Proper Motion Differences Research Articles

Astrometry in crowded fields towards the Galactic bulge

Context. The astrometry towards the Galactic bulge is hampered by high stellar crowding and patchy extinction. This effect is particularly severe for optical surveys such as the European Space Agency satellite Gala. Aims. In this study, we assess the consistency of proper motion measurements between optical (Gaia DR3) and near-infrared (VIRAC2) catalogues in comparison with proper motions measured with the Hubble Space Telescope (HST) observations in several crowded fields towards the Galactic bulge and in Galactic globular clusters. Methods. Assuming that the proper motion measurements are well characterised, the uncertainty-normalised proper motion differences between pairs of catalogues are expected to follow a normal distribution. A deviation from a normal distribution defines the inflation factor r. By multiplying the proper motion uncertainties with the appropriate inflation factor values, the Gaia (VIRAC2) proper motion measurements are brought into a 1σ agreement with the HST proper motions. Results. The inflation factor (r) depends on stellar surface density. For the brightest stars in our sample (G < 18), the dependence on G-band magnitude is strong, corresponding to the most precise Gaia DR3 proper motions. We used the number of observed Gaia DR3 sources as a proxy for the stellar surface density. Assuming that the HST proper motion measurements are well determined and free from systematic errors, we find that Gaia DR3 proper motion uncertainties are better characterised, having r < 1.5 in fields with a stellar number density with fewer than 200 Gaia DR3 sources per arcmin2, and are underestimated by up to a factor of 4 in fields with stellar densities higher than 300 sources per arcmin2. For the most crowded fields in VIRAC2, the proper motion uncertainties are underestimated by a factor of 1.1 up to 1.5, with a dependence on J-band magnitude. In all fields, the brighter sources have the higher r value. At the faint end (G > 19), the inflation factor is close to 1, meaning that the proper motions already fully agree with the HST measurements within 1σ. Conclusions. In the crowded fields common to both catalogues, VIRAC2 proper motions agree with HST proper motions and do not need an inflation factor for their uncertainties. Because of the depth and completeness of VIRAC2 in these fields, it is an ideal complement to Gaia DR3 for proper motion studies towards the Galactic bulge.

Link between the Optical and Radio Frames from Gaia DR3 Data and VLBI Measurements

Based on published data, we have assembled a sample of 126 radio stars with the trigonometric parallaxes and proper motions measured by VLBI and available in the Gaia DR3 catalogue (in fact, Gaia EDR3). Our analysis of the Gaia--VLBI proper motion differences for 84 radio stars based on the model of solid-body mutual rotation has revealed no rotation components differing significantly from zero, $(\omega_x,\omega_y,\omega_z)=(0.06,0.08,-0.10)\pm(0.06,0.07,0.08)$ mas yr$^{-1}.$ Based on the trigonometric parallax differences for 90 stars, we have obtained a new estimate of the systematic offset between the optical and radio frames, $\Delta\pi=-0.022\pm0.017$~mas, and showed that the parallax scale factor is close to unity, $b=1.001\pm0.002$.

Differential Proper Motion Spin of the Hipparcos and Gaia Celestial Reference Frames

The Hipparcos catalog provides the first epoch of the celestial reference frame (CRF) in the optical domain and serves as an indispensable tool to verify and improve the Gaia CRF for the brighter stars (V < 11 mag) and to identify the elusive astrometric binary stars with dim or invisible companions, including long-period exoplanets. The systems of positions in Hipparcos and Gaia cannot be directly compared, because they refer to two different mean epochs. It is shown that the proper motion systems for carefully filtered common stars are not statistically consistent within the given formal errors. The vector field of proper motion differences is fitted with 126 vector spherical harmonics up to degree 7 revealing a global pattern at high signal-to-noise ratios, including the three terms of rigid rotation. The origin of the differential spin and other large harmonic terms is investigated by producing a similar decomposition of the Gaia−HG proper motion field, where HG stands for the long-term proper motions derived from the Hipparcos and Gaia DR3 positions, for the same sample of stars. Hipparcos proper motions emerge as the largest source of sky-correlated distortions of the multiepoch optical CRF with a median value of ∼190 μas yr−1 and a global spin of ∼226 μas yr−1, while the Hipparcos positions and Gaia EDR3 proper motions are explicitly consistent by construction at a level of ∼10 μas yr−1. The latter, however, include multiple distortions of higher degree, which should be taken into account in astrometric applications using the HG field.

Results from The COPAINS Pilot Survey: Four new brown dwarfs and a high companion detection rate for accelerating stars

Abstract The last decade of direct imaging (DI) searches for sub-stellar companions has uncovered a widely diverse sample that challenges the current formation models, while highlighting the intrinsically low occurrence rate of wide companions, especially at the lower end of the mass distribution. These results clearly show how blind surveys, crucial to constrain the underlying planet and sub-stellar companion population, are not an efficient way to increase the sample of DI companions. It is therefore becoming clear that efficient target selection methods are essential to ensure a larger number of detections. We present the results of the COPAINS Survey conducted with SPHERE/VLT, searching for sub-stellar companions to stars showing significant proper motion differences (Δμ) between different astrometric catalogues. We observed twenty-five stars and detected ten companions, including four new brown dwarfs: HIP 21152 B, HIP 29724 B, HD 60584 B and HIP 63734 B. Our results clearly demonstrates how astrometric signatures, in the past only giving access to stellar companions, can now thanks to Gaia reveal companions well in the sub-stellar regime. We also introduce FORECAST (Finley Optimised REtrieval of Companions of Accelerating STars), a tool which allows to check the agreement between position and mass of the detected companions with the measured Δμ. Given the agreement between the values of the masses of the new sub-stellar companions from the photometry with the model-independent ones obtained with FORECAST, the results of COPAINS represent a significant increase of the number of potential benchmarks for brown dwarf and planet formation and evolution theories.

A Third-epoch Proper Motion Study of the Forward Shock in Kepler’s Supernova Remnant

Abstract We present measurements of the expansion of Kepler’s Supernova Remnant (SNR) over three epochs of Chandra X-ray observations from 2000, 2006, and 2014. As the remnant of a historical supernova (observed in 1604 CE), Kepler’s SNR presents the rare opportunity to study the dynamical evolution of such an object in real time. Measurements of the asymmetry in the forward shock velocity can also provide insight into the nature of the explosion and density of the circumstellar material. By combining data from 2014 with previous epochs in 2000 and 2006, we can observe the proper motion of filaments along the outer rim of the SNR. Prior studies of Kepler’s SNR have shown proper motion differences up to a factor of 3 between the northern and southern regions around the remnant. With the longer time baseline we use here, we find results that are consistent with previous studies, but with smaller uncertainties. Additionally, by adding a third epoch of observations, we search for any systemic change in the velocity in the form of a deceleration of the blast wave, as was recently reported in Tycho’s SNR. We find little to no conclusive evidence of such deceleration, and conclude that Kepler’s SNR is encountering circumstellar material that is roughly constant in density, though substantially varied around the periphery.

Dynamical Mass of the Young Substellar Companion HD 984 B

Abstract Model-independent masses of substellar companions are critical tools to validate models of planet and brown dwarf cooling, test their input physics, and determine the formation and evolution of these objects. In this work, we measure the dynamical mass and orbit of the young substellar companion HD 984 B. We obtained new high-contrast imaging of the HD 984 system with Keck/NIRC2 that expands the baseline of relative astrometry from 3 to 8 yr. We also present new radial velocities of the host star with the Habitable-Zone Planet Finder spectrograph at the Hobby-Eberly Telescope. Furthermore, HD 984 exhibits a significant proper motion difference between Hipparcos and Gaia EDR3. Our joint orbit fit of the relative astrometry, proper motions, and radial velocities yields a dynamical mass of 61 ± 4 M Jup for HD 984 B, placing the companion firmly in the brown dwarf regime. The new fit also reveals a higher eccentricity for the companion (e = 0.76 ± 0.05) compared to previous orbit fits. Given the broad age constraint for HD 984, this mass is consistent with predictions from evolutionary models. HD 984 B’s dynamical mass places it among a small but growing list of giant planet and brown dwarf companions with direct mass measurements.

Optimized modelling of Gaia–Hipparcos astrometry for the detection of the smallest cold Jupiter and confirmation of seven low-mass companions

ABSTRACT To fully constrain the orbits of low-mass circumstellar companions, we conduct combined analyses of the radial velocity data and the Gaia and Hipparcos astrometric data for eight nearby systems. Our study shows that companion-induced position and proper motion differences between Gaia and Hipparcos are significant enough to constrain orbits of low-mass companions to a precision comparable with previous combined analyses of direct imaging and radial velocity data. We find that our method is robust to whether we use Gaia Data Release 2 or Gaia Early Data Release 3, as well as whether we use all of the data or just proper motion differences. In particular, we fully characterize the orbits of HD 190360 b and HD 16160 C for the first time. With a mass of 1.8 ± 0.2 MJup and an effective temperature of 123–176 K and orbiting around a Sun-like star, HD 190360 b is the smallest Jupiter-like planet with well-constrained mass and orbit, belonging to a small sample of fully characterized Jupiter analogues. It is separated from its primary star by 0.25 arcsec and thus may be suitable for direct imaging by the coronagraph instrument of the Roman Space Telescope.

Looking for Astrometric Signals below 20 m s−1: A Candidate Exo-Jupiter in δ Pav

Abstract We use a combination of Hipparcos space mission data with the USNO dedicated ground-based astrometric program URAT-Bright designed to complement and verify Gaia results for the brightest stars in the south to estimate the small perturbations of observed proper motions caused by exoplanets. One of the 1423 bright stars in the program, δ Pav, stands out with a small proper motion difference between our long-term estimate and Gaia EDR3 value, which corresponds to a projected velocity of (−17, +13) m s−1. This difference is significant at a 0.994 confidence in the R.A. component, owing to the proximity of the star and the impressive precision of proper motions. The effect is confirmed by a comparison of long-term EDR3-Hipparcos and short-term Gaia EDR3 proper motions at a smaller velocity, but with formally absolute confidence. We surmise that the close Solar analog δ Pav harbors a long-period exoplanet similar to Jupiter.

The SUPERWIDE Catalog: A Catalog of 99,203 Wide Binaries Found in Gaia and Supplemented by the SUPERBLINK High Proper Motion Catalog

Abstract We present a catalog of 99,203 wide binary systems, initially identified as common proper motion (CPM) pairs from a subset of ∼5.2 million stars with proper motions , selected from Gaia data release 2 (DR2) and the SUPERBLINK high proper motion catalog. CPM pairs are found by searching for pairs of stars with angular separations &lt;1° and proper motion differences . A Bayesian analysis is then applied in two steps. In a first pass, we use proper motion differences and angular separations to distinguish between real binaries and chance alignments. In a second pass, we use parallax data from Gaia DR2 to refine our Bayesian probability estimates. We present a table of 119,390 pairs which went through the full analysis, 99,203 of which have probabilities &gt;95% of being real wide binaries. Of those 99,203 high-probability pairs, we estimate that only about 364 pairs are most likely to be false positives. In addition, we identify 57,506 pairs that have probabilities greater than 10% from the first pass but have high parallax errors and therefore were not vetted in the second pass. We examine the projected physical separation distribution of our highest probability pairs and note that the distribution is a simple exponential tail and shows no evidence of being bimodal. Among pairs with lower probability, wide binaries are detected at larger separations (&gt;104–105 au), consistent with the very wide population suggested in previous studies; however, our analysis suggests that these do not represent a distinct population, but instead represent either the exponential tail of the “normal” wide binary distribution or are simply chance alignments of unrelated field stars. We examine the Hertzsprung–Russell diagram of this set of high-probability wide binaries and find evidence for 980 overluminous components among 2227 K + K wide binaries; assuming these represent unresolved subsystems, we determine that the higher-order multiplicity fraction for K + K wide systems is at least 39.6%.

Parameters of the Link between the Optical and Radio Frames from Gaia DR2 Data and VLBI Measurements

Based on published data, we have assembled a sample of 88 radio stars for which there are both trigonometric parallax and proper motion measurements in the Gaia DR2 catalogue and VLBI measurements. A new estimate of the systematic offset between the optical and radio frames has been obtained by analyzing the GaiaDR2-VLBI trigonometric parallax differences: $\Delta\pi=-0.038\pm0.046$ mas (with a dispersion of 0.156 mas). This means that the Gaia DR2 parallaxes should be increased by this correction. The parallax scale factor is shown to be always very close to unity within $\sim$3 kpc of the Sun: $b=1.002\pm0.007.$ Our analysis of the proper motion differences for the radio stars based on the model of solid-body mutual rotation has revealed no rotation components differing significantly from zero: $(\omega_x,\omega_y,\omega_z)=(-0.14,0.03,-0.33)\pm(0.15,0.22,0.16)$ mas yr$^{-1}.$

The Hipparcos–Gaia Catalog of Accelerations

Abstract This paper presents a cross-calibrated catalog of Hipparcos and Gaia astrometry to enable their use in measuring changes in proper motion, i.e., accelerations in the plane of the sky. The final catalog adopts the reference frame of the second Gaia data release (DR2) and locally cross-calibrates both the scaled Hipparcos–Gaia DR2 positional differences and the Hipparcos proper motions themselves to this frame. This gives three nearly independent proper motion measurements per star, with the scaled positional difference usually being the most precise. We find that a linear combination of the two Hipparcos reductions is superior to either reduction on its own and address error inflation for both Hipparcos and Gaia DR2. Our adopted error inflation is additive (in quadrature) for Hipparcos and multiplicative for Gaia. We provide the covariance matrices along with the central epochs of all measurements. Our final proper motion differences are accurately Gaussian with the appropriate variances and are suitable for acceleration measurements and orbit fitting. The catalog is constructed with an eye toward completeness; it contains nearly 98% of the Hipparcos stars. It also includes a handful of spurious entries and a few stars with poor Hipparcos reductions that the user must vet by hand. Statistical distributions of accelerations derived from this catalog should be interpreted with caution.

Orbiting a binary

Aims. In this paper we present the results of the SPHERE observation of the HD 284149 system, aimed at a more detailed characterisation of both the primary and its brown dwarf companion. Methods. We observed HD 284149 in the near-infrared with SPHERE, using the imaging mode (IRDIS+IFS) and the long-slit spectroscopy mode (IRDIS-LSS). The data were reduced using the dedicated SPHERE pipeline, and algorithms such as PCA and TLOCI were applied to reduce the speckle pattern. Results. The IFS images revealed a previously unknown low-mass (~0.16 M⊙) stellar companion (HD 294149 B) at ~0.1′′, compatible with previously observed radial velocity differences, as well as proper motion differences between Gaia and Tycho-2 measurements. The known brown dwarf companion (HD 284149 b) is clearly visible in the IRDIS images. This allowed us to refine both its photometry and astrometry. The analysis of the medium resolution IRDIS long slit spectra also allowed a refinement of temperature and spectral type estimates. A full reassessment of the age and distance of the system was also performed, leading to more precise values of both mass and semi-major axis. Conclusions. As a result of this study, HD 284149 ABb therefore becomes the latest addition to the (short) list of brown dwarfs on wide circumbinary orbits, providing new evidence to support recent claims that object in such configuration occur with a similar frequency to wide companions to single stars.

Properties of the Tycho-2 catalogue from Gaia data release

Based on the measurements performed in the first 14 months of Gaia operation, we have solved the problem of obtaining the systematic differences between the stellar positions and proper motions of the TGAS (Tycho–Gaia Astrometric Solution) and Tycho-2 catalogues. By dividing the common stars from the TGAS and Tycho-2 catalogues into three G-magnitude groups for mean values of $$10\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 5,11\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 5,and13\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 0,$$ we have obtained the systematic differences between the stellar equatorial coordinates and proper motions of both catalogues in the form of a decomposition into vector spherical harmonics by taking into account the magnitude equation. The systematic components have been extracted from the individual differences with a probability of 0.977–0.999. The constructed model of systematic differences allows any position measurements performed using Tycho-2 as a reference catalogue to be transformed to the TGAS frame. An important fact is the existence of a magnitude equation in the systematic differences: when passing from bright (G = 10 m ) to faint (G = 13 m ) stars, the systematic position differences change within the range from approximately −40 to 15 mas, while the systematic proper motion differences change from −3 to 3 mas yr−1. The orientation and mutual rotation parameters of the Tycho-2 and TGAS frames have also been found to be different for stars of different magnitudes: when passing from bright to faint stars, the rotation angle of the Tycho-2 frame relative to TGAS changes from 3.51 to 5.63 mas, while the angular velocity of rotation changes from 0.35 to 1.22 mas yr−1. Based on the developed method that allows the extent to which the systematic errors in the equatorial propermotions of stars affect the results of a kinematic analysis of the Galactic proper motions to be estimated within the Ogorodnikov–Milne model, we have shown that the slope of the Galactic rotation curve and the Oort parameter C are most sensitive to the transition from the Tycho-2 frame to the TGAS one. Their relative changes after the transformation to the TGAS frame reach 56 and 100%, respectively. At the same time, the changes in the estimates of the Oort parameters A and B as well as the linear velocity of the Sun relative to the Galactic center, the Galactic rotation period, the ratio of the epicyclic frequency to the angular velocity of Galactic rotation, and the mass of the Galaxy within the Galactocentric distance of the Sun are not so large, being 2−10%.

Comparison of the XPM and UCAC4 catalogs

The systematic differences between the stellar positions and proper motions of the XPM and UCAC4 catalogs have been obtained in the form of decompositions into vector spherical harmonics by taking into account the magnitude equation. The systematic components have been extracted with a probability of at least 0.98 by dividing 41 316 676 stars into groups corresponding to 12 J magnitude bins with a width of 0.5m for mean values from 10m.25 to 15m.75. A study of the systematic differences between the equatorial coordinates suggests that the range of systematic differences between the XPM and UCAC4 positions exceeds the corresponding range of differences between PPMXL and UCAC4 by a factor of 5, especially for bright stars in the range being investigated. Analysis of the orientation of the XPM and UCAC4 reference frames has shown that their mutual rotation is 2–4 mas around the X axis and 7–10 mas around the Z axis. These angles depend on the magnitude of stars. Since two systems of proper motions are given in the XPM catalog, XPMx and XPMp, we have decomposed the proper motion differences XPMx–XPMp into vector spherical harmonics and found these differences to be free from the magnitude equation. An important fact is that the first-order zonal coefficients have turned out to be greatest in absolute value. The toroidal coefficient t1,0,1,0 found has shown that the XPMx and XPMp reference frames of proper motions rotate relative to each other around the Z axis with an angular velocity of 0.45 mas yr−1. It should be added that the range of systematic differences XPMx–XPMp is 2.1 mas yr−1 in right ascension and 1.7 mas yr−1 in declination. The angular velocities of mutual rotation of the XPMp and UCAC4 reference frames change within the range from 0.6 to 2.2 mas yr−1, while the analogous range for the XPMх and UCAC4 catalogs is 0.3–1.8 mas yr−1. The angular velocity and coordinates of the pole of the mutual rotation axis depend on the magnitude of stars. The parameters of the mutual rotation around the Z axis derived from the differences XPMx–UCAC4 and XPMp–UCAC4 change from 0.03 ± 0.06 to 1.73 ± 0.06 mas yr−1 and from 0.49 ± 0.06 to 2.19 ± 0.06 mas yr−1, respectively. Based on our analysis, we have shown that the XPMcatalog actually comprises two catalogs, XPM(XSC) and XPM(PSC), in which the stellar positions coincide at the standard epoch J2000 and differ at any other epoch. The decomposition coefficients of the systematic differences XPM–UCAC4 we obtained allow the stellar positions and proper motions from one catalog to be reduced to the system of the other catalog by taking into account this duality.

KINEMATICS OF THE X-SHAPED MILKY WAY BULGE: EXPECTATIONS FROM A SELF-CONSISTENTN-BODY MODEL

We explore the kinematics (both the radial velocity and the proper motion) of the vertical X-shaped feature in the Milky Way with an N-body bar/bulge model. From the solar perspective, the distance distribution of particles is double-peaked in fields passing through the X-shape. The separation and amplitude ratio between the two peaks qualitatively match the observed trends towards the Galactic bulge. We confirm clear signatures of cylindrical rotation in the pattern of mean radial velocity across the bar/bulge region. We also find possible imprints of coherent orbital motion inside the bar structure in the radial velocity distribution along l=0 degree, where the near and far sides of the bar/bulge show excesses of approaching and receding particles. The coherent orbital motion is also reflected in the slight displacement of the zero-velocity-line in the mean radial velocity, and the displacement of the maximum/minimum in the mean longitudinal proper motion across the bulge region. We find some degree of anisotropy in the stellar velocity within the X-shape, but the underlying orbital family of the X-shape cannot be clearly distinguished. Two potential applications of the X-shape in previous literature are tested, i.e., bulge rotation and Galactic center measurements. We find that the proper motion difference between the two sides of the X-shape can be used to estimate the mean azimuthal streaming motion of the bulge, but not the pattern speed of the bar. We also demonstrate that the Galactic center can be located with the X-shape, but the accuracy depends on the fitting scheme, the number of fields, and their latitudinal coverage.

AN ASYMMETRIC STREAMING MOTION IN THE GALACTIC BULGE X-SHAPED STRUCTURE REVEALED BY OGLE-III PROPER MOTIONS

The Galactic bulge shows a double red clump in sight lines with |b| > 5 deg and -3 deg < l < 4 deg. It is interpreted as a signature of an X-shaped structure seen almost edge-on. We measure the proper motions of the stars belonging to the closer and the further arm of the X-shaped structure. The intrinsic kinematic properties of the two arms are found by incorporating information taken from the luminosity function. At b = -5 deg we find that the proper motion difference between the two arms is a linear function of the Galactic longitude for -0.1 deg < l < 0.5 deg, which we interpret as a streaming motion of the stars within the X-shaped structure. Such a streaming motion has not previously been reported. The proper motion difference is constant for -0.8 deg < l < -0.1 deg, which gives us an estimate of bulge rotation speed of 87.9 +/- 8.2 km/s/kpc.

Error analysis of proper motions in decination obtained for 807 Hipparcos stars from PZT observations over many decades

AbstractAfter publication of the Hipparcos catalogue (in 1997), a few new astrometric catalogues have appeared (TYCHO‐2, ARIHIP, etc.), as a good combination of the Hipparcos satellite and ground‐based data, to get more accurate coordinates and proper motions of stars than the Hipparcos catalogue ones. There are also investigations on improving the Hipparcos coordinates and proper motions by using the astrometric observations of latitude and universal time variations (via observed stars referred to Hipparcos catalogue), together with Hipparcos data, carried out during the last few years. These kind of ground‐based data were collected at the end of the last century by J. Vondrák. There are about 4.4 million optical observations made worldwide at 33 observatories and with 47 instruments during 1899.7–1992.0; our Belgrade visual zenith telescope data (for the period 1949.0‐1986.0) were included. First of all, these data were used to determine the Earth Orientation Parameters – EOP, but they are also useful for the opposite task – to check the accuracy of coordinates and proper motions of Hipparcos stars which were observed from the ground over many decades. Here, we use the latitude part of ten Photographic Zenith Tubes – PZT data (more than 0.9 million observations made at 6 observatories during the time interval 1915.8–1992.0), and combine them with the Hipparcos catalogue ones, with suitable weights, in order to check the proper motions in declination for 807 common PZT/Hipparcos stars (and to construct the PZT catalogue of μδ for 807 stars). Our standard errors in proper motions in declination of these stars are less than or equal to the Hipparcos ones for 423 stars. The mean value of standard errors of 313 stars observed over more than 20 years by PZT is 0.40 mas/yr. This is 53% of 0.75 mas/yr (the suitable value from the Hipparcos catalogue). We used the Least Squares Method – LSM with the linear model. Our results are in good agreement with the Earth Orientation Catalogue – EOC‐2 and the new Hipparcos ones. The main steps of the method and the investigations of systematic errors in determined proper motions (the proper motion differences with respect to the Hipparcos values, the EOC‐2 ones and the new Hipparcos ones, as a function of α, δ, and magnitude) are presented here. A comparison of the four catalogues by pairs shows that there is no significant relationship between the differences of their μδ values and magnitudes and color indices of the common 807 stars. All catalogues have relatively small random and systematic errors which are close to each other. However, the comparison shows that our formal errors are too small. They are underestimated by a factor of nearly 1.7 (for EOC‐2, it is 2.0) if we take the new Hipparcos (or Hipparcos) data as reference (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Analysis of USNO-B1 proper motions of stars in open-cluster fields

USNO-B1 proper motions for stars in the fields of NGC 457 and χ and h Per open clusters are analyzed and compared to the the proper motions based on plate material taken with normal astrographs. USNO-B1 proper motions in central regions of clusters are found to have large errors due mostly to numerous misidentifications in crowded fields. At the same time, USNO-B1 proper motions for the rest of cluster stars proved to compare fairly well to those inferred in classical astrometric studies. Standard deviations of the proper-motion differences between USNO-B1 and cluster proper-motion studies are found to be on the order of 3.4–4.2 mas yr−1.

Statistical Constraints for Astrometric Binaries with Nonlinear Motion

Useful constraints on the orbits and mass ratios of astrometric binaries in the Hipparcos catalog are derived from the measured proper motion differences of Hipparcos and Tycho-2 (??), accelerations of proper motions (), and second derivatives of proper motions (). It is shown how, in some cases, statistical bounds can be estimated for the masses of the secondary components. Two catalogs of astrometric binaries are generated, one of binaries with significant proper motion differences and the other of binaries with significant accelerations of their proper motions. Mathematical relations between the astrometric observables ??, , and and the orbital elements are derived in the appendices. We find a remarkable difference between the distribution of spectral types of stars with large accelerations but small proper motion differences and that of stars with large proper motion differences but insignificant accelerations. The spectral type distribution for the former sample of binaries is the same as the general distribution of all stars in the Hipparcos catalog, whereas the latter sample is clearly dominated by solar-type stars, with an obvious dearth of blue stars. We point out that the latter set includes mostly binaries with long periods (longer than about 6 yr).

Catalog-to-catalog reductions - Results for the FK, N30, and GC catalogs

It is shown that previous computations of catalog-to-catalog differences, whether by the bin-and-average method or by least-squares adjustment combined with an expansion in an incomplete set of basis functions, have all been performed at inappropriate epoch(s) of place. As a consequence, such results are frequently much more informative regarding the systematic differences between the proper motion systems than they are with respect to the systematic differences between the positional systems. Moreover, and for the same reason, some sets of proper motion differences really do not portray the true state of the evolution of the proper motion systems. Finally, this has practical effects in other astrometrical work, for instance, in the analysis of earth rotation time series. This error is corrected by arguing for a more meaningful epoch of place for catalog-to-catalog comparisons. This method of infinitely overlapping circles is applied at these other epochs to the FK sequence of catalogs, the N30, and the GC, and as a corollary the effectiveness of this new technique is shown.