Astrometric Observations Research Articles

Inversion of Position for Low Speed Target in Near Space Using Non-resolved Photometric and Astrometric Data

A method for estimating the position parameters of a low speed target (LST) in near space was presented. The estimation method utilizes apparent magnitude, azimuthal angle, and angle of elevation to invert the position of a typical LST, such as a high altitude balloon (HAB). A photometric observation model composed of specular and diffuse reflection components for the LST was established. An astrometric observation model was discussed to deduce the azimuth and elevation of a ground-based opto-electronic detection system for the LST. An extended Kalman filter (EKF) was designed to invert the state parameters of the position and velocity of the LST from photometric and astrometric data. Two sets of simulations were carried out to validate this inversion method. The simulation results show that the simulation errors for the estimated parameters are small, the estimated positions agree with GPS very well, and the non-resolved photometric and angular data can be used to estimate the indirect observable position state parameters for a LST in near space.

Understanding Fomalhaut as a Cooper pair

Fomalhaut is a nearby stellar system and has been found to be a triple based on astrometric observations. With new radial velocity and astrometric data, we study the association between Fomalhaut A, B, and C in a Bayesian framework finding that the system is gravitationally bound or at least associated. Based on simulations of the system, we find that Fomalhaut C can be easily destabilized through combined perturbations from the Galactic tide and stellar encounters. Considering that observing the disruption of a triple is probably rare in the solar neighbourhood, we conclude that Fomalhaut C is a so-called "gravitational pair" of Fomalhaut A and B. Like the Cooper pair mechanism in superconductors, this phenomena only appears once the orbital energy of a component becomes comparable with the energy fluctuations caused by the environment. Based on our simulations we find (1) an upper limit of 8 km/s velocity difference is appropriate when selecting binary candidates and (2) an empirical formula for the escape radius, which is more appropriate than tidal radius when measuring the stability of wide binaries.

General relativistic effects on the orbit of the S2 star with GRAVITY

Context. The first observations of the GRAVITY instrument obtained in 2016, have shown that it should become possible to probe the spacetime close to the supermassive black hole Sagittarius A* (Sgr A*) at the Galactic center by using accurate astrometric positions of the S2 star. Aims. The goal of this paper is to investigate the detection by GRAVITY of different relativistic effects affecting the astrometric and/or spectroscopic observations of S2 such as the transverse Doppler shift, the gravitational redshift, the pericenter advance and higher-order general relativistic (GR) effects, in particular the Lense-Thirring effect due to the angular momentum of the black hole. Methods. We implement seven stellar-orbit models to simulate both astrometric and spectroscopic observations of S2 beginning near its next pericenter passage in 2018. Each model takes into account a certain number of relativistic effects. The most accurate one is a fully GR model and is used to generate the mock observations of the star. For each of the six other models, we determine the minimal observation times above which it fails to fit the observations, showing the effects that should be detected. These threshold times are obtained for different astrometric accuracies as well as for different spectroscopic errors. Results. Transverse Doppler shift and gravitational redshift can be detected within a few months by using S2 observations obtained with pairs of accuracies (σA,σV) = (10−100 μas, 1−10 km s-1) where σA and σV are the astrometric and spectroscopic accuracies, respectively. Gravitational lensing can be detected within a few years with (σA,σV) = (10 μas, 10 km s-1). Pericenter advance should be detected within a few years with (σA,σV) = (10 μas, 1−10 km s-1). Cumulative high-order photon curvature contributions, including the Shapiro time delay, affecting spectroscopic measurements can be observed within a few months with (σA,σV) = (10 μas, 1 km s-1). By using a stellar-orbit model neglecting relativistic effects on the photon path except the major contribution of gravitational lensing, S2 observations obtained with accuracies (σA,σV) = (10 μas, 10 km s-1), and a black hole angular momentum (a,i′,Ω′) = (0.99,45°,160°), the 1σ error on the spin parameter a is of about 0.4, 0.2, and 0.1 for a total observing run of 16, 30, and 47 yr, respectively. The 1σ errors on the direction of the angular momentum reach σi′ ≈ 25° and σΩ′ ≈ 40° when considering the three orbital periods run. We found that the uncertainties obtained with a less spinning black hole (a = 0.7) are similar to those evaluated with a = 0.99. Conclusions. The combination of S2 observations obtained with the GRAVITY instrument and the spectrograph SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) also installed at the VLT (Very Large Telescope) will lead to the detection of various relativistic effects. Such detections will be possible with S2 monitorings obtained within a few months or years, depending on the effect. Strong constraints on the angular momentum of Sgr A* (e.g., at 1σ = 0.1) with the S2 star will be possible with a simple stellar-orbit model without using a ray-tracing code but with approximating the gravitational lensing effect. However, long monitorings are necessary, and we thus must rely on the discovery of closer-in stars near Sgr A* if we want to efficiently constrain the black hole parameters with stellar orbits in a short time, or monitor the flares if they orbit around the black hole.

Uncovering the identities of compact objects in high-mass X-ray binaries and gamma-ray binaries by astrometric measurements

We develop a method for identifying a compact object in binary systems with astrometric measurements and apply it to some binaries. Compact objects in some high-mass X-ray binaries and gamma-ray binaries are unknown, which is responsible for the fact that emission mechanisms in such systems have not yet confirmed. The accurate estimate of the mass of the compact object allows us to identify the compact object in such systems. Astrometric measurements are expected to enable us to estimate the masses of the compact objects in the binary systems via a determination of a binary orbit. We aim to evaluate the possibility of the identification of the compact objects for some binary systems. We then calculate probabilities that the compact object is correctly identified with astrometric observation (= confidence level) by taking into account a dependence of the orbital shape on orbital parameters and distributions of masses of white dwarfs, neutron stars, and black holes. We find that the astrometric measurements with the precision of 70 $\mu$as for $\gamma$~Cas allows us to identify the compact object at 99% confidence level if the compact object is a white dwarf with 0.6 M$_{\odot}$. In addition, we can identify the compact object with the precision of 10 $\mu$as at 97% or larger confidence level for LS~I~+61$^{\circ}$~303 and 99% or larger for HESS~J0632+057. These results imply that the astrometric measurements with the 10-$\mu$as precision level can realize the identification of compact objects for $\gamma$~Cas, LS~I~+61$^{\circ}$~303, and HESS~J0632+057.

The Solar Neighborhood. XXXX. Parallax Results from the CTIOPI 0.9 m Program: New Young Stars Near the Sun

Abstract As a step toward completing and characterizing the census of the solar neighborhood, we present astrometric, photometric, and spectroscopic observations of 32 systems observed with the Cerro Tololo Inter-American Observatory 0.9 m and 1.5 m telescopes. Astrometry from the 0.9 m indicates that among the 17 systems that had no previous published trigonometric parallaxes, 14 are within 25 pc. In the full sample, nine systems have proper motions larger than 0.″5 yr−1, including 2MASS J02511490-0352459, which exceeds 2.″0 yr−1. VRI photometry from the 0.9 m and optical spectra from the 1.5 m indicate that the targets have V = 11–22 mag and spectral types M3.0V–L3.0V. For 2MASS J23062928-0502285 (TRAPPIST-1), we present updated astrometry and photometric variability based on over 12 years of observations. Of the nine binaries in the sample, two promise mass determinations in the next decade: LHS 6167AB, an M4.5V system for which we present an accurate parallax placing the binary at 9.7 pc, and 2MASS J23515048-2537367AB, an M8.5V system at 21.1 pc for which we present the first evidence of an unseen, low-mass companion. Most importantly, Na i and K i gravity indicators, Hα measurements, long-term photometric variability, locations on the H-R diagram, and kinematic assessments indicate that as many as 13 of the systems are young, including candidate members of young moving groups, with ages less than ∼120 Myr.

Astrometric VLBI Observations of the Galactic LPVs, Miras, and OH/IR stars

AbstractStudies of Galactic LPVs based on astrometric VLBI are presented. We use a VLBI array, “VERA”, to measure parallaxes and calibrate the K-band period luminosity relation (PLR) of the Galactic Miras. Since the PLR offers a distance indicator, its calibration is crucial to reveal their spatial distribution. Parallaxes of dozens of LPVs are presented. For the longer period stars, the mass-loss is high and the stars are obscured and recognized as OH/IR stars. We estimated mid-infrared absolute magnitudes of dozens of OH/IR stars and found that they show a loose concentration around −14 mag at λ of 11.6 μm, indicating an existence of PLR for OH/IR stars. Astrometry of OH/IR stars will also help us to study non-steady spiral arms as proposed from the latest simulation study of the galactic dynamics. We will start astrometric VLBI observation of two OH/IR stars NSV25875 and OH127.8+0.0 at 43 GHz with VERA.

VLBI astrometry of a water maser source in the Sgr B2 complex with VERA

AbstractWe have conducted astrometric observations toward a 22 GHz water maser source associated with the Sgr B2 complex in the Galactic center region with VERA (VLBI exploration of Radio Astrometry). We measured a trigonometric parallax and absolute proper motion of the Sgr B2 complex with respect to an extra-galactic source by observing the water maser source at 10 epochs from 2014 to 2017. The measured distance was 7.52+3.01−1.67 kpc for the Sgr B2M region.We also succeeded to measure internal motions of maser spots in Sgr B2M, and N region. The number of spots which we could measure the internal motions is about 400. The distribution of the maser spots shows that the maser spots are associated with envelope of HII region seen in radio continuum image obtained with VLA and ALMA. We discuss relative motions between Sgr B2M, and N by using the internal motion.

General post-Minkowskian expansion and application of the phase function

The phase function is a useful tool to study all observations of space missions, since it can give all the information about light propagation in a gravitational field. For the extreme accuracy of the modern space missions, a precise relativistic modeling of observations is required. So, we develop a recursive procedure enabling us to expand the phase function into a perturbative series of ascending powers of the Newtonian gravitational constant. Any $n$th-order perturbation of the phase function can be determined by the integral along the straight line connecting two point events. To illustrate the result, we carry out the calculation of the phase function outside a static, spherically symmetric body up to the order of ${G}^{2}$. Then, we develop a precise relativistic model that is able to calculate the phase function and the derivatives of the phase function in the gravitational field of rotating and uniformly moving bodies. This model allows the computing of the Doppler, radio science, and astrometric observables of the space missions in the Solar System. With the development of space technology, the relativistic corrections due to the motion of a planet's spin must be considered in the high-precision space missions in the near future. As an example, we give the estimates of the relativistic corrections on the observables about the space missions TianQin and BEACON.

Impact of basic angle variations on the parallax zero point for a scanning astrometric satellite

Determination of absolute parallaxes by means of a scanning astrometric satellite such as Hipparcos or Gaia relies on the short-term stability of the so-called basic angle between the two viewing directions. Uncalibrated variations of the basic angle may produce systematic errors in the computed parallaxes. We examine the coupling between a global parallax shift and specific variations of the basic angle, namely those related to the satellite attitude with respect to the Sun. The changes in observables produced by small perturbations of the basic angle, attitude, and parallaxes are calculated analytically. We then look for a combination of perturbations that has no net effect on the observables. In the approximation of infinitely small fields of view, it is shown that certain perturbations of the basic angle are observationally indistinguishable from a global shift of the parallaxes. If such perturbations exist, they cannot be calibrated from the astrometric observations but will produce a global parallax bias. Numerical simulations of the astrometric solution, using both direct and iterative methods, confirm this theoretical result. For a given amplitude of the basic angle perturbation, the parallax bias is smaller for a larger basic angle and a larger solar aspect angle. In both these respects Gaia has a more favourable geometry than Hipparcos. In the case of Gaia, internal metrology is used to monitor basic angle variations. Additionally, Gaia has the advantage of detecting numerous quasars, which can be used to verify the parallax zero point.

Asteroid mass estimation using Markov-chain Monte Carlo

Estimates for asteroid masses are based on their gravitational perturbations on the orbits of other objects such as Mars, spacecraft, or other asteroids and/or their satellites. In the case of asteroid-asteroid perturbations, this leads to an inverse problem in at least 13 dimensions where the aim is to derive the mass of the perturbing asteroid(s) and six orbital elements for both the perturbing asteroid(s) and the test asteroid(s) based on astrometric observations. We have developed and implemented three different mass estimation algorithms utilizing asteroid-asteroid perturbations: the very rough ‘marching’ approximation, in which the asteroids’ orbital elements are not fitted, thereby reducing the problem to a one-dimensional estimation of the mass, an implementation of the Nelder–Mead simplex method, and most significantly, a Markov-chain Monte Carlo (MCMC) approach. We describe each of these algorithms with particular focus on the MCMC algorithm, and present example results using both synthetic and real data. Our results agree with the published mass estimates, but suggest that the published uncertainties may be misleading as a consequence of using linearized mass-estimation methods. Finally, we discuss remaining challenges with the algorithms as well as future plans.

Use of long-term nongravitational force models for fitting astrometric observations of comet Encke

Based on the equations derived in (Usanin et al., 2016) a new solution combining the observations of 30 apparitions of the comet Encke from 1911 to 2010 is obtained. For the first time in the worldwide practice the solution is obtained by using converging differential correction of all 60 observed returns of the comet, however, the deviations are still unsatisfactory. The single solution has allowed to draw some preliminary conclusions. The contributions of planetary and nongravitational perturbations to the change of the elements of the orbit during the entire period of observation are determined. The extrapolation of the solution shows that for the past two thousand years the elements of the orbit orientation could change for a half of turnover, which should be taken into account when identifying the comet and associated meteor showers in ancient records. The predictions made by Z. Sekanina and I. Ferrín about oncoming termination of the comet activity are confirmed.

Volumes and bulk densities of forty asteroids from ADAM shape modeling

Disk-integrated photometric data of asteroids do not contain accurate information on shape details or size scale. Additional data such as disk-resolved images or stellar occultation measurements further constrain asteroid shapes and allow size estimates. We aim to use all available disk-resolved images of about forty asteroids obtained by the Near-InfraRed Camera (Nirc2) mounted on the W.M. Keck II telescope together with the disk-integrated photometry and stellar occultation measurements to determine their volumes. We can then use the volume, in combination with the known mass, to derive the bulk density. We download and process all asteroid disk-resolved images obtained by the Nirc2 that are available in the Keck Observatory Archive (KOA). We combine optical disk-integrated data and stellar occultation profiles with the disk-resolved images and use the All-Data Asteroid Modeling (ADAM) algorithm for the shape and size modeling. Our approach provides constraints on the expected uncertainty in the volume and size as well. We present shape models and volume for 41 asteroids. For 35 asteroids, the knowledge of their mass estimates from the literature allowed us to derive their bulk densities. We clearly see a trend of lower bulk densities for primitive objects (C-complex) than for S-complex asteroids. The range of densities in the X-complex is large, suggesting various compositions. Moreover, we identified a few objects with rather peculiar bulk densities, which is likely a hint of their poor mass estimates. Asteroid masses determined from the Gaia astrometric observations should further refine most of the density estimates.

Resolved astrometric orbits of ten O-type binaries

Aims. Our long-term aim is to derive model-independent stellar masses and distances for long period massive binaries by combining apparent astrometric orbit with double-lined radial velocity amplitudes (SB2). Methods. We followed-up ten O+O binaries with AMBER, PIONIER and GRAVITY at the VLTI. Here, we report on 130 astrometric observations over the last seven years. We combined this dataset with distance estimates to compute the total mass of the systems. We also computed preliminary individual component masses for the five systems with available SB2 radial velocities. Results. Nine of the ten binaries have their three-dimensional orbit well constrained. Four of them are known to be colliding wind, non-thermal radio emitters, and thus constitute valuable targets for future high angular resolution radio imaging. Two binaries break the correlation between period and eccentricity tentatively observed in previous studies. This suggests either that massive star formation produces a wide range of systems, or that several binary formation mechanisms are at play. Finally, we found that the use of existing SB2 radial velocity amplitudes can lead to unrealistic masses and distances. Conclusions. If not understood, the biases in radial velocity amplitudes will represent an intrinsic limitation for estimating dynamical masses from SB2+interferometry or SB2+Gaia. Nevertheless, our results can be combined with future Gaia astrometry to measure the dynamical masses and distances of the individual components with an accuracy of 5 to 15%, completely independently of the radial velocities.

Astrometrical Observations of the Selected WDS Stars

Astrometrical Observations of the Selected WDS Stars

Alerting observations of asteroids at the SBG telescope of the Kourovka Astronomical Observatory in the Gaia-FUN-SSO Network

AbstractRegular astrometric observations of small bodies of the Solar System are conducted using a SBG telescope of the Kourovka Astronomical Observatory of the Ural Federal University. The first results of participation in Gaia-FUN-SSO network are presented.

Astrometric Reduction of Cassini ISS Images of Enceladus in 2015 Based on Gaia DR1

AbstractThe Gaia DR1 catalogue stars are taken as reference ones to reduce the Cassini ISS images of Enceladus in 2015, and a total of 494 Cassini-centered astrometric observation are obtained in right ascension(α) and declination (δ) in the international Celestial Reference Frame(ICRF). Compared with JPL ephemerides SAT367, we derive that their mean residuals are a few tens meters in α*cos(δ) and a few kilometers in δ, and their standard deviation is not over 2 kilometers. Compared with the results from UCAC4 catalogue stars, The Gaia DR1 has the equivalent precision of reduction.

Astrometry for New Reductions: The ANR method

AbstractAccurate positional measurements of planets and satellites are used to improve our knowledge of their orbits and dynamics, and to infer the accuracy of the planet and satellite ephemerides. With the arrival of the Gaia-DR1 reference star catalog and its complete release afterward, the methods for ground-based astrometry become outdated in terms of their formal accuracy compared to the catalog's which is used. Systematic and zonal errors of the reference stars are eliminated, and the astrometric process now dominates in the error budget.We present a set of algorithms for computing the apparent directions of planets, satellites and stars on any date to micro-arcsecond precision. The expressions are consistent with the ICRS reference system, and define the transformation between theoretical reference data, and ground-based astrometric observables.

Characterization of the VVV Survey RR Lyrae Population across the Southern Galactic Plane

Abstract Deep near-IR images from the VISTA Variables in the Vía Láctea (VVV) Survey were used to search for RR Lyrae stars in the Southern Galactic plane. A sizable sample of 404 RR Lyrae of type ab stars was identified across a thin slice of the fourth Galactic quadrant (295° < ℓ < 350°, −2.°24 < b < −1.°05). The sample’s distance distribution exhibits a maximum density that occurs at the bulge tangent point, which implies that this primarily Oosterhoff type I population of RRab stars does not trace the bar delineated by their red clump counterparts. The bulge RR Lyrae population does not extend beyond ℓ ∼ 340°, and the sample’s spatial distribution presents evidence of density enhancements and substructure that warrants further investigation. Indeed, the sample may be employed to evaluate Galactic evolution models, and is particularly lucrative since half of the discovered RR Lyrae are within reach of Gaia astrometric observations.

The impact trajectory of asteroid 2008 TC3

The impact of asteroid 2008 TC3 was an unprecedented event—the first ever predicted impact of a near-Earth object. When it was first detected about 20 h before impact, 2008 TC3 was still farther away than the Moon. Once it was recognized as an impactor and announced as such, 2008 TC3 began to receive considerable attention from astronomical observers. Using the unprecedented dataset of nearly 900 astrometric observations and the latest observation debiasing and weighting techniques, we estimate the precise trajectory of 2008 TC3 and its impact ground track. At the entry point into the atmosphere, the 3-σ formal uncertainty in predicted position is an ellipse only 1.4 km × 0.15 km in size. The locations of the many meteorites recovered from the desert floor mark the asteroid’s actual ground track and provide a unique opportunity to validate trajectory models. We find that the second-order zonal harmonics of the Earth gravity field moves the ground track by more than 1 km and the location along the ground track by more than 2 km, while non-zonal and higher order harmonics change the impact prediction by less than 20 m. The contribution of atmospheric drag to the trajectory of 2008 TC3 is similar to the numerical integration error level, a few meters, down to an altitude of 50 km. Integrating forward to lower altitudes and ignoring the break-up of 2008 TC3, atmospheric drag causes an along-track deviation that can be as large as a few kilometers at sea level.

Frequency-dependent tidal dissipation in a viscoelastic Saturnian core and expansion of Mimas’ semi-major axis

Regarding tidal dissipation in Saturn, usually parameterized by Saturn's quality factor Q, there remains a discrepancy between conventional estimates and the latest determination that has been derived from astrometric observations of Saturn's inner satellites. If dissipation in Saturn is as large as the astrometric observations suggest, conventional models predict that Mimas' initial orbit should be located inside Saturn's synchronous orbit or even inside its Roche limit. Using simple structure models and assuming Saturn's core to be viscoelastic, we look for dissipation models which are consistent with both the latest astrometric observations and with Mimas' orbital migration. Firstly, using a two-layer model of Saturn's interior structure, we constrain the ranges of rigidity and viscosity of Saturn's core which are consistent with Saturn's dissipation derived from astrometric observations at the tidal frequencies of Enceladus, Tethys, and Dione. Next, within the constrained viscosity and rigidity ranges, we calculate Mimas' semi-major axis considering the frequency dependence of viscoelastic dissipation in Saturn's core. By the two calculations, we evaluate (1) Saturnian models which can explain the astrometrically determined Saturnian dissipation, and (2) whether Mimas' initial semi-major axis is larger than the synchronous orbit. We show that if the core is assumed to be solid with a viscosity of 10^{13}-10^{14} Pa s, the lower boundary of the observed Saturnian dissipation at tidal frequencies of Enceladus, Tethys, and Dione can be explained by our model. In this viscosity range, Mimas can stay outside the synchronous orbit and the Roche limit for 4.5 billion years of evolution. In the case of a frequency dependent viscoelastic dissipative core, the lower boundary of the observed Saturnian dissipation can be consistent with the orbital expansion of Mimas.