**Thong, Le**

We report new stringent limit relating to the space-time variation in the fine-structure constant from an analysis of quasar J110325-264515 towards zabs = 1.8389 . We find delta alpha/alpha = (1.56+-1.78)X10-6 from a comparison of laboratory wavelengths of Fe II with those found in quasar spectra. The obtained result proposes how inferred measurements of cosmic can improve these limitations in the future and can also constrain space-time variations.

**Zschocke, Sven**

We report on recent advancement in the theory of light propagation in the Solar System aiming at the sub-micro-arcsecond level of astrometric accuracy. Trajectories of light signals in the gravitational field of N Solar System bodies in arbitrary motion are determined in the post-Newtonian (PN) approach. In line with the IAU recommendations, harmonic coordinates are used and the gravitational fields of the Solar System bodies are expressed in terms of their intrinsic mass-multipoles and spin-multipoles, allowing for arbitrary shape, inner structure, oscillations, and rotational motion of these bodies. In particular:_x000D_ (1) The solution for light rays in 1.5PN approximation is obtained in the gravitational field of N arbitrarily moving bodies of arbitrary shape, inner structure, oscillations, and rotational motion._x000D_ (2) The solution for light rays in 2PN approximation is obtained in the gravitational field of one arbitrarily moving pointlike body.

**Arora, Monika**

The present paper explores the e ect of the perturbations of Coriolis and Centrifugal forces in the restricted four-body problem when all the primaries are oblate in the Lagrange equilateral triangle con guration. We have numerically investigated the number of libration points, viz. the in-plane and out of plane and investigated their stability. Zero velocity curves have also been drawn and the regions of motion have been explored for various values of the Jacobi constant. Newton Raphson basins of attraction have also been drawn and investigated.

**Gross, Richard**

The Jet Propulsion Laboratory (JPL) is pursuing an approach to determining ITRF-like terrestrial reference frames based upon the use of a Kalman filter/smoother. Kalman filters are commonly used to estimate the parameters of some system when a stochastic model of the system is available and when the data contain noise. For the purpose of determining a terrestrial reference frame, the system consists of the positions and velocities of geodetic observing stations and associated EOPs along with their full covariance matrices. The data consist of time series of observed VLBI, SLR, GNSS, and DORIS station positions and EOPs along with the data measurement covariance matrices. In addition, measurements from ground surveys of the positions of reference marks of co-located stations are used as constraints to tie the technique-specific measurements to each other. JPL's Kalman filter and smoother for reference frame determination (KALREF) combines these measurements to determine ITRF-like reference frames subject to constraints imposed on the allowed evolution of the station positions. KALREF includes options to model the station motion as linear, linear and annual, or linear, annual, and semiannual. Through the use of stochastic models for the process noise, the station positions can be constrained to follow these models of the station motion (by setting the process noise to zero), to recover the observed station positions (by setting the process noise to a large value), or to follow a smoothed path (by setting the process noise to some intermediate value). The sequential estimation approach to determining terrestrial reference frames that is being pursued at JPL will be described along with its use to determine JTRF2014, JPL's realization of a terrestrial reference frame using the ITRF2014 input data sets.

**Mittal, Amit**

We numerically investigated the circular autonomous restricted four-body problem where the fourth particle of variable mass is moving under the gravitational influence of three bodies always known as primaries which move in_x000D_ circular orbits around their their common center of mass in such a way that their configuration remains always an equilateral triangle. The effect of the variable mass parameter on the existence as well as on the locations of the libration points are investigated. The parametric variation of the position of libration points and zero velocity curve are also revealed when the variable mass parameter increases. Moreover, the Newton-Raphson basins of convergence corresponding to libration points are unveiled numerically when the variable mass parameter increases. The correlations between the basins of attraction and the required number of iterations associated with them are also evaluated and discussed numerically. The obtained results strongly suggests that the study of the evolution of the attracting domains of the proposed dynamical system is worth studying in spite of their complexity.

**Suraj, M S**

The Newton-Raphson basins of convergence, corresponding to the libration points also known as attractors, are unveiled in the Copenhagen problem, where instead of the Newtonian potential and forces the quasi-homogeneous potential created by two primaries are considered. The multivariate version of the Newton-Raphson iterative scheme is used to reveal the attracting domain associated with libration points on the various type of the two-dimensional configuration planes.The correlations between the basins of convergence and the required number of iterations corresponding to it are also presented and discussed in detail. The present numerical analysis reveal that the evolution of the attracting domains in this dynamical system is enormously complicated but worth studying issue.

**Aggarwal, Rajiv**

In the resent paper, we have numerically explored the fractal basins of convergence associated with the libration points in the axisymmetric restricted problem of five-bodies. The four bodies are supposed to be in axisymmetric central configurations as discussed by Érdi and Czirják (2016) and the dynamics of the fifth body moving under the gravitational influence of them are analyzed. We have considered the convex case which is one of the cases of the three basic axisymmetric central configurations proposed by Érdi and Czirják. This can be visualized by taking a system of three masses on a straight line and then dividing one of the masses into two equal halves to put them up and down in such a manner that the resulting four point masses configuration is symmetric about the x-axis. The convex case is taken into consideration which can be obtained by splitting the central mass to form a four-sided polygon. We have also unveiled numerically the domain of convergence corresponding to libration points by using the multivariate version of the Newton-Raphson iterative scheme. The correlations between the basins of convergence associated with libration points and the required number of iterations corresponding to it are also presented and discussed in detail. The present numerical analysis reveal that the evolution of the attracting domains in this dynamical system is enormously complicated but worth studying issue._x000D_ References:_x000D_ Érdi, B., Czirják, Z.: Central configurations of four bodies with an axis of symmetry. Celest. Mech. Dyn. Astron. 125(1), 33–70 (2016) _x000D_ Gao, C., Yuan, J., Sun, C.: Equilibrium points and zero velocity surfaces in the axisymmetric restricted five-body problem. Astrophys Space Sci. 362:72 (2017)

**Sachan, Prachi**

In present paper, we have drawn and studied the Newton-Raphson basins of convergence associated with the libration points in the axisymmetric restricted five-body problem. We have discussed the motion of the infinitesimal mass (fifth body) moving under the mutual gravitational influence of four bodies which are fixed on the central configurations proposed by Érdi and Czirják (2016). The proposed central configuration can be easily visualized by taking a system of three masses on a straight line and then dividing the one of the mass into two equal halves to put them up and down in such a manner that the resulting four point masses configuration is symmetric about the x-axis. We have taken the concave case into consideration which can be obtained by splitting one of the other two peripheral masses to form a four-sided. Furthermore, we obtain two different type of concave configuration according to the fact that whether the center of mass of the system is enclosed by the polygon or not. We have numerically unveiled the domain of convergence corresponding to libration points by using the multivariate version of the Newton-Raphson iterative scheme. Moreover, we have successfully presented the correlations between the basins of convergence associated with the equilibrium point and the required number of iterations corresponding to it are also presented and discussed in detail. The present numerical analysis reveals that the evolution of the attracting domains in this complex dynamical system is worth studying._x000D_ Érdi, B., Czirják, Z.: Central configurations of four bodies with an axis of symmetry. Celest. Mech. Dyn. Astron. 125(1), 33–70 (2016) _x000D_ Gao, C., Yuan, J., Sun, C.: Equilibrium points and zero velocity surfaces in the axisymmetric restricted five-body problem. Astrophys Space Sci. 362:72 (2017)

**Gouda, Naoteru**

We are now focusing on the development of two missions; those are Nano-JASMINE and Small-JASMINE whose missions are complementary to Gaia mission.The Nano-JASMINE, with a primary mirror aperture of 5-cm, will produce scientific results based on the astrometric information of bright stars in the neighboring space. Nano-JASMINE will operate in zw-band (0.6~1.0 micron) to perform an all sky survey with a precision of 3 mas for positions, parallaxes and proper motions. The combination of the observational data from Nano-JASMINE and the Hipparcos Catalogue is expected to produce more precise data on proper motions (~0.1mas/yr) .We are making progress on a Launch Services Agreement for Nano-JASMINE with a company for small rockets.An additional plan is underway to launch Small-JASMINE, with a primary mirror aperture of 30-cm, in around 2024. Small-JASMINE will provide positions and parallaxes accurate to ~20 micro-as for stars, brighter than Hw=12.5 mag (Hw-band: 1.1 ~ 1.7 micron). Proper motion precision of ~20 micro-as/yr is expected. This satellite will engage in observations of a limited area around the Galactic nuclear bulge and certain specific astronomical objects. One example of the main scientific objectives is to clarify the dynamical structure of the Galactic nuclear bulge and search for observational relics of a sequential merger of multiple supermassive black holes to form Sgr A* at the Galactic center. We have been aiming at the realization of the Small-JASMINE mission as a mission of the small science satellite program (Medium Class mission) executed by JAXA. We should have multiple steps of reviews by JAXA. Small-JASMINE successfully passed the Mission Definition Review (MDR) and an international review which was executed as a part of a review of planning by the executive officer of ISAS/JAXA. We are now proceeding the further detailed investigation and technical demonstration tests to pass following reviews by JAXA.

**Hohenkerk, Catherine**

The Division A functional working group, Standards of Fundamental Astronomy (SOFA), continues its task of establishing and maintaining an accessible and authoritative set of algorithms and procedures that implement standard models used in fundamental astronomy. This poster highlights the current software collection that SOFA makes freely available in thirteen different areas; astrometry, time, precession-nutation, to name but a few. This will include the new routines that were added during the last triennium.

**Yano, Taihei**

Small-JASMINE (hear after SJ), infrared astrometric satellite developed mainly at National Astronomical Observatory of Japan, will measure the positions and the proper motions which are located around the Galactic center, by operating at near infrared wave-lengths. SJ will clarify the formation process of the super massive black hole (hear after SMBH) at the Galactic center. In particular, SJ will determine whether the SMBH was formed by a sequential merging of multiple black holes. The clarification of this formation process of the SMBH will contribute to a better understanding of merging process of satellite galaxies into the Galaxy, which is suggested by the standard galaxy formation scenario. A numerical simulation (Tanikawa and Umemura, 2014) suggests that if the SMBH was formed by the merging process, then the dynamical friction caused by the black holes have influenced the phase space distribution of stars. The phase space distribution measured by SJ will make it possible to determine the occurrences of the merging process. SJ will determine whether the SMBH was formed through merging process of multiple black holes with high confidence level.

**Escapa, Alberto**

There is no theory accounting for the crossed-nutation effect for the non-rigid Earth. Crossed-nutation is a second order effect in the sense of perturbation theories arising from the coupling between the lunisolar nutations and the gravitational potential generating them. Although current IAU nutation model IAU 2000 (Mathews et al. 2002) tried to incorporate this effect in its formulation, its treatment presents some deficiencies. First, since the crossed-nutation is constructed from the rigid theory (Souchay et al. 1999), it lacks from the part associated with the Oppolzer terms. This part might play a relevant role at the micro arc second level due to the resonances of the fluid core. Second, the derived Poisson terms in IAU 2000 can be misleading. The reason is that the transfer function approach used to construct IAU 2000 is linear, whereas the crossed-nutation effect is intrinsically non-linear, so its application to derive these terms is more than doubtful. These issues can be solved by extending the Hamiltonian theory of the non-rigid Earth to the second order._x000D_ We present the first of such extensions (Getino et al. 2018) for a non-rigid Earth model composed of a rigid mantle enclosing a fluid core, the so-called Poincaré model. In particular, to run a parallel way to the rigid Earth, it has been necessary to modify previous results (Getino 1995) in order to find a canonical set of variables suitable for performing the cumbersome second order analytical computations._x000D_ The formulation of the Hamiltonian of the system in this new canonical set has allowed us to obtain the crossed-nutation effect through the Hori perturbation method. It has been found that there are marked differences, relevant at the micro arc second level, with respect to the rigid model. This fact represents a big difference with respect to the first order results where the nutations of the angular momentum axis are independent of the Earth interior structure.

**ARLOT, Jean-Eudes**

The former working group “Motions and observations of the planetary satellites” fulfilled its tasks and all astrometric observations together with several ephemerides are now provided to the astronomical community. However, some problems are not solved: the databases of observations are in different format contrarily to the asteroid database which adopted a standard format. In spite of the different sources of types of observations, it will be useful to choose a standard format (linked with VO) in order to make easier the research for improving the dynamical models and ephemerides. A discussion should be made in Division A and Commission X2 Ephemerides to decide on standards to be applied in the future.

**de Bruijne, Jos**

We study the Hyades open cluster using Gaia DR2 data complemented by Hipparcos-2 data for bright stars not contained in DR2. By assuming that all cluster members move with the mean cluster velocity to within the velocity dispersion, we compare the observed and expected motions of the stars, based on Gaia DR2 astrometry and radial velocities, to determine individual cluster membership probabilities. Our membership list extends the Gaia DR1-based list at the faint end by many magnitudes. This membership forms an invaluable basis for a continued assessment of the cluster, for instance using Gaia DR2 photometric data.

**Nurul Huda, Ibnu**

Nutation is mainly produced by cyclic gravitational forces exerted by the Sun and the Moon on the Earth equatorial bulge. In turn it is composed of several harmonic components. These components are commonly estimated from celestial pole offset (CPO) time series, which are produced from VLBI time delay analysis. However, as CPO are determined independently for each 24 hour VLBI session, the celestial pole offsets can present uncorrected systematic effect stemming from the geometry of the antenna network. The instability of the observed sources also makes this approach questionable. On the other hand the nutation components can be treated as global parameters and directly fitted to VLBI time delay. In this work, we apply both approaches over 30 years of VLBI observations and show that the global approach give more consistent and precise estimates of the nutation components.

**Soja, Benedikt**

The conventional approach to determine celestial reference frames (CRFs) is by estimating constant radio source coordinates in a least-squares adjustment. However, several radio sources exhibit coordinate variations that go beyond the traditional constant coordinate model considering nowadays’ growing accuracy requirements. Since it is currently very difficult to correct for the effects causing such variations (e.g., source structure) on the observational level, we have decided to use a time series representation of the radio source coordinates that constitute our CRF solutions. We treat radio source coordinates as stochastic processes and estimate them in a Kalman filter and smoother. The selection of the process noise model, which regulates the temporal variations of the resulting time series, is essential in the application of Kalman filtering. We test different approaches to derive the process noise model, for example based on the variability of the source flux and the jet direction. We compare the Kalman filter CRF solutions with traditional ones by assessing their performance in the VLBI data analysis, in particular concerning the estimated source coordinates and nutation parameters.

**Kwak, Younghee**

Conventionally, the International Celestial Reference System (ICRS) is realized solely by the Very Long Baseline Interferometry (VLBI). On the other hand, the International Terrestrial Reference System (ITRS) is realized by the four space geodetic techniques: VLBI, Global Navigation Satellite System (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). The VLBI-derived Terrestrial Reference Frame (TRF) is not consistent with the inter-technique combined TRF due to different data sets, its network geometry, and the impact of local ties at co-location sites. The same holds for the EOP series which provide the link between the Celestial Reference Frame (CRF) and TRF. Apart from consistency issues, the VLBI-derived EOP (especially terrestrial x/y-pole coordinates) have less precise time series than those derived from GNSS observations since GNSS is the dominant technique in determining terrestrial x/y-pole coordinates among the space geodetic techniques. In the combination, the precise GNSS EOP series can beneficially stabilize the VLBI-derived EOP and consequently indirectly the CRF.In this contribution, we estimate CRF, TRF, and EOP simultaneously based on the VLBI input data for ICRF3 (1979-2017), and the GNSS input data for ITRF2014 (1994-2014) extended for the period 2015-2017. We present the possible correlation between CRF parameters (right ascension and declination) and EOP. The influence of various combination strategies on different types of radio sources is also studied.

**Hilton, James**

The U.S. Nautical Almanac Office is constructing the IAU 2000A nutation theory ephemeris in a binary-PCK kernel for use in situations where speed of execution is critical. The nutation ephemeris will cover the time-span of the Jet Propulsion Laboratory's DE430 planetary ephemeris, JD 2287184.5-2688976.5 TDB (~1550.0-2650.0 CE). The method used in constructing this ephemeris also permits the evaluation of the instantaneous time rate of change of the nutation angles. Putting the theory in this format reduces the number of additions and multiplications required by about 2.75 orders of magnitudes each, and eliminates the need to evaluate any transcendental functions. The drawbacks of this representation are its limited time-span and the overhead of locating and reading the correct set of coefficients from the binary-PCK kernel. Extensive testing of numerous scenarios will be required to evaluate the true improvement in evaluation time. Currently, the conversion to a binary-PCK kernel is nearing completion. The project report at the General Assembly will include up-to-date information on testing this technique.

**Hunt, Lucas**

We present results from a Very Long Baseline Array (VLBA) campaign imaging sources used in the third realization of the International Celestial Reference Frame (ICRF 3). Imaging these sources allows us to determine spectral index, peak flux density, compactness and source structure index. This information is crucial to understanding source structure and variability which better allows us to determine if a source is suitable for inclusion in the ICRF and suitability as a phase reference calibrator.

**Bellini, Andrea**

We take advantage of the exquisite quality of the Hubble Space Telescope to distill the main sequence of ? Cen into its constituent populations. To this end, we restrict ourselves to the five most useful filters: the magic “trio” of F275W, F336W, and F438W, along with F606W and F814W. We develop a strategy for identifying color systems where different populations stand out most distinctly, then we isolate those populations and examine them in other filters where their subpopulations also come to light. In this way, we have identified at least 15 subpopulations, each of which has a distinctive fiducial curve through our five-dimensional photometric space. We confirm the MSa to be split into two subcomponents, and find that both the bMS and the rMS are split into three subcomponents. Moreover, we have discovered two additional MS groups: the MSd (which has three subcomponents) shares similar properties with the bMS, and the MSe (which has four subcomponents) has properties more similar to those of the rMS. We examine the fiducial curves together and use synthetic spectra to infer relative heavy-element, light-element, and helium abundances for the populations. Our findings show that the stellar populations and star formation history of ? Cen are even more complex than inferred previously.

**fienga, agnes**

In this poster we will present first results obtained with a trtacking campaign of mainly GNSS satellites simultaneously with the Meo laser tracking facility and the C2PU telescope both at the Calern site.CCD images of the satellites were obtained during the laser tracking sessions and flashes of the laser reflections are visible. In using these flashes, astrometry was obtained in using the GAIA DR1 and accurate angular positions related to the GAIA reference frames were obtained.

**Tang, Kai**

Tang et al. (2015) provided a numerical solution of the Earth’s precession in the relativistic framework for a long time span. Now we use a new hybrid integrator to calculate the motion of the solar system in the BCRS and the Earth's rotation in the GCRS. All the main relativistic effects are included following Klioner et al. (2010), especially we considered several relativistic reference systems with corresponding time scales, scaled constants and parameters. We improve this work to give new parameters to represent the precession. The results are still consistent with other long-term precession theories. The relativistic influences are obtained and analyzed here.

**Nastula, Jolanta**

An assessment of the impact of hydrological effects on polar motion, through seasonal soil moisture changes, ice and snow loading and melting, is based on the determination of geodetic residuals (GAO). Being difference between Geodetic Angular Momentum (GAM) and the sum of Atmospheric and Oceanic Angular Momentum (AAM and OAM, respectively), geodetic residuals reflect hydrological signals in observed polar motion excitation. However, while GAM is determined from precise geodetic measurements such as SLR, VLBI and GNSS, AAM and OAM are based on different models of atmosphere and ocean. Consequently, the errors of geodetic hydrological excitations are mainly related to inaccurate geophysical models. The mass terms of AAM and OAM, related to air pressure and ocean bottom pressure, as well as motion terms connected with wind speed and currents, vary from one model to another.Here, we would like to compare the results of different geodetic hydrological excitation functions, that are computed by removing modelled atmospheric and oceanic effects from precise observations of polar motion excitations. To do this, we use several models of AAM and OAM. Additionally, we compare the resulting geodetic residuals with hydrological excitation functions based on Gravity Recovery and Climate Experiment (GRACE) satellite mission and hydrological models. The polar motion budged is analysed here at different time scales – decadal, interannual, seasonal and short term oscillations. This analysis could let us indicate which components of different models of atmosphere and ocean cause the biggest errors in geodetic budget at specific time scales.

**Taris, François**

As in the radio domain, it can be reasonably postulated that quasar optical flux variations can alert us to potential changes in the source structure. These changes could have important implications for the position of the targets photocenters (together with the evolution in time of these centers) and in parallel have consequences for the link of the reference systems.In the case of some targets that are well observed by the TAROT telescopes, the Allan time variance shows that the largest averaging period of the magnitudes is in the range 20d-70d. The observation period by Gaia, for a single target, largely exceeds these values which might be a problem when the magnitude variations exhibit flicker or random walk noises.The Allan time variance could also help to quantify the optical stability of a target and, then, could help to ?nd new suitable targets for the link between the reference frames.Preliminary computations show that if the coordinates of the targets studied in this work were affected by a white phase noise with a formal uncertainty of about 1 mas (due to astrophysical processes that are put in evidence by the magnitude variations of the sources), it would affect the precision of the link at the level of 50 µas.In order to obtain long and well sampled time series, that could also be of interest to bring constraints to astrophysical models of AGN, the SYRTE department of the Paris Observatory has the project to build a 1m robotic telescope to monitor the magnitude variation of AGN. The results of the seeing campaign and the first images, obtained in remote mode with a 50cm test telescope, will be presented.

**Park, Ryan**

Several distant scattered Kuiper belt objects have similar perihelion geometries that might be aligned due to the influence of an unknown planet well outside the orbit of Neptune (Batygin & Brown, 2016 Astronomical J. 151:22). Such a planet, with a mass up to an order of magnitude larger than the Earth, would dynamically affect the rest of the solar system. Saturn, which is well observed from radio range and Very Long Baseline Interferometry (VLBI) observations of the Cassini spacecraft, provides an opportunity to look for these perturbations. An unknown large planet would be expected to affect the orbit of Saturn, but this effect might be partially absorbed in the estimation of parameters used to fit the planetary ephemerides. Ephemeris parameters include the planetary orbital elements, the mass of the Sun and the masses of asteroids that perturb the orbit of Mars. Earlier analysis of the Cassini data showed no non-Newtonian effect as suggested by the Modified Newtonian Dynamics theory. We present an updated Cassini data set, with the accuracy of ranges to Saturn improved through updated estimates of the Cassini spacecraft orbit, and an analysis of the largest possible perturbing distant planet mass consistent with the ranging.

**Gordon, David**

VLBA observations have enabled us to greatly improve the precisionof ICRF3, which will be presented for approval at the 2018 IAU meeting.Of the 3414 sources in the Second Realization of the InternationalCelestial Reference Frame (ICRF2), approximately 2/3 were singleepoch VLBA Calibrator Survey (VCS) sources that had average positionuncertainties nearly 5 times worse than the other 1/3. Several VLBAobserving campaigns since 2014 have been undertaken to re-observe thesefor ICRF3. Some 32 X/S 24-hr VLBA astrometry sessions have now been runand analyzed. For the sources in the original ICRF2 VCS class,formal position errors have been reduced by an average factor of nearly7. The VLBA observations have also included many other sources resultingin the addition of 436 new sources to the X/S reference frame for ICRF3.

**Li, Jian**

The ratio of partial solar eclipse and central solar eclipse can be induced from the “eclipse limit”, but the ratio of the three types of central eclipses: annular, total and total-annular eclispe, can not be reached by the same way. We found that considering the distance of Earth-Moon and the distance of Earth-Sun (during central eclipses) as random variables, it’s easy to find the proximate ratio of the central eclipses through the probability theory.By constructing the probability density function of the random variables, we calculate that the ratio of annular, total-annular and total eclispe (to central eclipses) is respectively 58.1%, 10.5% and 31.4%, which is consisted with the statistical result from AD 1001 to AD 2000 given by NASA in the rough: 49.5%(annular), 10.7%(total-annular) and 39.8%(total). It showes that the probability method maybe a relatively simple way to let the public know more about why there are more annular solar eclipses, fewer total solar eclpses and much fewer total-annular solar eclipses.

**LIAO, Shilong**

Quasars are essential for astrometric in the sense that they are spatial stationary because of their large distance from the Sun. The European Space Agency (ESA) space astrometric satellite Gaia is scanning the whole sky with unprecedented accuracy up to a few µas level. However, Gaia’s two fields of view observations strategy may introduce a parallax bias in the Gaia catalog. Since it presents no significant parallax, quasar is perfect nature object to detect such bias. More importantly, quasars can be used to construct a Celestial Reference Frame in the optical wavelengths in Gaia mission. In this paper, we compile the most reliable quasars existing in literatures. The final compilation (designated as Known Quasars Catalog for Gaia mission, KQCG) contains 1843850 objects, among of them, 779349 objects are found in Gaia DR2 after cross-identifications. This catalog will be very useful in Gaia mission.

**de Witt, Aletha**

A K-band (24 GHz) celestial reference frame of 825 sources covering the full sky has been constructed using 461K observations from 55 observing sessions from the VLBA and HartRAO-Hobart. Observations at K-band are motivated by their ability to access more compact source morphology and reduced core shift relative to observations at the historically standard S/X-band (2.3/8.4 GHz). The factor of three increase in interferometer resolution at K-band should resolve out source structure which is a concern for AGN centroid stability. K-band median precision is now better than the ICRF-2 precision (for common sources) thereby raising the question of which frame is more accurate. The accuracy of the K CRF is quantified by comparison of 707 sources in common with the current S/X-band producing wRMS agreement of 86 micro-arcseconds as in RA cos(Dec) and 123 micro-arcseconds in Dec. There is evidence for systematic errors at the ~100 micro-arcsecond level. The success of the Gaia optical astrometric satellite motivates work to tie the radio and optical frames. K-band data and Gaia Data Release-2 data give a frame tie precision of ~15 micro-arcsecond (1-sigma, per 3-D rotation component). If K-band precision can be pushed below ~20-30 micro-arcsecond, the K frame has potential to produce a tie to Gaia that is superior to S/X due to reduced astrophysical systematics at K relative to S/X.

**Noerdlinger, Peter**

Does the Newtonian Gravity "Constant" G VaryA series of measurements of Newton's gravity constant, G, dating back as far as 1893, yielded widely varying values, the variation greatly exceeding the stated error estimates (Gillies, 1997; Quinn, 2000, Mohr et al 2008). The value of G is usually said to be unrelated to other physics, but we point out that the 8B Solar Neutrino Rate ought to be very sensitive. Improved pulsar timing could also help settle the issue as to whether Greally varies. We claim that the variation in measured values over time (1893-2014 C.E.) is a more serious problem than the failure of the error bars to overlap; it appears that challenging or adjusting the error bars hardly masks the underlying disagreement in central values. We have assessed whether variations in the gravitational potential due to (for example) local dark matter (DM) could explain the variations. We note that true variations in G would be associated with variations in clock rates (Derevianko and Pospelov 2014; Loeb and Maoz 2015), which could mask changes in orbital dynamics. Bernabei et al (2003) also found evidence for DM penetrating deep underground at Gran Sasso. If, indeed, variations in G can be tied to variations in gravitational potential, we have a new tool to assess the DM density.

**NISHI, Ryoichi**

We analyze the Gaia DR2 stellar data, comparing with the large area rado map of Orion A Giant Molecular Cloud (GMC), to investigate the aggregate star formation process in GMCs. As a part of Nobeyama Radio Observatory of NAOJ Star Formation Project, we paerformed large area mapping observation of Orion A, in 13CO, C18O and some other molecular lines using the Nobeyama 45-m radio telescope. We investigated both the frame structure of Orion A and the detailed structure of dense cores in Orion A. On the other hand, we have picked up stellar associations around Orion A with the Gaia DR2 data, and are investigating the physical characteristics of them, such as the structure, the overall motion, the interenal motion, the age, and so on. We will investigate the relation of thess characteristics and the structure of Orion A and will discuss star formation process in GMCS.

**Fang, Bess Yiyuan**

Recent progress in time and frequency metrology has led us to optical clocks with unprecedented performance. One of the key constituent of these clocks is the ultra-stable lasers whose fractional frequency fluctuations are typically around one part in 10^16 today, hindering further improvement of the stability of optical clocks. Frequency-locking a laser to a spectral hole in rare-earth doped crystals at cryogenic temperature has been shown to be a promising alternative to the more traditional use of high finesse Fabry-Perot cavities when seeking a laser with a very high short term stability. We demonstrate here a novel technique for achieving such stabilization, based on generating a heterodyne beat-note between a master laser and a co-propagating slave laser whose dephasing caused by propagation near a spectral hole generate the error signal of the frequency lock. The master laser is far detuned from the center of the inhomogeneous absorption profile, and therefore exhibits only limited interaction with the crystal despite a potentially high optical power. The demodulation and frequency corrections are generated digitally with a hardware and software implementation based on a field-programmable gate array and a Software Defined Radio platform, making it straightforward to address several frequency channels (spectral holes) in parallel. Our technique can potentially improve the frequency stability of stablized lasers by orders of magnitude, thereby improving the performance of optical clocks and related time and frequency distribution.

**Lammers, Uwe**

The backbone of Gaia Data Release 2 (DR2) consists of the astrometric data for 1,692,919,135 sources (1,331,909,727 of which with parallax and proper motion) produced with the Astrometric Global Iterative Solution scheme. Although the solution is overall of excellent quality it is not free of systematic errors and correlations of small, but noticeable levels. Some of the found features in the data are expected and just reflect the fact that Gaia DR2 is based on merely 22 months of mission data and an immature calibration of the astrometric instrument. Other effects in the data were unexpected and are still under investigation. In this paper we characterize and discuss the quality of the solution, outlining its limitations that all users of the data should be aware of.

**Bruursema, Justice**

We introduce and present the status of the United States Naval Observatory (USNO) and UKIRT K-band Hemisphere Survey, currently underway using the Wide Field Camera (WFCAM) installed on UKIRT on Maunakea. This survey is a collaborative effort undertaken by USNO, the Institute for Astronomy, University of Hawaii, the Cambridge Astronomy Survey Unit (CASU), and the Wide Field Astronomy Unit (WFAU) in Edinburgh. In addition to the K-band survey, we hope to obtain H-band imaging for the same area. The principal objective of these surveys is to provide continuous northern hemisphere K- and H-band coverage over a declination range from δ=0° to +60° by combining over 12,700 deg2 of new imaging with the existing UKIRT Infrared Deep Sky Survey (UKIDSS), Large Area Survey (LAS), Galactic Plane Survey (GPS) and Galactic Cluster Survey (GCS). The expected 5-σ point source sensitivity is K~18.4 mag and H~18.9 mag (Vega); over three magnitudes deeper than the Two Micron All Sky Survey (2MASS). The combined survey data are meant to complement and combine with the recently obtained J-band UKIRT Hemisphere Survey (UHS) as well as complement the southern hemisphere VISTA Hemisphere Survey (VHS). In this contribution we discuss survey design, data acquisition and processing, calibration and quality control, and the current progress of the observations. The data obtained by these surveys will be made publicly available through the Wide Field Science Archive (WSA) maintained by the WFAU.

**Levkina, Polina**

Objects in the near-Earth space can be conditionally divided into two groups: operating spacecrafts - there are currently about 2000 objects, and space debris - about 17000 objects, the size of which is more than 10 cm. As a rule, operating objects have compact dimensions, and their area-to-mass ratio is small. Therefore, orbit determination for such satellites is not a laborious task. On the other hand, space debris objects can have high area-to-mass ratio (HAMR), for example, 111 m2/kg. In this case, significant perturbations of the orbits arise, including radiation pressure. Such large values of AMR and uncontrolled triaxial rotation of the object lead to fluctuations in the rate of influence of the radiation pressure force with time, and, consequently, to unmodifiable errors, not allowing to determine the orbit of these objects with the necessary accuracy. Note that the orbital evolution of objects with HAMR is significantly different from objects with small AMR. Orbital motion modeling for objects with a large but constant AMR (sun-oriented objects) revealed that the amplitude of the inclination variations is proportional to the magnitude of the AMR value. These orbits also show significant periodic changes in eccentricity. Orbits of such objects eventually expand from the initial circular shape to highly elliptical. Our research showed that for objects of space debris with a constant area-to-mass ratio equal to 25 m2/kg, the eccentricity after 5 days increases from 0.001 to 0.005.This paper presents the results of orbital determination for objects with HAMR, using optical observations obtained at the peak Terskol observatory and Zvenigorod observatory of Instutute of Astronomy of the RAS and provides numerical modelof the object motion in near-Earth space.This research supported by grant of the President of the Russian Federation No. MK-6640.2018.2.

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