Search

The nature of the spiral structure of the Milky Way has long been debated. Only in the last decade have astronomers been able to accurately measure distances to a substantial number of high-mass star-forming regions, the classic tracers of spiral structure in galaxies. We report distance measurements at radio wavelengths using the Very Long Baseline Array for eight regions of massive star formation near the Local spiral arm of the Milky Way. Combined with previous measurements, these observations reveal that the Local Arm is larger than previously thought, and both its pitch angle and star formation rate are comparable to those of the Galaxy's major spiral arms, such as Sagittarius and Perseus. Toward the constellation Cygnus, sources in the Local Arm extend for a great distance along our line of sight and roughly along the solar orbit. Because of this orientation, these sources cluster both on the sky and in velocity to form the complex and long enigmatic Cygnus X region. We also identify a spur that branches between the Local and Sagittarius spiral arms.

We discovered 83 asteroids at the Baldone Astrophysical Observatory (MPC code 069) in 2017–2022. We studied one of the dynamically interesting Apollo (Near Earth object) observed at the Baldone Astronomical Observatory, namely 428694 Saule (2008 OS9) and the Centaur-type asteroid 330836 Orius (2009 HW77). We studied the evolution of the asteroid Saule’s rotation period, obliquity, and spin axis together with its non-gravitational parameter da∕dt{\rm{d}}a/{\rm{d}}t connected with the Yarkovsky effect. Additionally, we studied the orbit of the Amor-type asteroid 2017 UW42, which has the significant non-gravitational parameter A2A2.

In 1953 I heard of an experiment in 1925 by Bengt Strömgren where he observed transit times with the meridian circle at the Copenhagen University Observatory measuring the current in a photocell behind slits when a star was crossing. In 1954 just 22 years old I was given the task as a student to make first test observations with a new meridian circle of the observatory. I became fascinated by the instrument and by the importance of astrometry for astronomy. Work at four meridian circles, two in Denmark, one in Hamburg, one in Lund, and Pierre Lacroute's vision of space astrometry in France had by 1973 created the foundation for development of the Hipparcos satellite, and Gaia followed. In 2013 I proposed a successor satellite which has gained momentum especially thanks to the efforts of David Hobbs and it has a good chance to be launched by ESA about 2045. – But 70 years ago, optical astrometry was considered a dying branch of astronomy, unattractive compared with astrophysics. The following growth built on the still active interest in astrometry in Europe in those years and it was supported by ESA, the European Space Agency. – This review is only about astrometry where I was personally involved. [ABSTRACT FROM AUTHOR]

Introduction: In recent decades, numerous large survey projects have been initiated to enhance our understanding of the cosmos. Among these, the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) stands out as a flagship project of the Stage IV cosmology imaging surveys, offering an open-source framework for data management and processing adaptable to various instruments.Methods: In this paper, we introduce the ‘obs_mccd’ software package, designed to serve as a bridge linking raw data from generic mosaic-CCD instruments to the LSST data management framework. The package also facilitates the deployment of tailored configurations to the pipeline middleware. To validate our data processing pipeline, we processed a batch of realistic data from a commissioning wide-field telescope.Results: We established a prototype of the quality control (QC) system capable of assessing image quality parameters such as PSF size, ellipticity, and astrometric calibration. Our findings indicate that using a fifth-order polynomial for astrometric calibration effectively characterizes geometric distortion, achieving a median average geometric distortion residual of 0.011 pixel.Discussion: When comparing the performance of our pipeline to our in-house pipeline applied to the same dataset, we observed that our new ‘obs_mccd’ pipeline offers improved precision, reducing the median average geometric distortion residual from 0.016 pixel to 0.011 pixel. This enhancement in performance underscores the benefits of the obs_mccd package in managing and processing data from wide-field surveys, and it opens up new avenues for scientific exploration with smaller, flexible survey systems complementing the LSST.

Analyzing the succession of the beginning and end years of the Saros - 13, -12 ... 190 series, we find that most of the beginning or end years are arranged in ascending order, but from time to time some series begin or end before the series which it follows. The values for time travel are on average 183.8 years for the beginning of the Saros series and -153.7 years for the end of the Saros series. These occur with a recurrence of 587.99 years for the start of the Saros series and with a recurrence of 560 years for the end of the Saros series. It is shown that the number of years after which a new Saros series begins - steps at start, is 600.05 year and the number of years after wich a saros series end - steps at end, is 563.74. On the other hand, the chart of the intervals between the start years of the Saros series highlights the fact that at about 595 years the Saros series have high values around 1500 years. It also shown that the increase in the duration of the Saros series, also with 18 years, from 1226 years to 1514 years is accompanied by a decrease in Saros cycles on average by 2h 57m 26s. The size chart of the Saros series shows that their size decreases slowly over time. Thus around 3000 B.C. the large series have values around 1533 years, and around 2500 A.D. they have values around 1393 years. In the same periods the small series have values of 1399 years and 1249 years. Therefore, the size of the Saros series decreases slowly over time until extinction. [ABSTRACT FROM AUTHOR]

In the field of astronomy and its broader scientific context of space sciences, the English and Romanian languages use terms and sentences on various levels of specialization and therefore, accessibility for the general public as well as for the scientists. The flow of scientific data and information is generated in both languages, which raises the subsequent need of a correct equivalence in specific translations from English to Romanian and vice-versa. In this respect, this study discusses in a multi-disciplinary context a list of terms and expressions translated from English to Romanian that are nowadays used in the fields of astronomy and space sciences. In these scientific areas, the authors of this article considered the English language as being more specific than Romanian and therefore, they aimed to deconstruct the scientific meaning behind each of these specialized English language entries, and point out through additional comments, the rationality of the translation choice they have made, according to the thinking, the ways of expressing and converging ideas in both languages. [ABSTRACT FROM AUTHOR]

The paper presents the results obtained in four points regarding the astro-geodetic vertical deviation determination. The four points located in the metropolitan area of Bucharest city and they are stationed twice, in 2015 and 2023. Observations were realized by a platform composed by an electronic total station coupling with a micro-CCD camera and a GPS time receiver, all controlled by a portable PC. For every station were realized more sessions of observations in both years. For vertical deviation calculus we used geodetic coordinates extracted from models, while astronomical coordinates were measured in the field. The goal of these astro-geodetic determinations is to serve for testing and evaluating global geoid models as EGM2008 and GOCE, as well as to validate the historical astro-geodetic determinations in our country. [ABSTRACT FROM AUTHOR]

The objective of this study is to explore the potential of the Chinese Space Station Telescope (CSST) in asteroid mass determination with asteroid-asteroid close encounters. The CSST is expected to observe some asteroids with an accuracy of several milliarcseconds and has a limiting magnitude of 26 (AB mag) or higher in the g and r bands. By combining CSST observations with existing ground-based observations, significant improvements in asteroid mass precision can be achieved. To quantify the CSST’s capability in asteroid mass determination, three types of simulations are conducted. In Type A simulation, 58 close encounters with available Gaia DR2 observations were considered, assuming CSST observes asteroids at a frequency similar to Gaia’s. After using the simulated CSST observations, asteroid mass precision is improved substantially. In seven events, the determined precision are better than 5%. Type B simulation is performed based on a tentative optical survey plan of CSST, but the limited opportunities to observe asteroids involved in a close encounter with strong perturbation from to-be-determined masses. As a result, the precision of mass determination is low, though the improvement brought by CSST data is obvious. This implies that the dedicated observations are necessary for CSST to contribute masses with high precision. Type C simulation is performed with a small amount of CSST observing time, to be specific for a strong encounter, 144 observations spanning 3 years centered at the encounter time, totaling about 7.2 observation hours. In this case, CSST can determine a number of asteroid masses, of which 10 asteroid’s precision are expected to be better than 10%.

Introduction: The China Space Station Telescope (CSST) will enter a low Earth orbit around 2024 and operate for 10 years, with seven of those years devoted to surveying the area of the median-to-high Galactic latitude and median-to-high Ecliptic latitude of the sky. To maximize the scientific output of CSST, it is important to optimize the survey schedule. We aim to evaluate the astrometric capability of CSST for a given survey schedule and to provide independent suggestions for the optimization of the survey strategy.Methods: We first construct the astrometric model and then conduct simulated observations based on the given survey schedule. The astrometric solution is obtained by analyzing the simulated observation data. And then we evaluate the astrometric capability of CSST by analyzing the properties of the astrometric solution.Results: The accuracy of parallax and proper motion of CSST is better than 1 mas (⋅ yr−1) for the sources of 18–22 mag in g band, and about 1∼10 mas (⋅ yr−1) for the sources of 22–26 mag in g band, respectively. The results from real survey could be worse since the assumptions are optimistic and simple.Discussion: Optimizing the survey schedule can improve the astrometric accuracy of CSST. In the future, we will improve the astrometric capability of CSST by continuously iterating and optimizing the survey schedule.

The application of clustering algorithms to the Gaia astrometric catalog has revolutionized our census of stellar populations in the milky Way, including the discovery of many new dispersed structures. We focus on one such structure, Theia 456 (COIN-Gaia-13), a loosely bound collection of ∼320 stars spanning ∼120 pc that has previously been shown to exhibit kinematic, chemical, and gyrochronal coherency, indicating a common origin. We obtain follow-up radial velocities and supplement these with Gaia astrometry to perform an in-depth dynamical analysis of Theia 456. By integrating stellar orbits through a Milky Way potential, we find the currently dispersed structure coalesced into a small cluster in the past. Via Bayesian modeling, we derive a kinematic age of 245 ± 3 Myr (statistical), a half-mass–radius of 9 ± 2 pc, and an initial one-dimensional velocity dispersion of 0.14 ± 0.02 km s ^−1 . Our results are entirely independent of model isochrones, details of stellar evolution, and internal cluster dynamics, and the statistical precision in our age derivation rivals that of the most precise age-dating techniques known today, though our imperfect knowledge of the Milky Way potential and simple spherical model for Theia 456 at birth add additional uncertainties. Using posterior predictive checking, we confirm these results are robust under reasonable variations to the Milky Way potential. Such low-density structures that are disrupted by the Galactic tides before virializing may be ubiquitous, signifying that Theia 456 is a valuable benchmark for studying the dynamical history of stellar populations in the Milky Way.

The Milky Way’s central parsec is a highly extinguished region with a population of high-proper-motion stars. We have tracked 145 stars for ∼10 yr at wavelengths between 1 and 4 μ m to analyze extinction effects in color–magnitude space. Approximately 30% of this sample dims and reddens over the course of years, likely from the motion of sources relative to an inhomogeneous screen of dust. We correct previous measurements of the intrinsic variability fraction for differential extinction effects, resulting in a reduced stellar variability fraction of 34%. The extinction variability subsample shows that the extinguishing material has subarcsecond scales, much smaller variations than previously reported. The observed extinction events imply a typical cross section of 500 au and a density of around 3 × 10 ^4 atoms cm ^−3 for the extinguishing material, which are consistent with measurements of filamentary dust and gas at the Galactic center. Furthermore, given that the stars showing extinction variability tend to be more highly reddened than the rest of the sample, the extinction changes are likely due to material localized to the Galactic center region. We estimate the relative extinction between 1 and 4 μ m as ${{A}}_{{H}}:{{A}}_{{K}^{\prime} }:{{A}}_{{L}^{\prime} }=1.67\pm 0.05:1:0.69\pm 0.03$ . Our measurement of extinction at longer wavelengths— L ’ (3.8 μ m)—is inconsistent with recent estimations of the integrated extinction toward the central parsec. One interpretation of this difference is that the dust variations this experiment are sensitive to—which are local to the Galactic center—are dominated by grains of larger radius than the foreground.

Superfast rotators (SFRs) are small solar system objects that rotate faster than generally possible for a cohesionless rubble pile. Their rotational characteristics allow us to make inferences about their interior structure and composition. Here, we present the methods and results from a preliminary search for SFRs in the DECam Ecliptic Exploration Project (DEEP) data set. We find three SFRs from a sample of 686 main-belt asteroids, implying an occurrence rate of ${0.4}_{-0.3}^{+0.1}$ %—a higher incidence rate than has been measured by previous studies. We suggest that this high occurrence rate is due to the small sub-kilometer size regime to which DEEP has access: the objects searched here were as small as ∼500 m. We compute the minimum required cohesive strength for each of these SFRs and discuss the implications of these strengths in the context of likely evolution mechanisms. We find that all three of these SFRs require strengths that are more than that of weak regolith but consistent with many cohesive asteroid strengths reported in the literature. Across the full DEEP data set, we have identified ∼70,000 Main-Belt Asteroids and expect ∼300 SFRs—a result that will be assessed in a future paper.

Following the Pluto flyby of the New Horizons spacecraft, the mission provided a unique opportunity to explore the Kuiper Belt in situ. The possibility existed to fly by a Kuiper Belt object (KBO), as well as to observe additional objects at distances closer than are feasible from Earth-orbit facilities. However, at the time of launch no KBOs were known about that were accessible by the spacecraft. In this paper we present the results of 10 yr of observations and three uniquely dedicated efforts—two ground-based using the Subaru Suprime Camera, the Magellan MegaCam and IMACS Cameras, and one with the Hubble Space Telescope—to find such KBOs for study. In this paper we overview the search criteria and strategies employed in our work and detail the analysis efforts to locate and track faint objects in the Galactic plane. We also present a summary of all of the KBOs that were discovered as part of our efforts and how spacecraft targetability was assessed, including a detailed description of our astrometric analysis, which included development of an extensive secondary calibration network. Overall, these efforts resulted in the discovery of 85 KBOs, including 11 that became objects for distant observation by New Horizons and (486958) Arrokoth, which became the first post-Pluto flyby destination.

The LHS 1610 system consists of a nearby ( d = 9.7 pc) M5 dwarf hosting a candidate brown dwarf companion in a 10.6 days, eccentric ( e ∼ 0.37) orbit. We confirm this brown dwarf designation and estimate its mass ( ${49.5}_{-3.5}^{+4.3}$ M _Jup ) and inclination (114.5° ${}_{-10.0}^{+7.4}$ ) by combining discovery radial velocities (RVs) from the Tillinghast Reflector Echelle Spectrograph and new RVs from the Habitable-zone Planet Finder with the available Gaia astrometric two-body solution. We highlight a discrepancy between the measurement of the eccentricity from the Gaia two-body solution ( e = 0.52 ± 0.03) and the RV-only solution ( e = 0.3702 ± 0.0003). We discuss possible reasons for this discrepancy, which can be further probed when the Gaia astrometric time series become available as part of Gaia Data Release 4. As a nearby mid-M star hosting a massive short-period companion with a well-characterized orbit, LHS 1610 b is a promising target to look for evidence of sub-Alfvénic interactions and/or auroral emission at optical and radio wavelengths. LHS 1610 has a flare rate (0.28 ± 0.07 flares per day) on the higher end for its rotation period (84 ± 8 days), similar to other mid-M dwarf systems such as Proxima Cen and YZ Ceti that have recent radio detections compatible with star–planet interactions. While available Transiting Exoplanet Survey Satellite photometry is insufficient to determine an orbital phase dependence of the flares, our complete orbital characterization of this system makes it attractive to probe star–companion interactions with additional photometric and radio observations.

We characterize asteroid (16) Psyche using high-precision astrometry, including the recent Gaia Focused Product Release. The gravitational perturbations of Psyche on other asteroids can be observable in the case of mutual encounters. Using a least squares approach, we estimate the mass of Psyche by fitting astrometric data of asteroids that come within 0.05 au of Psyche. Combining the resulting individual estimates, we find GM = 1.601 ± 0.017 km ^3 s ^−2 . This result is robust against variations in the orbit determination setup and specific data set used. The volume and equivalent radius of Psyche are currently constrained by occultations and radar and optical imaging to (5.75 ± 0.19) × 10 ^6 km ^3 and ${111}_{-0.5}^{+2}$ km, respectively. Given the volume of Psyche, our mass estimate corresponds to a bulk density of 4172 ± 145 kg m ^−3 , which is compatible with an M-type taxonomic classification. Finally, the phase-dependent photocenter offset is visible in the residuals of Gaia astrometric observations of Psyche. This effect is consistent with the size of Psyche.

A large number of candidate binary stars with apparent acceleration on the sky has emerged from analysis of astrometric data collected by the Hipparcos, Tycho-2, and Gaia space missions. Although the apparent acceleration can serve as a relatively reliable indicator of binarity, it provides scarce information about the orbital and physical parameters of the components. With an emphasis on the search for stellar-mass black holes and neutron stars hidden in binary systems, we start a broader effort to characterize the most promising candidates using follow-up ground-based observations. Accurate quantification of orbital and physical parameters of systems with dim or invisible companions requires combination of Hipparcos, Gaia, and precision spectroscopic measurements. In this paper, we review the necessary steps in this implementation and describe the improved Hipparcos–Gaia sample of long-term astrometric accelerations, which includes correction of sky-correlated systematic errors using the vector spherical decomposition method. As an example, we study one Hipparcos star with a large acceleration, HIP 44842, where the companion is revealed to be a normal main-sequence star.

The Gaia mission has provided astrometric observations of unprecedented accuracy for more than 156,000 asteroids. The reported astrometric uncertainties are of the order of milliarcseconds, about 2 orders of magnitude smaller than that of traditional ground-based observations. The accuracy of Gaia data requires a high-fidelity orbit determination process, especially in the observation modeling. We present a statistical analysis of Gaia Focused Product Release to test the accuracy of the reported positions and associated uncertainties. We find that center-of-light offsets due to phase variations need to be modeled to properly fit the observational data. Prediction tests show that the uncertainty in the fitted orbits can be optimistic unless the observational uncertainty is inflated to account for errors in finding the center-of-mass of the body. Moreover, errors in the masses of small-body perturbers can cause differences in the orbital solution that exceed formal uncertainties of the best constrained orbits. As an example, we provide an update of the impact hazard analysis of 1950 DA, one of the asteroids observed by Gaia, and find that the impact probability in the year 2880 increases to 3.8 × 10 ^−4 .

We report the identification of 89 new systems containing ultracool dwarf companions to main-sequence stars and white dwarfs, using the citizen science project Backyard Worlds: Planet 9 and cross-reference between Gaia and CatWISE2020. 32 of these companions and 33 host stars were followed up with spectroscopic observations, with companion spectral types ranging from M7–T9 and host spectral types ranging from G2–M9. These systems exhibit diverse characteristics, from young to old ages, blue to very red spectral morphologies, potential membership to known young moving groups, and evidence of spectral binarity in nine companions. 20 of the host stars in our sample show evidence for higher-order multiplicity, with an additional 11 host stars being resolved binaries themselves. We compare this sample’s characteristics with those of the known stellar binary and exoplanet populations, and find our sample begins to fill in the gap between directly imaged exoplanets and stellar binaries on mass ratio–binding energy plots. With this study, we increase the population of ultracool dwarf companions to FGK stars by ∼42%, and more than triple the known population of ultracool dwarf companions with separations larger than 1000 au, providing excellent targets for future atmospheric retrievals.

We present ethraid , an open-source Python package designed to measure the mass ( m _c ) and separation ( a ) of a bound companion from measurements covering a fraction of the orbital period. ethraid constrains m _c and a by jointly modeling radial velocity, astrometric, and/or direct imaging data in a Bayesian framework. Partial orbit data sets, especially those with highly limited phase coverage, are represented well by a few method-specific summary statistics. By modeling these statistics rather than the original data, ethraid optimizes computational efficiency with minimal reduction in accuracy. ethraid uses importance sampling to efficiently explore the often broad posteriors that arise from partial orbits. The core computations of ethraid are implemented in Cython for speed. We validate ethraid 's performance by using it to constrain the masses and separations of the planetary companions to HD 117207 and TOI-1694. We designed ethraid to be both fast and simple, as well as to give broad, “quick look” constraints on companion parameters using minimal data. ethraid is pip installable and available on Zenodo and GitHub.

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 μ as 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.

We present the direct-imaging discovery of a substellar companion (a massive planet or low-mass brown dwarf) to the young, γ Doradus ( γ Dor)-type variable star HIP 39017 (HD 65526). The companion’s SCExAO/CHARIS JHK (1.1–2.4 μ m) spectrum and Keck/NIRC2 $L^{\prime} $ photometry indicate that it is an L/T transition object. A comparison of the JHK + L $^{\prime} $ spectrum to several atmospheric model grids finds a significantly better fit to cloudy models than cloudless models. Orbit modeling with relative astrometry and precision stellar astrometry from Hipparcos and Gaia yields a semimajor axis of ${23.8}_{-6.1}^{+8.7}$ au, a dynamical companion mass of ${30}_{-12}^{+31}$ M _J , and a mass ratio of ∼1.9%, properties most consistent with low-mass brown dwarfs. However, its mass estimated from luminosity models is a lower ∼13.8 M _J due to an estimated young age (≲115 Myr); using a weighted posterior distribution informed by conservative mass constraints from luminosity evolutionary models yields a lower dynamical mass of ${23.6}_{-7.4}^{+9.1}$ M _J and a mass ratio of ∼1.4%. Analysis of the host star’s multifrequency γ Dor-type pulsations, astrometric monitoring of HIP 39017 b, and Gaia Data Release 4 astrometry of the star will clarify the system age and better constrain the mass and orbit of the companion. This discovery further reinforces the improved efficiency of targeted direct-imaging campaigns informed by long-baseline, precision stellar astrometry.

The Differential Speckle Survey Instrument (DSSI) was relocated to the Astrophysical Research Consortium 3.5 m telescope at Apache Point Observatory (APO) in early 2022. Here we present results from the first year of observations along with an updated instrument description for DSSI at APO, including a detailed description of a new internal slit mask assembly used to measure the instrument plate scale from first principles. Astrometric precision for DSSI at APO during this time was measured to be 2.06 ± 0.11 mas, with a photometric precision of 0.14 ± 0.04 mag. Results of 40 resolved binary systems are reported, including two that were previously unknown to be binaries: HIP 7535 and HIP 9603. We also present updated orbital fits for two systems: HIP 93903 and HIP 100714. Finally, we report updated or confirmed dispositions for five Kepler Objects of Interest (KOIs) that were previously explored in Colton et al., using speckle imaging to discern common proper motions pairs from line of sight companions: KOI-270, KOI-959, KOI-1613, KOI-1962, and KOI-3214AB.

This article is devoted to the discovery of comets at the Vilnius Observatory together with the orbital analysis of dynamically interesting comets, namely 322P. We studied the orbital evolution of comet 322P with and without non-gravitational effects. It turned out that many of the comet’s orbital clones go into and out of retrograde orbits, sometimes repeatedly. The reason for such dramatic changes in the inclination of the orbit is the origin of comet 322P close to mean motion resonance 3:1 with Jupiter, ejecting them from there and, consequently, bringing the clones closer to the terrestrial group of planets. In this way, the clones of comet 322P enter retrograde orbits and reside there several ky to several My.

Optical interferometry has always been seen as a promising but difficult technique for astronomy. Until the year 2018, it was excrucially limited in sensitivity to magnitudes below 10. However, thanks to the advent of the GRAVITY instrument, fringe tracking and dual field interferometry made it possible to observe very faint target. This paper presents the different techniques used by optical interferometry to perform astrometry. It shows why dual field interferometry has become the key technique for faint companions. Taking the example of the HD95086 b exoplanet, we show how dual field interferomtry can detect an exoplanet as faint as magnitude 19.5, and how its astrometry was extracted. Use this example this paper explains how and why an astrometric accuracy of 10 µas is possible, and describes the remaining biases that can hinder this measurement. Last, we conclude by presenting the orbital trajectory of 10 exoplanets in 4 systems, and conclude with the short and longer term perspectives of the technique. [ABSTRACT FROM AUTHOR]

The issue of preservation and improvement of the Gaia catalogue throughout the next few decades is addressed. The goal of this study is to evaluate the feasibility of astrometric parameter update, for objects already in the catalogue, by inclusion of new observations from other space missions, in particular CSST. The proposed approach consists in modeling the astrometric fit of sources in the extragalactic region of the Gaia sample with new observations, using the stars in the local field as reference for each target. The concept is verified by simulation, in the framework of the expectations on the Gaia astrometric performance and on the data from the forthcoming CSST Optical Survey. This approach can mitigate the natural degradation of the initial precision on coordinates of Gaia sources with time by improving proper motions by a factor >3, thus improving the positional precision in future epochs. Moreover, the catalogue is densified by the inclusion of objects below the Gaia limiting magnitude, improving on the galactic population census and on extragalactic population. CSST-OS data will reduce the Gaia precision degradation on positions by a factor of 2.7 over 30 years and increase the number of available reference sources over 40% of the sky. Future observations from other missions may further improve on the Gaia catalogue by extending the sky coverage and temporal baseline.

The data set collected during the night of the discovery of a minor body constitutes a too-short arc (TSA), resulting in failure of the differential correction procedure. This makes it necessary to recover the object during subsequent nights to gather more observations that will allow a preliminary orbit to be calculated. In this work, we present a recovery technique based on sampling the admissible region (AdRe) by the constrained Delaunay triangulation. We construct the AdRe in its topocentric and geocentric variants, using logarithmic and exponential metrics, for the following near-Earth-asteroids: (3122) Florence, (3200) Phaethon, 2003 GW, (1864) Daedalus, 2003 BH84 and 1977 QQ5; and the main-belt asteroids: (1738) Oosterhoff, (4690) Strasbourg, (555) Norma, 2006 SO375, 2003 GE55 and (32811) Apisaon. Using our sampling technique, we established the ephemeris region for these objects, using intervals of observation from 25 minutes up to 2 hours, with propagation times from 1 up to 47 days. All these objects were recoverable in a field of vision of 95’ × 72’, except for (3122) Florence and (3200) Phaethon, since they were observed during their closest approach to the Earth. In the case of 2006 SO375, we performed an additional test with only two observations separated by 2 minutes, achieving a recovery of up to 28 days after its discovery, which demonstrates the potential of our technique.

Given their extremely large distances and small apparent sizes, quasars are generally considered as objects with near-zero parallax and proper motion. However, some special quasars may have abnormal astrometric characteristics, such as quasar pairs, lensed quasars, AGNs with bright parsec-scale optical jets, which are scientifically interesting objects, such as binary black holes. These quasars may come with astrometric jitter detectable with Gaia data, or significant changes in the position at different wavelengths. In this work, we aim to find these quasar candidates from Gaia EDR3 astrometric data combining with Sloan Digital Sky Survey (SDSS) spectroscopic data to provide a candidate catalog to the science community. We propose a series of criteria for selecting abnormal quasars based on Gaia astrometric data. We obtain two catalogs containing 155 sources and 44 sources, respectively. They are potential candidates of quasar pairs.

The paper investigates the refraction as it concerns position astronomy and the attempts of generations of astronomers since Tycho Brahe to estimate its size as a function of the apparent zenith distance of the sources. A few historical models are introduced and their value discussed by comparison to modern tables. The fundamental equation of the light-rays is established starting from the Fermat principle, together with a sample of analytical approximations of its solution down to the end of the XIXth century. Finally the two numerical approaches used today are presented with some details alongside the standard model of the Earth atmosphere. Theoretical expressions are developed step by step to allow the coding into a computer program. [ABSTRACT FROM AUTHOR]

This paper is devoted to the discovery of 37 asteroids at the Baldone Astrophysical Observatory (MPC 069) from 2015 to 2018, and one of dynamically interesting Mars-crosser (MC) observed at the Baldone Astrophysical Observatory, namely 2008 LX16. In Baldone Observatory, was independently discovered the Near-Earth Object 2018 GE3 on the image of 13 April 2018. Also, the NEO 2006 VB14 was observed doing its astrometry and photometry. Moreover, we observed asteroids 1986 DA and 2014 LJ1. We computed orbits and analyzed the orbital evolution of these asteroids. 566 positions and photometric observations of NEO objects 345705 (2006 VB14) and 6178 (1986 DA) were obtained with Baldone Schmidt telescope in 2018 and 2019. We detected their rotation period and other physical characteristics. Also, a Fourier transform was applied to determine the rotation period of asteroid 6178 (1986 DA). Value (3.12 ± 0.02)h was obtained. Our observations confirm the previously obtained rotation periodP=3.25hfor2006VB14.

This paper presents a methodology for Initial Orbit Determination (IOD) based on a modification of the Laplace’s geocentric method. The orbital elements for Near-Earth asteroids (1864) Daedalus, 2003 GW, 2019 JA8, a Hungaria-type asteroid (4690) Strasbourg, and the asteroids of the Main Belt (1738) Oosterhoff, (2717) Tellervo, (1568) Aisleen and (2235) Vittore were calculated. Input data observations from the Minor Planet Center MPC database and Astronomical Observatory of the Technological University of Pereira (OAUTP; MPC code W63) were used. These observations cover observation arcs of less than 22 days. The orbital errors, in terms of shape and orientation for the estimated orbits of the asteroids, were calculated. The shape error was less than 53 × 10–3 AU, except for the asteroid 2019 JA8. On the other hand, errors in orientation were less than 0.1 rad, except for (4690) Strasbourg. Additionally, we estimated ephemerides for all bodies for up to two months. When compared with actual ephemerides, the errors found allowed us to conclude that these bodies can be recovered in a field of vision of 95’ × 72’ (OAUTP field). This shows that Laplace’s method, though simple, may still be useful in the IOD study, especially for observatories that initiate programs of minor bodies observation.

The accuracy of stellar distances inferred purely from parallaxes degrades rapidly with distance. Proper motion measurements, when combined with some idea of typical velocities, provide independent information on stellar distances. Here, I build a direction- and distance-dependent model of the distribution of stellar velocities in the Galaxy, then use this together with parallaxes and proper motions to infer kinegeometric distances and transverse velocities for stars in Gaia DR3. Using noisy simulations, I assess the performance of the method and compare its accuracy to purely parallax-based (geometric) distances. Over the whole Gaia catalog, kinegeometric distances are on average 1.25 times more accurate than geometric ones. This average masks a large variation in the relative performance, however. Kinegeometric distances are considerably better than geometric ones beyond several kpc, for example. On average, kinegeometric distances can be measured to an accuracy of 19% and velocities ( $\sqrt{{v}_{\alpha * }^{2}+{v}_{\delta }^{2}}$ ) to 16 km s ^−1 (median absolute deviations). In Gaia DR3, kinegeometric distances are smaller than geometric ones on average for distant stars, but the pattern is more complex in the bulge and disk. With the much more accurate proper motions expected in Gaia DR5, a further improvement in the distance accuracy by a factor of (only) 1.35 on average is predicted (with kinegeometric distances still 1.25 times more accurate than geometric ones). The improvement attained from proper motions is limited by the width of the velocity prior, in a way that the improvement from better parallaxes is not limited by the width of the distance prior.

Giant exoplanets have been directly imaged over orders of magnitude of orbital separations, prompting theoretical and observational investigations of their formation pathways. In this paper, we present new VLTI/GRAVITY astrometric data of HIP 65426 b, a cold, giant exoplanet which is a particular challenge for most formation theories at a projected separation of 92 au from its primary. Leveraging GRAVITY’s astrometric precision, we present an updated eccentricity posterior that disfavors large eccentricities. The eccentricity posterior is still prior dependent, and we extensively interpret and discuss the limits of the posterior constraints presented here. We also perform updated spectral comparisons with self-consistent forward-modeled spectra, finding a best-fit ExoREM model with solar metallicity and C/O = 0.6. An important caveat is that it is difficult to estimate robust errors on these values, which are subject to interpolation errors as well as potentially missing model physics. Taken together, the orbital and atmospheric constraints paint a preliminary picture of formation inconsistent with scattering after disk dispersal. Further work is needed to validate this interpretation. Analysis code used to perform this work is available on GitHub: https://github.com/sblunt/hip65426 .

On 2022 March 4, the object known as WE0913A crashed into the Moon after several close flybys of the Earth and the Moon in the previous three months. Leading up to impact, the identity of the lunar impactor was up for debate, with two possibilities: the Falcon 9 from the DSCOVR mission or the Long March 3C from the Chang’e 5-T1 mission. In this paper, we present a trajectory and spectroscopic analysis using ground-based telescope observations to show conclusively that WE0913A is the Long March 3C rocket body (R/B) from the Chang’e 5-T1 mission. Analysis of photometric light curves collected before impact give a spin period of 185.221 ± 6.540 s before the first close Earth flyby on 2022 January 20 and a period of 177.754 ± 0.779 s, both at a 1 σ confidence level, before the second close Earth flyby on 2022 February 8. Using Markov Chain Monte Carlo sampling and a predictive light curve simulation based on an anisotropic Phong reflection model, we estimate both physical and dynamical properties of the Chang’e 5-T1 R/B at the start of an observation epoch. The results from the Bayesian analysis imply that there may have been additional mass on the front of the rocket body. Using our predicted impact location, the Lunar Reconnaissance Orbiter was able to image the crater site approximately 7.5 km from the prediction. Comparing the pre- and post-impact images of the location shows two distinct craters that were made, supporting the hypothesis that there was additional mass on the rocket body.

The Earth close approach of near-Earth asteroid 2005 LW3 on 2022 November 23 represented a good opportunity for a second observing campaign to test the timing accuracy of astrometric observation. With 82 participating stations, the International Asteroid Warning Network collected 1046 observations of 2005 LW3 around the time of the close approach. Compared to the previous timing campaign targeting 2019 XS, some individual observers were able to significantly improve the accuracy of their reported observation times. In particular, U.S. surveys achieved good timing performance. However, no broad, systematic improvement was achieved compared to the previous campaign, with an overall negative bias persisting among the different observers. The calibration of observing times and the mitigation of timing errors should be important future considerations for observers and orbit computers, respectively.

One possible formation mechanism for Hot Jupiters is that high-eccentricity gas giants experience tidal interactions with their host star that cause them to lose orbital energy and migrate inwards. We study these types of tidal interactions in an eccentric Hot Jupiter called HAT-P-2 b, which is a system where a long-period companion has been suggested and hints of orbital evolution were detected. Using 5 additional years of radial velocity (RV) measurements, we further investigate these phenomena. We investigated the long-period companion by jointly fitting RVs and Hipparcos-Gaia astrometry and confirmed this long-period companion, significantly narrowed down the range of possible periods ( ${P}_{2}={8500}_{-1500}^{+2600}$ days), and determined that it must be a substellar object ( ${10.7}_{-2.2}^{+5.2}$ M _j ). We also developed a modular pipeline to simultaneously model rapid orbital evolution and the long-period companion. We find that the rate and significance of evolution are highly dependent on the long-period companion modeling choices. In some cases the orbital rates of change reached ${de}/{dt}\,=\,{3.28}_{-1.72}^{+1.75}\times {10}^{-3}$ yr ^−1 , d ω / dt = 1.12° ± 0.22° yr ^−1 , which corresponds to a ∼321 yr apsidal precession period. In other cases, the data is consistent with de / dt = 7.67 ± 18.6 × 10 ^−4 yr ^−1 , d ω / dt = 0.76° ± 0.24° yr ^−1 . The most rapid changes found are significantly larger than the expected relativistic precession rate and could be caused by transient tidal planet–star interactions. To definitively determine the magnitude and significance of potential orbital evolution in HAT-P-2 b, we recommend further monitoring with RVs and precise transit and eclipse timings.

We combined the ground-based and Gaia Data Release 3 (DR3) asteroid observations to determine the masses of 20 asteroids with asteroid–asteroid close encounters. In order to take full advantage of the high-precision observations from Gaia, we use the Fisher information to select appropriate model parameters and the modified Encke’s equation of motion to construct a dynamical model complete at the level of observation precision. With diameters from literature, bulk densities of 20 asteroids are derived. The results indicate that the utilization of Gaia DR3 provides substantial benefits in terms of improving mass precision. Among the 20 asteroids analyzed in our study, we find that 10 asteroids achieved a mass precision better than 5%, and 15 asteroids better than 10%.

Spatiotemporal techniques for signal coordination with actively transmitting extraterrestrial civilizations, without the need for prior communication, can constrain technosignature searches to a significantly smaller coordinate space. With the variable star catalog from Gaia Data Release 3, we explore two related signaling strategies: the SETI Ellipsoid, and that proposed by Seto, which are both based on the synchronization of transmissions with a conspicuous astrophysical event. This data set contains more than 10 million variable star candidates with light curves from the first three years of Gaia’s operational phase, between 2014 and 2017. Using four different historical supernovae as source events, we find that less than 0.01% of stars in the sample have crossing times, the times at which we would expect to receive synchronized signals on Earth, within the date range of available Gaia observations. For these stars, we present a framework for technosignature analysis that searches for modulations in the variability parameters by splitting the stellar light curve at the crossing time.

The fundamental celestial reference frame (CRF) is based on two catalogs of astrometric positions: the third realization of the International Celestial Reference Frame (ICRF3), and the much larger Gaia CRF, built from the third data release (DR3). The objects in common between these two catalogs are mostly distant AGNs and quasars that are both sufficiently optically bright for Gaia and radio loud for the VLBI. This limited collection of reference objects is crucially important for the mutual alignment of the two CRFs and the maintenance of all of the other frames and coordinate systems branching from the ICRF. In this paper, we show that the three components of ICRF3 ( S/X, K , and X/Ka band catalogs) have significantly different sky-correlated vector fields of position offsets with respect to Gaia DR3. When iteratively expanded in the vector spherical harmonics up to degree 4 on a carefully vetted set of common sources, each of these components includes several statistically significant terms. The median sky-correlated offsets from the Gaia positions are found to be 56 μ as for the S/X , 100 μ as for the K , and 324 μ as for the X/Ka catalogs. The weighted mean vector field is subtracted from the Gaia reference positions, while the deviations from that field are added to each of the ICRF3 components. The corrected positions from each of the four input catalogs are combined into a single weighted mean catalog, which we propose to be the current most accurate realization of an inertial radio-optical CRF.

In this paper, we report on the follow-up of six potential exoplanets detected with Gaia astrometry and provide an overview of what is currently known about the nature of the entire Gaia astrometric exoplanet candidate sample, 72 systems in total. We discuss the primary false-positive scenario for astrometric planet detections: binary systems with alike components that produce small photocenter motions, mimicking exoplanets. These false positives can be identified as double-lined binaries (SB2) through analysis of high-resolution spectra. Doing so we find that three systems, Gaia DR3 1916454200349735680, Gaia DR3 2052469973468984192, and Gaia DR3 5122670101678217728, are indeed near-equal-mass double-star systems rather than exoplanetary systems. The spectra of the other two analyzed systems, HD 40503 and HIP 66074, are consistent with the exoplanet scenario in that no second set of lines can be found in the time series of publicly available high-resolution spectra. However, their Gaia astrometric solutions imply radial-velocity semiamplitudes ∼3 (HD 40503) and ∼15 (HIP 66074) larger than what was observed with ground-based spectrographs. The Gaia astrometry orbital solutions and ground-based radial-velocity measurements exhibit inconsistencies in six out of a total of 12 exoplanet candidate systems where such data are available, primarily due to substantial differences between observed ground-based radial-velocity semiamplitudes and those implied by the Gaia orbits. We investigated various hypotheses as to why this might be the case, and although we found no clear perpetrator, we note that a mismatch in orbital inclination offers the most straightforward explanation.

Dynamical masses of giant planets and brown dwarfs are critical tools for empirically validating substellar evolutionary models and their underlying assumptions. We present a measurement of the dynamical mass and an updated orbit of PZ Tel B, a young brown dwarf companion orbiting a late-G member of the β Pic moving group. PZ Tel A exhibits an astrometric acceleration between Hipparcos and Gaia EDR3, which enables the direct determination of the companion’s mass. We have also acquired new Keck/NIRC2 adaptive optics imaging of the system, which increases the total baseline of relative astrometry to 15 yr. Our joint orbit fit yields a dynamical mass of ${27}_{-9}^{+25}\,{M}_{\mathrm{Jup}}$ , semimajor axis of ${27}_{-4}^{+14}\,\mathrm{au}$ , eccentricity of ${0.52}_{-0.10}^{+0.08}$ , and inclination of ${91.73}_{-0.32}^{{+0.36}^\circ} $ . The companion’s mass is consistent within 1.1 σ of predictions from four grids of hot-start evolutionary models. The joint orbit fit also indicates a more modest eccentricity of PZ Tel B than previous results. PZ Tel joins a small number of young (

Using Fisher information matrices, we forecast the uncertainties σ _M on the measurement of a “Planet X” at heliocentric distance d _X via its tidal gravitational field’s action on the known planets. Using planetary measurements currently in hand, including ranging from the Juno, Cassini, and Mars-orbiting spacecraft, we forecast a median uncertainty (over all sky positions) of ${\sigma }_{M}=0.22\,{M}_{\oplus }{({d}_{x}/400{\rm{au}})}^{3}.$ A 5 σ detection of a 5 M _⊕ Planet X at d _X = 400 au should be possible over the full sky but over only 5% of the sky at d _X = 800 au. The gravity of an undiscovered Earth- or Mars-mass object should be detectable over 90% of the sky to a distance of 260 or 120 au, respectively. Upcoming Mars ranging improves these limits only slightly. We also investigate the power of high-precision astrometry of ≈8000 Jovian Trojans over the 2023–2035 period from the upcoming Legacy Survey of Space and Time (LSST). We find that the dominant systematic errors in optical Trojan astrometry (photocenter motion, nongravitational forces, and differential chromatic refraction) can be solved internally with minimal loss of information. The Trojan data allow cross-checks with Juno/Cassini/Mars ranging, but do not significantly improve the best achievable σ _M values until they are ≳10× more accurate than expected from LSST. The ultimate limiting factor in searches for a Planet X tidal field is confusion with the tidal field created by the fluctuating quadrupole moment of the Kuiper Belt as its members orbit. This background will not, however, become the dominant source of uncertainty until the data get substantially better than they are today.

We present the latest and most precise characterization of the architecture for the ancient (≈11 Gyr) Kepler-444 system, which is composed of a K0 primary star (Kepler-444 A) hosting five transiting planets and a tight M-type spectroscopic binary (Kepler-444 BC) with an A–BC projected separation of 66 au. We have measured the system’s relative astrometry using the adaptive optics imaging from Keck/NIRC2 and Kepler-444 A’s radial velocities from the Hobby-Eberly Telescope and reanalyzed relative radial velocities between BC and A from Keck/HIRES. We also include the Hipparcos-Gaia astrometric acceleration and all published astrometry and radial velocities in an updated orbit analysis of BC’s barycenter. These data greatly extend the time baseline of the monitoring and lead to significant updates to BC’s barycentric orbit compared to previous work, including a larger semimajor axis ( $a={52.2}_{-2.7}^{+3.3}$ au), a smaller eccentricity ( e = 0.55 ± 0.05), and a more precise inclination ( $i=85\buildrel{\circ}\over{.} {4}_{-0\buildrel{\circ}\over{.} 4}^{+0\buildrel{\circ}\over{.} 3}$ ). We have also derived the first dynamical masses of B and C components. Our results suggest that Kepler-444 A’s protoplanetary disk was likely truncated by BC to a radius of ≈8 au, which resolves the previously noticed tension between Kepler-444 A’s disk mass and planet masses. Kepler-444 BC’s barycentric orbit is likely aligned with those of A’s five planets, which might be primordial or a consequence of dynamical evolution. The Kepler-444 system demonstrates that compact multiplanet systems residing in hierarchical stellar triples can form at early epochs of the universe and survive their secular evolution throughout cosmic time.

Benchmark brown dwarf companions with well-determined ages and model-independent masses are powerful tools to test substellar evolutionary models and probe the formation of giant planets and brown dwarfs. Here, we report the independent discovery of HIP 21152 B, the first imaged brown dwarf companion in the Hyades, and conduct a comprehensive orbital and atmospheric characterization of the system. HIP 21152 was targeted in an ongoing high-contrast imaging campaign of stars exhibiting proper-motion changes between Hipparcos and Gaia, and was also recently identified by Bonavita et al. (2022) and Kuzuhara et al. (2022). Our Keck/NIRC2 and SCExAO/CHARIS imaging of HIP 21152 revealed a comoving companion at a separation of 0.″37 (16 au). We perform a joint orbit fit of all available relative astrometry and radial velocities together with the Hipparcos-Gaia proper motions, yielding a dynamical mass of ${24}_{-4}^{+6}\,{M}_{\mathrm{Jup}}$ , which is 1–2 σ lower than evolutionary model predictions. Hybrid grids that include the evolution of cloud properties best reproduce the dynamical mass. We also identify a comoving wide-separation (1837″ or 7.9 × 10 ^4 au) early-L dwarf with an inferred mass near the hydrogen-burning limit. Finally, we analyze the spectra and photometry of HIP 21152 B using the Saumon & Marley (2008) atmospheric models and a suite of retrievals. The best-fit grid-based models have f _sed = 2, indicating the presence of clouds, T _eff = 1400 K, and $\mathrm{log}g=4.5\,\mathrm{dex}$ . These results are consistent with the object’s spectral type of T0 ± 1. As the first benchmark brown dwarf companion in the Hyades, HIP 21152 B joins the small but growing number of substellar companions with well-determined ages and dynamical masses.

Sagittarius A* (Sgr A*) is a strong, compact radio source believed to be powered by a supermassive black hole at the galactic center. Extinction by dust and gas in the galactic plane prevents observing it optically, but its position and proper motion have previously been estimated using radio interferometry. We present new VLBI absolute astrometry measurements of its precise position and proper motion in the frame of the third realization of the International Celestial Reference Frame (ICRF3). The observations used were made at 52 epochs on the VLBA at K band (24 GHz) between 2006 June and 2022 August. We find the proper motion of Sgr A* to be −3.128 ± 0.042 mas yr ^−1 in R.A. and −5.584 ± 0.075 mas yr ^−1 in decl., or 6.400 ± 0.073 mas yr ^−1 at a position angle of 209.°26 ± 0.°51. We also find its J2000 ICRF3 coordinates at the 2015.0 proper motion epoch to be 17 ^h 45 ^m 40.ˢ034047 ± 0.ˢ000018, −29°00′28.″21601 ± 0.″00044. In galactic coordinates, Sgr A* shows proper motion of −6.396 ± 0.071 mas yr ^−1 in galactic longitude and −0.239 ± 0.045 mas yr ^−1 in galactic latitude, indicating solar motion of 248.0 ± 2.8 km s ^−1 in the galactic plane and 9.3 ± 1.9 km s ^−1 toward the north galactic pole.

The rotational reflex velocity (RRV) method was proposed by Heinze and Metchev in 2015 and was used to measure the distances of main-belt asteroids (MBAs). Later, Lin et al. generalized this method using spherical astronomy in 2016. The method measures the distances of MBAs using the observations from a single ground-based telescope over two nights. We refined this method and extend it further to the distance measurement of near-Earth asteroids (NEAs). In practice, we measure the distance of the potentially hazardous asteroid (99942) Apophis from the acquired CCD frames using the newly refined method. According to the requirement of the newly refined method, we also simulate the distance measurements of the four typical NEAs, (1221) Amor, (1862) Apollo, (2062) Aten, and (163693) Atira, on their discovery dates and follow-up dates. The measurement results of Apophis based on the newly refined RRV method show that the mean relative errors for the independent exposure frames on the successive two nights is ∼0.08% (about a factor of 2 improvement in comparison with the research of Lin et al.) compared with the distance from JPL ephemeris. Our simulation results also show that this refined method can accurately and precisely measure the distances of newly discovered NEAs in an astrometric way without performing orbital determination. The accurate and precise distances of newly discovered asteroids help us to conveniently evaluate their impact risks within a shorter time, leaving us more time to take defense precautions.

Accurately comparing two celestial reference frames based on the observed position of a number of common objects requires to detect and appropriately process outliers, lest they spuriously influence the results. It is thus of practical importance to use algorithms able to minimize the impact of those outliers when comparing radio and/or optical astrometric catalogs. In this paper, we investigate and compare the performances of some well-established and more recent robust algorithms when fitting a simple rotation vector between two reference frames. We particularly focus on two aspects: the variance of the resulting estimates, and the ability of the estimators to deal with outlying leverage points. We ran a number of Monte Carlo simulations with synthetic objects, varying their number, as well as the fraction and dispersion of outliers. Since the distribution of catalog objects in the sky is sometimes markedly nonuniform as in the case of the ICRF3 catalog, and because the position of outliers in the sky might cause issues when fitting rotation models, we also ran simulations representative of the observed distribution of objects. We compare the ICRF3 S/X, ICRF2 and Gaia EDR3 reference frames. Our results, based on the synthetic simulations and the comparison between the existing celestial frames, show that the M estimator, with a scale obtained from a least absolute deviations estimate, is the best among all the robust estimators compared.