Latitude observations of the last century to obtain accurate independent latitude stations (INDLS) catalog of µ_δ for 682 Hipparcos stars and comparison with Gaia DR2

Abstract

The independent latitude stations (INDLS) catalog of proper motion in declination μδ for 682 bright Hipparcos (HIP) stars is presented. It is obtained in the Hipparcos reference frame, and only the Hipparcos mean position at epoch 1991.25 is used for INDLS data to calculate independent μδ (the HIP μδ values are not included in the INDLS data). Catalog comparisons were made in pairs for the four catalogs (HIP, new Hipparcos—NHIP, Gaia DR2, INDLS) to look for possible systematic differences as a function of brightness, color, and position coordinates (α, δ). All catalogs were found to be in agreement, except 0.21 mas year−1 for differences in INDLS–DR2 in line with the brightness; −0.13 and 0.24 mas year−1 for differences in HIP–DR2 in line with the free term and brightness, respectively; and −0.16, 0.23, and 0.13 mas year−1 for differences NHIP–DR2 in line with the free term, brightness, and color, respectively. A slightly better consistency is observed in the cases of INDLS–NHIP and INDLS–DR2 than HIP–DR2 and NHIP–DR2. The systematic error is in line with brightness on the level 0.1 mas year−1 in DR2 and, near 0.1 mas year−1, in line with color. An indication that the bright reference frame of DR2 rotates by about 0.1 mas year−1 relative to the faint quasar frame is in line with a presented sinusoidal curve of differences μδ as a function of α. Its amplitude is A = 0.53 ± 0.11 mas year−1 for INDLS–DR2 (0.26 ± 0.07 for INDLS–NHIP); about 0.2 belongs to DR2. The level of formal errors is near 1.9 mas year−1 for the case INDLS–DR2 and near 1.5 for: INDLS–NHIP, HIP–DR2, and NHIP–DR2. The values μδ of bright DR2 stars could be underestimated; the astrometry is unreliable for G ≤ 6 mag due to detector saturation.