Systematic effect of the Galactic aberration on the ICRS realization and the Earth orientation parameters

Abstract

Context. The curvature of the motion of the solar system barycenter around the Galactic center induces an aberration effect varying linearly with time. It can be called the “Galactic aberration” and is also known as the “secular aberration (drift)” or “aberration in proper motions”. This results in a systematic dipole pattern of the apparent proper motions of an ensemble of distant extragalactic objects, which are used to define the International Celestial Reference System (ICRS).Aims. The purpose of this paper is to investigate the effect of the Galactic aberration on the ICRS realization and on the Earth orientation parameters (EOP), which refer to the ICRS.Methods. We first computed the global rotation of the celestial reference system resulting from the Galactic aberration effect on the apparent proper motions of the ensemble of extragalactic objects that realize this system. Then we evaluated the influence of the Galactic aberration on the EOP using CIO based ICRS-to-ITRS coordinate transformation. Numerical evaluations of the effect were performed with the ICRF1 and ICRF2 catalogs over short and long time intervals.Results. We show that the effect of the Galactic aberration strongly depends on the distribution of the sources that are used to realize the ICRS. According to different distributions of sources (of the ICRF1 and ICRF2 catalogs) the amplitude of the apparent rotation of the ICRS is included between about 0.2 and 1 microarcsecond per year (μ as yr-1 ). We show that this rotation has no component around the axis pointing to the Galactic center and has an zero amplitude in the case of uniform distribution of sources. The effect on the coordinates of the Celestial intermediate pole (CIP) is included between about 1 to 100 μ as after one century from J2000.0, while the effects on the Earth rotation angle (ERA) are from 4 to several tens of μ as after one century. Conclusions. We demonstrate that the Galactic aberration is responsible for a variation with time of the orientation of the ICRS axes and consequently for systematic errors on the determination of the EOP, which refer to the ICRS. The effect on the ICRS and EOP increases with time and is not negligible after several tens of years. With high-accuracy astrometry and the increasing length of the available VLBI observation time series, this effect should be considered, particularly in constructing the next realization of the ICRS. Observations of more radio sources, especially in the southern hemisphere should be developed to more homogeneously distribute defining sources in the ICRF to minimize that effect.