Independent analysis of μδ data of ILS and INDLS catalogs to obtain the spin of the bright Gaia DR2 reference frame

Abstract

The Gaia DR2 reference frame should be without relative rotation to the quasars (QSOs) and consistent with the International Celestial Reference System (ICRS). For the faint part of DR2 (stars with Gaia magnitude G ≥ 16) that task was done via Gaia's observations of QSOs (G ≥ 17 mag), but the bright DR2 (G ≤ 13 mag) is difficult to validate and it rotates relative to the faint DR2 at rate of the order of 0.1 mas/yr. Very bright DR2 stars (G ≤ 6 mag) mostly have inferior astrometry. Here, the aim is to determine two spin components (ωX and ωY ) of the bright DR2 using International Latitude Service (ILS, for 387 stars) and independent latitude stations (INDLS, for 682 stars) catalogs of proper motion in declination μδ; both are referred to the Hipparcos reference frame and their stars are mostly from 4 to 8 mag in the V-band (critical part of DR2). Also, using the new Hipparcos (NHIP) values μδ for ILS and INDLS stars, we can see that the merit of the ILS and INDLS is the long time baseline (Δt ≈ 90 years) important for μδ because the standard deviation of μδ is opposite to Δt. Applying the least squares method (LSM) to the differences of μδ between two catalogs (ILS-DR2, INDLS-DR2, etc.), our results support the mentioned spin. The 3σ criterion and Tukey's fences method were used to reject some stars, the Abbe criterion to explain the variability in ILS-DR2 and other μδ differences, and the Shapiro-Wilk test to check the standard distribution of differences. The obtained ωγ is significant at the 2σ level, and the ILS and INDLS catalogs could be useful for validation of the bright reference frame of Gaia DR2.

Keywords

• catalogs
• surveys
• proper motions
• reference systems