Aims. In order to investigate the systematic errors in the very long baseline interferometry (VLBI) positions of extragalactic sources (quasars) and the global differences between Gaia and VLBI catalogs, we use the first data release of Gaia (Gaia DR1) quasar positions as the reference and study the positional offsets of the second realization of the International Celestial Reference Frame (ICRF2) and the Goddard VLBI solution 2016a (gsf2016a) catalogs. Methods. We select a sample of 1032 common sources among three catalogs and adopt two methods to represent the systematics: considering the differential orientation (offset) and declination bias; analyzing with the vector spherical harmonics (VSH) functions. Results. Between two VLBI catalogs and Gaia DR1, we find that: i) the estimated orientation is consistent with the alignment accuracy of Gaia DR1 to ICRF, of ~0.1 mas, but the southern and northern hemispheres show opposite orientations; ii) the declination bias in the southern hemisphere between Gaia DR1 and ICRF2 is estimated to be +152 μas, much larger than that between Gaia DR1 and gsf2016a which is +34 μas. Between two VLBI catalogs, we find that: i) the rotation component shows that ICRF2 and gsf2016a are generally consistent within 30 μas; ii) the glide component and quadrupole component report two declination-dependent offsets: dipolar deformation of ~+50 μas along the Z-axis, and quadrupolar deformation of ~−50 μas that would induce a pattern of sin2δ. Conclusions. The significant declination bias between Gaia DR1 and ICRF2 catalogs reported in previous studies is possibly attributed to the systematic errors of ICRF2 in the southern hemisphere. The global differences between ICRF2 and gsf2016a catalogs imply that possible, mainly declination-dependent systematics exit in the VLBI positions and need further investigations in the future Gaia data release and the next generation of ICRF.

Aims. The first Gaia data release (Gaia DR1) provides 2 191 ICRF2 sources with their positions in the auxiliary quasar solution and five astrometric parameters -positions, parallaxes, and proper motions -for stars in common between the Tycho-2 catalogue and Gaia in the joint Tycho-Gaia astrometric solution (TGAS). We aim to analyze the overall properties of Gaia DR1 reference frame. Methods. We compare quasar positions of the auxiliary quasar solution with ICRF2 sources using different samples and evaluate the influence on the Gaia DR1 reference frame owing to the Galactic aberration effect over the J2000.0-J20015.0 period. Then we estimate the global rotation between TGAS with Tycho-2 proper motion systems to investigate the property of the Gaia DR1 reference frame. Finally, the Galactic kinematics analysis using the K-M giant proper motions is performed to understand the property of Gaia DR1 reference frame.Results. The positional comparison between the auxiliary quasar solution and ICRF2 shows negligible orientation and validates the declination bias of ∼−0.1 mas in Gaia quasar positions with respect to ICRF2. Galactic aberration effect is thought to cause an offset ∼0.01 mas of the Z axis direction of Gaia DR1 reference frame. The global rotation between TGAS and Tycho-2 proper motion systems, obtained by different samples, shows a much smaller value than the claimed value 0.24 mas yr −1 . For the Galactic kinematics analysis of the TGAS K-M giants, we find possible non-zero Galactic rotation components beyond the classical Oort constants: the rigid part ω Y G = −0.38±0.15 mas yr −1 and the differential part ω ′ Y G = −0.29±0.19 mas yr −1 around the Y G axis of Galactic coordinates, which indicates possible residual rotation in Gaia DR1 reference frame or problems in the current Galactic kinematical model. Conclusions. The Gaia DR1 reference frame is well aligned to ICRF2, and the possible influence of the Galactic aberration effect should be taken into consideration for the future Gaia-ICRF link. The cause of the rather small global rotation between TGAS and Tycho-2 proper motion systems is unclear and needs further investigation. The possible residual rotation in Gaia DR1 reference frame inferred from the Galactic kinematic analysis should be noted to and examined in future data release.

We use high-precision kinematic data from Gaia data release 2 (DR2) and spectroscopic data from Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) data release 5 (DR5) to explore the kinematics of solar neighborhood dwarf stars in the northern and southern sides of the Galactic plane. The metallicity range of the sampled stars is −0.5 < [Fe/H] < 0.5. Systematic north–south asymmetries in stellar radial motion and rotation are detected: stars in the north disk are moving toward the outer disk, while stars in the south disk have inward-streaming motion toward the Galactic center; stars in the north disk rotate slower than stars in the south disk in general. The tilt angle is antisymmetric about the Galactic plane: for an assumed Galactocentric distance of the Sun, R 0 = 8.122 kpc, the increase in the tilt angle α Rz with the vertical distance z is well described by the relation . The variation of stellar kinematics with the vertical distance to the Galactic plane has no essential dependence on the metallicity.

Context. The celestial reference frame is established on the basis of the absolute positions of extragalactic sources that are assumed to be fixed in space. The fixing of the axes is one of the crucial points for the concept behind the International Celestial Reference System (ICRS). However, due to various effects such as its intrinsic activity, the apparent position of the extragalactic sources may vary with time, resulting in a time-dependent deviation of the frame axes that are defined by the positions of these sources. Aims. We aim to evaluate the axis stability of the third realization of the International Celestial Reference Frame (ICRF3). Methods. We first derived the extragalactic source position time series from observations of very long baseline interferometry (VLBI) at the dual S∕X-band (2.3/8.4 GHz) between August 1979 and December 2020. We measured the stability of the ICRF3 axes in terms of the drift and scatter around the mean: (i) we estimated the global spin of the ICRF3 axes based on the apparent proper motion (slope of the position time series) of the defining sources of the ICRF3; (ii) we also constructed the yearly representations of the ICRF3 through annually averaged positions of the defining sources of the ICRF3 and estimated the dispersion in the axes orientation of these yearly frames. Results. The global spin is no higher than 0.8 μas yr-1 for each ICRF3 axis with an uncertainty of 0.3 μas yr-1, corresponding to an accumulated deformation smaller than 30 μas for the celestial frame axes during 1979.6–2021.0. The axes orientation of the yearly celestial frame becomes more stable as time elapses, with a standard deviation of 10 μas–20 μas for each axis. Conclusions. The axes of the ICRF3 are stable at approximately 10 μas–20 μas from 1979.6–2021.0 and the axes stability does not degrade after the adoption of the ICRF3.

Aims. In order to check the astrometric solution quality, dependences of parallaxes and proper motions on precision, reliability, and consistency of sample solutions are studied for the quasars in the celestial reference frame of the second release of Gaia data (Gaia-CRF2). Methods. Astrometric statistics (the number of visibility periods, the semi-major axis of the astrometric five-dimensional error ellipse σ 5d, max , the unit weight error u, the correlation coefficients ρ µ α * , and ρ µ δ , ) 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. We also investigated mean values of astrometric statistics in equal-area spherical cells to study the impact of the scanning law.Results. 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, ρ µ α * , and ρ µ δ , obtained from the astrometric solutions show a generally ideal distribution for the full sample. 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 periods. The quasars with visibility periods in the domain [13, 16], with ρ µ α * , and ρ µ δ , 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.