrometric parameters of the most precise sources deviate from the others. Astrometric parameters are stable for those quasars fitting the five-parameter model well. The correlation coefficients, ρ_µα*,ϖ and ρ_µδ,ϖ obtained from the astrometric solutions show a generally ideal distribution for the full sample. Spherical-cell mean values of these correlation coefficients are found to have a centrally asymmetric distribution. Distributions of two correlation coefficients are found to correlate with the number of visibility periods. The quasars with visibility periods in the domain [13, 16], with ρ_µα*,ϖ and ρ_µδ,ϖ generally well-distributed have more reliable astrometric parameters. Magnitudes and colours are found to have little influence on the irregular patterns of the correlation coefficients.