A Research On Astrometric Parameters Of Quasars In Gaia-CRF2

One of the key science objectives of the European Space Agency' s Gaia mission is to build a rotation-free celestial reference frame in the visible wavelengths,which is called the Gaia Celestial Reference Frame(Gaia-CRF).The second release of Gaia data(Gaia DR2)contains the astrometric parameters for more than half a million quasars.These sources and their positions are a version of the Gaia-CRF called Gaia-CRF2.The astrometric properties of the quasars selected to represent Gaia-CRF2 need to be investigated carefully.In this paper,dependences of parallaxes and proper motions on precision,reliability,and consistency of sample solutions are studied for the quasars in Gaia-CRF2.Astrometric statistics(the number of visibility periods,the semi-major axis of the astrometric five-dimensional error ellipse ?5d,max,the unit weight error u,the correlation coefficients ???*,? and ???,?)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.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,???*,?and???,? obtained from the astrometric solutions show a generally ideal distribution for the full sample.We also investigated mean values of astrometric statistics in equal-area spherical cells to study the impact of the scanning law.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 pe-riods.The quasars with visibility periods in the domain[13,16],with ???*,? and ???,?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.With a decomposition on a set of orthogonal vector spherical harmonics(VSH),we studied the proper motion components of a quasar sample with different visibility periods.The quasar samples with visibility periods in the domain[13,16]have a spherical vector field close to the theoretical value while the proper motion vector fields of quasars with other visibility periods are less reliable.The main contents of this paper are:1.The dependences of the astrometric parameters,parallaxes and proper motions,on indicators of solution qualities are evaluated.Some strange systematic trends of these astrometric parameters are found.2.We investigated mean values of astrometric statistics in equalarea spherical cells to study the impact of the scanning law.Distributions of two correlation coefficients are found to correlate with the number of visibility periods,which can give rise to the strange systematic trends of the astrometric parameters.3.We use a decomposition on VSH to study the proper motion vector fields of quasar samples with different visibility periods.The quasar samples with visibility periods in the domain[13,16]have a spherical vector field close to the theoretical value.