Comparison of proper motions in declination for 387 Gaia DR2 and HIPPARCOS stars from ILS observations over many decades
Authors: Damljanović G. , Taris F. 
Published: October 2019

Context. The second solution of the Gaia catalog, which has been available since April 2018, plays an important role in the realization of the future Gaia reference frame. Since 1997, the reference frame has been materialized by the optical HIPPARCOS positions of about 120 000 stars. The HIPPARCOS has been compared with and linked to the International Celestial Reference Frame (ICRF). The ICRF is materialized by means of the radio positions of extragalactic sources using very large baseline interferometry observations. Both, the HIPPARCOS and Gaia missions belong to the European Space Agency, and it is important to note that the Gaia catalog is going to replace the HIPPARCOS catalog.
Aims. It has been shown that the International Latitude Service zenith telescope data pertaining to ground-based surveys that span a time baseline of about 80 yr, and which are also key when measuring proper motions, could be useful for the accurate determination of μδ for 387 ILS stars. Therefore, in this study we aim first to reduce these stars to the HIPPARCOS reference system; second, to made our original catalog of μδ, which we refer to as the ILS catalog, for these 387 bright stars; third, to present comparison results of the four catalogs by pairs (the ILS, HIPPARCOS or HIP, new HIPPARCOS or NHIP, and Gaia DR2); and fourth, to analyze the differences in μδ between pairs of catalogs to characterize the μδ errors for these catalogs with a special focus on the Gaia DR2 and ILS catalogs.
Methods. At seven ILS sites around the world at latitude 39.°1, a set of seven telescopes was used to monitor the latitude variation via observations of the same stars for about 80 yr. Here, the inverse task was applied to improve μδ values of the 387 HIPPARCOS stars using the previously mentioned observations. Due to the specific Horrebow-Talcott method of the measured star pair, it is difficult to determine μδ for each single star. However, we achieved this by developing the original method and in combination with the HIPPARCOS data. We used the previously developed least squares method and formula to determine the coefficients, which describe the systematic part of differences in μδ between the pairs of catalogs.
Results. We calculated the coefficients with the aforementioned formula (in line with the coordinates, stellar magnitude, and color index of every star) to compare ILS, HIP, NHIP, and Gaia DR2 data of μδ against each other by using the set of 387 stars. The presented differences of μδ show that the systematic errors in the four catalogs are nearly at the same level of 0.1 mas yr−1. This means that the DR2 and ILS μδ values are in good agreement with each other, and with values from the HIPPARCOS and new HIPPARCOS catalogs. Also, the random errors of differences are small ones; they are near 1 mas yr−1 for ILS-HIP and ILS-NHIP, and about 2 mas yr−1 for ILS-DR2, HIP-DR2, and NHIP-DR2. It is important to note that there is a similar level of proper motion formal errors in HIPPARCOS and new HIPPARCOS catalogs.