Astrometry From Hipparcos to Gaia: Parallax Discrepancy

In spite of the fact that Gaia astrometric space mission promises a dramatic improvement in the available data of the parallax measurements, we still find discrepancy in the measurements between Gaia and Hipparcos catalogues that cannot be ignored. We present the results of comparing parallax measurements given by the catalogues of Hipparcos 2, Gaia DR2, and Gaia EDR3. We focused on stars with parallax discrepancy between Hip 2 and Gaia DR2 greater than 5 mas, as clear cases which need to be solved using either new measurements or another computational technique. Among a sample of around 1700 BMSSS, with known orbits from the sixth.., we found that there are 55 systems have parallax discrepancies between Gaia DR2 and Hip 2. 10 of these these systems have no parallax measurements in Gaia EDR3. The study shows that there is a clear problem in Gaia DR2 parallax measurements for BMSSs. And in spite of the fact that Gaia ERD3 improved the measurements, there are still some systems with a clear problems in their parallaxes, with favoring to Hip 2. We present a solution for one of these cases as an example. It is the stellar system Hip 45858. Where we found that Hip 2 parallax measurement is the closest to the dynamical one calculated in this work, which is the best way for identifying parallaxes of BMSSs. Our main recommendation is that Gaia parallax measurements of BMSSs need special attention in the data reduction process