International Celestial Reference System Research Articles

How Well is the International Celestial Reference System Maintained in Official IAU Implementations?

The International Celestial Reference Frame (ICRF) based on the VLBI-derived positions of 608 extragalactic radio sources was adopted by the International Astronomical Union (IAU) in 1998 as the first realization of the International Celestial Reference System (ICRS). Later, in 2009 and 2020, two extended ICRF versions, ICRF2 and ICRF3, respectively, were released. The latter is adopted by the IAU as the current implementation of the ICRF in the radio band. In the meantime, the Gaia mission delivered three versions of the optical ICRS realization in 2016, 2018, and 2022 with an accuracy similar to that achieved by VLBI-based ICRF. The Gaia-CRF catalogs were linked to the ICRF under no-rotation conditions and thus may suffer from ICRF systematic instability if the latter is substantial. In this work, a new analysis was performed to assess the long-term stability of radio and optical ICRS realizations. Based on the 16-parameter vector spherical harmonics expansion of the differences between the three ICRF catalogs, it can be concluded that the mutual orientation between them is at a level of a few tens of microarcseconds, while the components of the glide vector and E 2,0 term are several times greater. A comparison of the three Gaia-CRF catalogs with the ICRF3-SX showed that for the latest Gaia-CRF catalog, Gaia-CRF3, all rotational and deformation components are below 20 μas except for the E 2,0 term, which is several times greater. For both ICRF and Gaia-CRF catalogs, the evolution of the source position errors is also tracked.

Enhancing the alignment of the optically bright Gaia reference frame with respect to the International Celestial Reference System

Context. The link of the Gaia frame in terms of non-rotation with respect to the International Celestial Reference System (ICRS), which is realized via very long baseline interferometry (VLBI) at radio wavelengths, has to be conducted for the wide range of optical magnitudes in which the spacecraft observes. There is a sufficient number of suitable counterparts between the two measurement systems for optically faint objects. However, the number of common optically bright (G ≤ 13 mag) objects is sparse as most are faint at radio frequencies, and only a few objects suitable for astrometry have been observed by VLBI in the past. As a result, rotation parameters for the optically bright Gaia reference frame are not yet determined with sufficient accuracy. Aims. The verification of the Gaia bright frame of DR2 and EDR3 is enhanced by the reevaluation of existing VLBI observations and the addition of newly acquired data for a sample of optically bright radio stars. Methods. Historical data from the literature were reevaluated, ensuring that the calibrator positions and uncertainties (used for the determination of the absolute star positions in the phase-referencing analysis) were updated and homogeneously referred to the ICRF3, the third realization of the International Celestial Reference Frame. We selected 46 suitable optically bright radio stars from the literature for new radio observations, out of which 32 were detected with the VLBA in continuum mode in the X or C band, along with radio-bright calibrators in the ICRF3. Improved Gaia-VLBI rotation parameters were obtained by adding new observations and utilizing more realistic estimates of the absolute position uncertainties for all phase-referenced radio observations. Results. The homogenization greatly improved the steadiness of the results when the most discrepant stars were rejected one after another through a dedicated iterative process. For Gaia DR2, this homogenization reduced the magnitude of the orientation parameters to less than 0.5 mas but increased that of the spin parameters, with the largest component being the rotation around the Y axis. An adjustment of the position uncertainties improved the reliability of the orientation parameters and the goodness of fit for the iterative solutions. Introducing the new single-epoch positions to the analysis reduced the correlations between the rotation parameters. The final spin for Gaia DR2 as determined by VLBI observations of radio stars is (−0.056, −0.113, +0.033) ± (0.046, 0.058, 0.053) mas yr−1. A comparison of the new results with external, independently derived spin parameters for Gaia DR2 reveals smaller differences than when using the historical data from the literature. Applying the VLBI data to Gaia EDR3, which was already corrected for spin during Gaia processing, the derived residual spin is (+0.022, +0.065, −0.016) ± (0.024, 0.026, 0.024)mas yr−1, showing that the component in Y is significant at the 2.4σ level. Conclusions. Even though our analysis provides a more accurate frame tie, more VLBI data are needed to refine the results and reduce the scatter between iterative solutions.

Effect of Galactic aberration on Earth orientation parameters: From the ICRF2 to the ICRF3

Aims. At the beginning of 2019 the third realization of the International Celestial Reference Frame (ICRF3) replaced the previous version (ICRF2). In the current International Celestial Reference Frame (ICRF), a major improvement is the inclusion of the Galactic aberration (GA) effect. The Earth orientation parameters (EOP), which link the celestial and terrestrial reference frames, are thus accordingly affected. This paper investigates the influence of the ICRF evolution and the modeling of the GA effect on the determination of the EOP. Methods. The EOP time series derived within the frame of two official ICRF realizations, as well as different ways of handling the GA effect, were estimated based on the very long baseline interferometry (VLBI) observational data obtained over the past 40 yr. The correlation between the station network and the GA contribution to the EOP was then analyzed by comparison of IVS R1 and R4 routine observations, which have a different distribution of antennas. We also studied the effect of GA on nutation and free core nutation amplitudes by least-squares fits. Results. The application of different reference frames (ICRF2 and ICRF3) in VLBI solutions leads to constant offsets of 315 microarcsec in the components of dX, dY, and dUT 1. This difference is mainly the reflection of the orientation offsets between the two realizations of the International Celestial Reference System. In a separate study of the GA effect, an approximate 0.3 ± 0.3 uas yr−1 bias is found in dY, while other components are not significantly affected. This bias results in an increasing offset in precession which is non-negligible after several tens of years. We further found that the bias caused by the GA effect will decrease when using a set of more uniformly distributed sources or stations in the most recent decades.

Evaluation of the ICRF stability from a position time series analysis

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.

The orbits of Triton and Nereid and the pole orientation of Neptune from Voyager,HubbleSpace Telescope, and Earth-based astrometry in 1847–2020

Context.New observations and new planetary and satellite ephemerides provide opportunities to improve the ephemerides for Triton and Nereid as well as relevant parameters. In particular, the observations include a lot of new accurate Earth-based positions reduced withGaiaastrometic catalogs and accurate positions obtained fromHubbleSpace Telescope.Aims.We aim to reliably improve the ephemerides for Triton and Nereid along with some parameters by using all the available astrometric data from 1847 to 2020 and by updating the dynamical model. We also aim to improve the geometrical descriptions based on the improved orbits of the two satellites and the pole orientation of Neptune.Methods.The orbits of Triton and Nereid are determined by fitting dynamical and observational model parameters to observations in a weighted least-squares sense. The dynamical model makes use of the new ephemerides from Jet Propulsion Laboratory for planets, DE440, and those for the inner satellites of Neptune, NEP090. For completeness, in addition to the gravitational effects considered by NEP081, the model also includes perturbations from inner satellites and a revised model for the motion of the pole orientation of Neptune. Moreover, model simplifications are investigated to speed up the motion equation integration. Since the pole orientation angles of Neptune at epoch are possibly improvable according to the preliminary post-fit sensitivity analysis, these angles are adjusted together with the satellite state vectors at epoch. Linear mapping of the covariance matrix is a measure of formal uncertainties of our orbit and pole solutions. However, to obtain more reliable accuracy estimations, it is necessary to consider the uncertainties in the observations and the unadjusted model parameters. To accomplish this, a method (BR-RS) that performs bootstrap resampling of observations (BR) and random sampling of unadjusted model parameters (RS) is used. Analytical representations are fitted to the orbit and pole solutions to provide their geometric descriptions.Results.The model we use can be fitted to the observations with their estimated accuracies. The new ephemerides,FORCES-8-MAIN-2020, covering years 1600–2650 are available online in SPICE format. The orbits are well determined with the orbital uncertainties expected to be within 200 km (about 10 mas as seen from the Earth) for Triton and 1000 km (50 mas) for Nereid for the next 100 yr as estimated by the BR-RS method. In particular, the correction in the Nereid mean orbit motion from the NEP081 solution is +4.′′9 yr−1, and has a BR-RS uncertainty of 0.′′24 yr−1. In the fitting process, we also determine the pole orientation of Neptune. At the initial epoch 1989 September 1 TDB, the right ascension and declination of the new pole orientation referred to the International Celestial Reference System areαp= 299.°339 ± 0.°012 (formal)∕ ± 0.°014 (BR-RS) andδp= 42.°985 ± 0.°016 (formal)∕ ± 0.°045 (BR-RS), respectively. From 1800 to 2200, the motion of the pole orientation is well constrained with a BR-RS uncertainty of about 0.°01–0.°05. We also provide geometrical descriptions for the new orbits and pole orientation.

Parsec-scale alignments of radio-optical offsets with jets in AGNs from multifrequency geodetic VLBI, Gaia EDR3, and the MOJAVE program

Aims. We aim to study the relative positions of quasar emission centers at different wavelengths in order to help link the various realizations of the International Celestial Reference System (ICRS), and to unveil systematic uncertainties and individual source behavior at different wavelengths. Methods. We based our study on four catalogs representing the ICRS, the ICRF3 positions in the three VLBI bands X, K, and Ka, and the Gaia EDR3 catalog in optical wavelengths. We complemented radio source positions with jet kinematics results from the MOJAVE team, allowing us to obtain jet directions on the sky. A six-parameter deformation model was used to remove systematic uncertainties present in the different catalogs. Results. For a set of 194 objects common to the four catalogs and to the objects whose jet kinematics was studied by the MOJAVE team, we computed the orientation between positions at the different wavelengths and with respect to the directions of the jets. We find that the majority of these objects have their radio-to-optical vector along the jet, with the optical centroid downstream from the radio centroids, and that the K and Ka centroids are preferably upstream in the jet with respect to the X centroid, which is consistent with the paradigm of a simple core–jet model. For a population of multiwavelength positions aligned along the jet, astrometric information can therefore be used to measure the direction of the jet independently of imaging. In addition, we find several sources for which the optical centroid coincides with stationary radio features with a relatively high fraction of polarization, indicating optical emission dominated by a synchrotron process in the jet.

New precise positions in 2013–2019 and a catalog of ground-based astrometric observations of 11 Neptunian satellites (1847–2019) based on Gaia-DR2

Context. Developing high-precision ephemerides for Neptunian satellites requires not only the continuation of observing campaigns but also the collection and improvement of existing observations. So far, no complete catalogs of observations of Neptunian satellites are available. Aims. We aim to provide new, precise positions, and to compile a catalog including all available ground-based astrometric observations of Neptunian satellites. The observations are tabulated in a single and consistent format and given in the same timescale, the Terrestrial Time (TT), and reference system, the International Celestial Reference System (ICRS), including necessary changes and corrections. Methods. New CCD observations of Triton and Nereid were made at Lijiang 2.4-m and Yaoan 0.8-m telescopes in 2013–2019, and then reduced based on Gaia-DR2. Furthermore, a catalog called OCNS2019 (Observational Catalog of Neptunian Satellites (2019 version)) was compiled, after recognizing and correcting errors and omissions. Furthermore, in addition to what was considered for the COSS08 catalog for eight main Saturnian satellites, all observed absolute and relative coordinates were converted to the ICRS with corrections for star catalog biases with respect to Gaia-DR2. New debiasing tables for both the modern and old star catalogs, which were previously not provided based on Gaia-DR2, are developed and applied. Treatment of missing positions of comparison bodies in conversions of observed relative coordinates are proposed. Results. OCNS2019 and the new debiasing tables are publicly available online. OCNS2019 includes 24996 observed coordinates of 11 Neptunian satellites obtained over 3741 nights from 1847 to 2019. All observations are given in TT and ICRS. The star catalog biases are removed, which are significant for Nereid and outer satellites. We obtained 880 (5% of total now available) new coordinates for Triton over 41 nights (1% of total observation nights so far), and 790 (14%) for Nereid over 47 nights (10%). The dispersions of these new positions are about 0.″03 for Triton and 0.″06 for Nereid. Conclusions. OCNS2019 should be useful in improving ephemerides for the above-mentioned objects.

Independent analysis of μδ data of ILS and INDLS catalogs to obtain the spin of the bright Gaia DR2 reference frame

The Gaia DR2 reference frame should be without relative rotation to the quasars (QSOs) and consistent with the International Celestial Reference System (ICRS). For the faint part of DR2 (stars with Gaia magnitude G ? 16) that task was done via Gaia's observations of QSOs (G ? 17 mag), but the bright DR2 (G ? 13 mag) is difficult to validate and it rotates relative to the faint DR2 at rate of the order of 0.1 mas/yr. Very bright DR2 stars (G ? 6 mag) mostly have inferior astrometry. Here, the aim is to determine two spin components (?X and ?Y ) of the bright DR2 using International Latitude Service (ILS, for 387 stars) and independent latitude stations (INDLS, for 682 stars) catalogs of proper motion in declination ??; both are referred to the Hipparcos reference frame and their stars are mostly from 4 to 8 mag in the V-band (critical part of DR2). Also, using the new Hipparcos (NHIP) values ?? for ILS and INDLS stars, we can see that the merit of the ILS and INDLS is the long time baseline (?t ? 90 years) important for ?? because the standard deviation of ?? is opposite to ?t. Applying the least squares method (LSM) to the differences of ?? between two catalogs (ILS-DR2, INDLS-DR2, etc.), our results support the mentioned spin. The 3? criterion and Tukey's fences method were used to reject some stars, the Abbe criterion to explain the variability in ILS-DR2 and other ?? differences, and the Shapiro-Wilk test to check the standard distribution of differences. The obtained ?? is significant at the 2? level, and the ILS and INDLS catalogs could be useful for validation of the bright reference frame of Gaia DR2.

The third realization of the International Celestial Reference Frame by very long baseline interferometry

A new realization of the International Celestial Reference Frame (ICRF) is presented based on the work achieved by a working group of the International Astronomical Union (IAU) mandated for this purpose. This new realization follows the initial realization of the ICRF completed in 1997 and its successor, ICRF2, adopted as a replacement in 2009. The new frame, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry at the standard geodetic and astrometric radio frequencies (8.4 and 2.3 GHz), supplemented with data collected at higher radio frequencies (24 GHz and dual-frequency 32 and 8.4 GHz) over the past 15 years. State-of-the-art astronomical and geophysical modeling has been used to analyze these data and derive source positions. The modeling integrates, for the first time, the effect of the galactocentric acceleration of the solar system (directly estimated from the data) which, if not considered, induces significant deformation of the frame due to the data span. The new frame includes positions at 8.4 GHz for 4536 extragalactic sources. Of these, 303 sources, uniformly distributed on the sky, are identified as “defining sources” and as such serve to define the axes of the frame. Positions at 8.4 GHz are supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources. In all, ICRF3 comprises 4588 sources, with three-frequency positions available for 600 of these. Source positions have been determined independently at each of the frequencies in order to preserve the underlying astrophysical content behind such positions. They are reported for epoch 2015.0 and must be propagated for observations at other epochs for the most accurate needs, accounting for the acceleration toward the Galactic center, which results in a dipolar proper motion field of amplitude 0.0058 milliarcsecond yr−1(mas yr−1). The frame is aligned onto the International Celestial Reference System to within the accuracy of ICRF2 and shows a median positional uncertainty of about 0.1 mas in right ascension and 0.2 mas in declination, with a noise floor of 0.03 mas in the individual source coordinates. A subset of 500 sources is found to have extremely accurate positions, in the range of 0.03–0.06 mas, at the traditional 8.4 GHz frequency. Comparing ICRF3 with the recently releasedGaiaCelestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames, in agreement with the ICRF3 noise level. Significant positional offsets between the three ICRF3 frequencies are detected for about 5% of the sources. Moreover, a notable fraction (22%) of the sources shows optical and radio positions that are significantly offset. There are indications that these positional offsets may be the manifestation of extended source structures. This third realization of the ICRF was adopted by the IAU at its 30th General Assembly in August 2018 and replaced the previous realization, ICRF2, on January 1, 2019.

Stellar occultations enable milliarcsecond astrometry for Trans-Neptunian objects and Centaurs

Context. Trans-Neptunian objects (TNOs) and Centaurs are remnants of our planetary system formation, and their physical properties have invaluable information for evolutionary theories. Stellar occultation is a ground-based method for studying these distant small bodies and has presented exciting results. These observations can provide precise profiles of the involved body, allowing an accurate determination of its size and shape. Aims. The goal is to show that even single-chord detections of TNOs allow us to measure their milliarcsecond astrometric positions in the reference frame of the Gaia second data release (DR2). Accurate ephemerides can then be generated, allowing predictions of stellar occultations with much higher reliability. Methods. We analyzed data from various stellar occultation detections to obtain astrometric positions of the involved bodies. The events published before the Gaia era were updated so that the Gaia DR2 stellar catalog is the reference, thus providing accurate positions. Events with detection from one or two different sites (single or double chord) were analyzed to determine the event duration. Previously determined sizes were used to calculate the position of the object center and its corresponding error with respectto the detected chord and the International Celestial Reference System propagated Gaia DR2 star position. Results. We derive 37 precise astrometric positions for 19 TNOs and four Centaurs. Twenty-one of these events are presented here for the first time. Although about 68% of our results are based on single-chord detection, most have intrinsic precision at the submilliarcsecond level. Lower limits on the diameter of bodies such as Sedna, 2002 KX14, and Echeclus, and also shape constraints on 2002 VE95, 2003 FF128, and 2005 TV189 are presented as valuable byproducts. Conclusions. Using the Gaia DR2 catalog, we show that even a single detection of a stellar occultation allows improving the object ephemeris significantly, which in turn enables predicting a future stellar occultation with high accuracy. Observational campaigns can be efficiently organized with this help, and may provide a full physical characterization of the involved object, or even the study of topographic features such as satellites or rings.

The Gaia reference frame for bright sources examined using VLBI observations of radio stars (Corrigendum)

Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude this is achieved thanks to Gaia's direct observations of quasars. At brighter magnitudes it is difficult to validate the quality of the reference frame due to the scarcity of comparison data. This paper examines the use of VLBI observations of radio stars to determine the spin and orientation of the bright reference frame of Gaia. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results for the 26 best-fitting sources indicate that the bright reference frame of Gaia DR2 is rotating relative to the faint quasars at a rate of about 0.1 mas/yr, significant at 2-sigma level. This supports a similar conclusion based on a comparison with stellar positions in the Hipparcos frame. The accuracy is currently limited by the small number of radio sources used, by uncertainties in the Gaia DR2 proper motions, and by the astrophysical nature of the radio stars. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.

The Gaia reference frame for bright sources examined using VLBI observations of radio stars

Context. Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude, this is achieved through Gaia’s direct observations of quasars. At brighter magnitudes, it is difficult to validate the quality of the reference frame because comparison data are scarce. Aims. The aim of this paper is to examine the use of very long baseline interferometry (VLBI) observations of radio stars to determine the spin and orientation of the bright reference frame of current and future Gaia data releases. Methods. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results. The VLBI data for the best-fitting 26 sources indicate that the bright reference frame of Gaia DR2 rotates relative to the faint quasars at a rate of about 0.1 mas yr−1, which is significant at the 2σ level. This supports a similar conclusion based on a comparison with stellar positions in the HIPPARCOS frame. The accuracy is currently limited because only a few radio sources are included in the solution, by uncertainties in the Gaia DR2 proper motions, and by issues related to the astrophysical nature of the radio stars. Conclusions. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.

Gaia Data Release 2

Context. Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. Aims. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these resultsperformed within the astrometry task. Methods. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. These data were calculated in two steps. First, the satellite attitude and the astrometric calibration parameters of the CCDs were obtained in an astrometric global iterative solution for 16 million selected sources, using about 1% of the input data. This primary solution was tied to the extragalactic International Celestial Reference System (ICRS) by means of quasars. The resulting attitude and calibration were then used to calculate the astrometric parameters of all the sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. Results. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G < 14 mag) sources, 0.1 mas at G = 17 mag, and 0.7 masat G = 20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas yr−1, respectively.The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas yr−1. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr−1 in proper motion are seen on small (< 1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.

Gaia Data Release 2

Context. The second release of Gaia data (Gaia DR2) contains the astrometric parameters for more than half a million quasars. This set defines a kinematically non-rotating reference frame in the optical domain. A subset of these quasars have accurate VLBI positions that allow the axes of the reference frame to be aligned with the International Celestial Reference System (ICRF) radio frame. Aims. We describe the astrometric and photometric properties of the quasars that were selected to represent the celestial reference frame of Gaia DR2 (Gaia-CRF2), and to compare the optical and radio positions for sources with accurate VLBI positions. Methods. Descriptive statistics are used to characterise the overall properties of the quasar sample. Residual rotation and orientation errors and large-scale systematics are quantified by means of expansions in vector spherical harmonics. Positional differences are calculated relative to a prototype version of the forthcoming ICRF3. Results. Gaia-CRF2 consists of the positions of a sample of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean, dense, and homogeneous set of extragalactic point sources in the magnitude range G ≃ 16 to 21 mag with accurately known optical positions. The median positional uncertainty is 0.12 mas for G < 18 mag and 0.5 mas at G = mag. Large-scale systematics are estimated to be in the range 20 to 30 μas. The accuracy claims are supported by the parallaxes and proper motions of the quasars in Gaia DR2. The optical positions for a subset of 2820 sources in common with the ICRF3-prototype show very good overall agreement with the radio positions, but several tens of sources have significantly discrepant positions. Conclusions. Based on less than 40% of the data expected from the nominal Gaia mission, Gaia-CRF2 is the first realisation of a non-rotating global optical reference frame that meets the ICRS prescriptions, meaning that it is built only on extragalactic sources. Its accuracy matches the current radio frame of the ICRF, but the density of sources in all parts of the sky is much higher, except along the Galactic equator.

Link between the VLBI and Gaia Reference Frames

Abstract The link between the International Celestial Reference Frame at radio wavelength and the forthcoming Gaia optical reference frame is a mandatory task after the completion of the Gaia mission. Starting from the provisional reference frame in which Gaia astrometric solutions were obtained, we discuss the ways to correct the residual rotation and acceleration effects and investigate three potential options for linking the two frames realized by extragalactic sources. We have estimated the accuracy for the frame alignment assuming different astrometric models of quasar proper motions observed by very long baseline interferometry (VLBI). Using about 500,000 high-precision proper motions of extragalactic sources, the residual rotation of the Gaia reference frame is evaluated under 1 μas yr−1. In view of its favorable properties, Gaia should be given priority to be considered as the future fundamental reference frame that is consistent with the principle of the International Celestial Reference System. The VLBI reference frame can be linked to Gaia based on thousands of common quasars with an accuracy of 10 μas for each axis.

Test of source selection for constructing a more stable and uniform celestial reference frame

We aim to evaluate the possibility of improving the International Celestial Reference System realization starting from the ICRF2 catalogue by investigating the coordinate time series of radio sources observed by the very long baseline interferometry between 1979 and 2016. Sources with long observational history are selected as the candidates and the least-squares fits with special handling of the weights are performed to derive the linear drifts of the source coordinates. Then the sources are sorted, based on the normalized linear drift (i) over the whole sky, and (ii) in four homolographic areas divided by declinations. The axial stability of the reference system and sky distribution defined by the selected sources are evaluated, which are acted as the criterion for the final source lists. With our improved source selection scheme, two groups of sources are proposed and considered suitable for defining a more stable and homogeneous celestial reference system compared to the second version of the current International Celestial Reference Frame (ICRF2). The number of sources in the final lists are 323 and 294, respectively, and the global rotations of the axes derived from apparent motion of the sources are about two times better than the ICRF2.

Connecting VLBI and Gaia Celestial Reference Frames

The current state of the link problem between radio and optical celestial reference frames is considered. The main objectives of the investigations in this direction during the next few years are the preparation of a comparison and the mutual orientation and rotation between the optical {\it Gaia} Celestial Reference Frame (GCRF) and the 3rd generation radio International Celestial Reference Frame (ICRF3), obtained from VLBI observations. Both systems, ideally, should be a realization of the ICRS (International Celestial Reference System) at micro-arcsecond level accuracy. Therefore, the link accuracy between the ICRF and GCRF should be obtained with similar error level, which is not a trivial task due to relatively large systematic and random errors in source positions at different frequency bands. In this paper, a brief overview of recent work on the GCRF--ICRF link is presented. Additional possibilities to improve the GCRF--ICRF link accuracy are discussed. The suggestion is made to use astrometric radio sources with optical magnitude to 20$^m$ rather than to 18$^m$ as currently planned for the GCRF--ICRF link. In addition, the use of radio stars is also a prospective method to obtain independent and accurate orientation between the Gaia frame and the ICRF.

A multipole analysis of the apparent motion of reference radio sources

In this paper, the apparent motions of quasars, which are the reference sources of the international celestial reference system (ICRS), are analyzed. Kinematic parameters from four catalogs compiled by different research groups are used. Apparent motions are expanded on a special set of vector functions on the sphere that are an irreducible representation of the rotation group O(3). The degree of the noninertiality of the barycentric reference system caused by the rotation of the solar system around the galactic center is estimated according to expansion coefficients. The direction and magnitude of the acceleration vector are calculated and compared with the alternative estimates. This method is discussed as a way to test Newton’s law on a large scale.

ABSOLUTE PROPER MOTIONS OUTSIDE THE PLANE (APOP)—A STEP TOWARD THE GSC2.4

We present a new catalog of absolute proper motions and updated positions derived from the same Space Telescope Science Institute digitized Schmidt survey plates utilized for the construction of Guide Star Catalog II. As special attention was devoted to the absolutization process and the removal of position, magnitude, and color dependent systematic errors through the use of both stars and galaxies, this release is solely based on plate data outside the galactic plane, i.e., vertical bar b vertical bar >= 27 degrees. The resulting global zero point error is less than 0.6 mas yr(-1), and the precision is better than 4.0 mas yr(-1) for objects brighter than R-F = 18.5, rising to 9.0 mas yr(-1) for objects with magnitudes in the range 18.5 < R-F < 20.0. The catalog covers 22,525 square degrees and lists 100,774,153 objects to the limiting magnitude of R-F similar to 20.8. Alignment with the International Celestial Reference System was made using 1288 objects common to the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths. As a result, the coordinate axes realized by our astrometric data are believed to be aligned with the extragalactic radio frame to within +/- 0.2 mas at the reference epoch J2000.0. This makes our compilation one of the deepest and densest ICRF-registered astrometric catalogs outside the galactic plane. Although the Gaia mission is poised to set the new standard in catalog astronomy and will in many ways supersede this catalog, the methods and procedures reported here will prove useful to remove astrometric magnitude-and color-dependent systematic errors from the next generation of ground-based surveys reaching significantly deeper than the Gaia catalog.

Global survey of star clusters in the Milky Way

Context. The global Milky Way Star Clusters (MWSC) survey provided new cluster membership lists and mean cluster parameters for nearly 80% of all previously known Galactic clusters. The MWSC data reduction pipeline involved the catalogue of positions and proper motions (PPMXL) on the International Celestial Reference System (ICRS) and near-infrared photometry from the Two Micron All Sky Survey (2MASS).