# Symposium 348 - Abstracts

## Probing the Galactic population of black holes with OGLE and Gaia

Wyrzykowski, Lukasz

As shown by recent gravitational wave detections, galaxies harbour an unknown population of black holes at high masses. In our Galaxy such dark objects can be found and studied solely via gravitational microlensing method. We describe our search for black hole lenses in both archival OGLE data and among on-going microlensing events found by OGLE and Gaia. Combination of superb time-domain astrometry from Gaia and photometry will allow to uniquely derive masses and distances to these dark lenses and describe the demographics of the unseen component of the Milky Way.

## New Astrometric Directions in Globular Star Cluster Research

Richer, Harvey

In this talk I will cover a number of new areas of research on globular star clustersthat are being made possible with astrometric observations. Among these are the influence of Gaia, the use of ultraviolet imaging and the enormous effect thatJWST will have. I will cover issues ranging from the distances to the clusters,the search for Intermediate Mass Black Holes and cluster brown dwarfs.

## Dynamics of multi-planet systems and the importance of astrometric measurements

Correia, Alexandre

Dynamical studies are very important to fully understand and characterise multi-planetary systems. However, the complete architecture of these systems can only be obtained through astrometric measurements. In particular, the classification of the orbital types and the determination of the habitable zones rely on an accurate determination of the mutual inclinations. In this talk, we will revisit the main limitations of traditional detection techniques, and illustrate the importance of astrometry with some real examples.

## Modern statistical and machine learning methodologies applied to astrometric data

de Souza, Rafael

Astrometry and statistics have a long entangled history, and Astrostatistics is at the same time one of the oldest disciplines and one of the youngest. Of particular note was Hipparchus (190–120 BCE) who, following on the earlier work of Aristarchus of Samos (310–230 BCE and Eratosthenes (276–147 BCE), is widely regarded as the first to clearly apply statistical principles to the analysis of astronomical events.  In 1801 Hungarian Franz von Zach applied the new least squares regression algorithm developed by Carl Gauss for predicting the position of Ceres as it came into view from its orbit behind the Sun.Nowadays, given the sheer volume and quality of astrometric data, modern methodologies are imperative to enable an optimal knowledge extraction from such data. Many different approaches, frequentist and Bayesian alike, are adopted by the astronomical community to exploit astrometric data, and to extract information from regression, clustering and classification solutions. These results have a broad impact in astronomy and astrophysics: from dynamical studies of open clusters, dwarf galaxies, and stellar streams, to stellar and galaxy astrophysics studies leading to near-field cosmology, and to more distant cosmological inferences from multiple calibrations of the distance ladder. In this talk,  I will present an overview of some methods that are being adopted to probe astrometric data, and in particular, the ones currently used to explore the Gaia Data Release 2.

## Hypervelocity stars as powerful dynamical tracers of dark matter

Rossi, Elena Maria

In this talk, I will review theoretical and observational efforts to use a unique class of objects to trace the amount and shape of dark matter in our Galaxy: hypervelocity stars. These are observed in the halo with velocities in excess of the local escape speed and trajectories consistent with coming from the Galactic Centre. The current sample of ~20 has been found in the SDSS survey and I will show how it has been exploited to constrain the dark matter halo. I will also look into the near future and show the prospects for Gaia to increase the sample by a factor of 10 and our forecasting on how dark matter constraints can be improved using the Gaia larger and less bias sample.

## Astrometric Measurements for Studies of Dwarf Galaxies

Watkins, Laura

Where is the mass in galaxies? From extensive observational campaigns, we have a good understanding of the distributions of the stellar and gaseous components, but the distribution of dark matter is still uncertain, especially in the centres of galaxies. Cosmological simulations suggest that galaxies form with a cuspy central dark matter profile, but that baryonic feedback processes can flatten the profiles toward cores. So determining accurate mass profiles not only provides tests of the cosmological model on galaxy scales, but also places strong constraints on the baryonic processes that have shaped galaxies. Since we cannot measure dark matter profiles directly, we use the kinematics of the stars in the galaxies. So far kinematic studies have been limited to line-of-sight velocities, which are sparse and probe only one component of the three-dimensional motion of the stars. Proper motion studies provide the other two components of motion and thus are crucial to make progress. Aside from stars in our Milky Way, proper motions can only be measured for stars in nearby dwarf galaxies. Even then it is a challenge that demands extremely high astrometric precision, better even than the Hubble Space Telescope and Gaia. Such datasets also demand similarly high standards in the modelling machinery used for their analysis.  My talk will focus on the future prospects for measuring accurate proper motions in nearby dwarf galaxies with which their internal dynamics may be fully characterised. I will also discuss some work we have been doing using Hubble and Gaia proper motions to study other stellar systems surrounding the Milky Way and the state-of-the-art dynamical models we have developed to analyse them.

## Probing the nature of dark matter with Theia: a mission concept for ultra precise differential astrometry

MAMON, Gary

Space-astrometry is a widely transversal discipline, touching most areas of Astronomy. Theia is a space-astrometry mission concept dedicated to differential astrometric measurements in targeted fields reaching 60 times the accuracy of Gaia: from hundreds of nano-arcseconds to 10 micro-arcseconds, in targets from magnitudes 5 to 20. Such astrometric precision is due to a highly stable, continuously monitored, payload concept that relies on state-of-the-art materials and mechanical design, a stable optical design, multiple robust metrology systems and an overall optimized mission architecture. I will describe how, with proper motions and parallaxes achieved with such ultra-sharp astrometry, Theia should achieve breakthroughs on the nature of Dark Matter, the equation of state of neutron stars and possible quark stars, and a complete sample of habitable-zone Earth-like planets in the nearest stars, with a focus on the cusp/core dark matter debate in dwarf spheroidal galaxies.

## Exoplanet Astrometry Methods for WFIRST

Bennett, David

The Wide Field Infrared Survey Telescope (WFIRST) is NASA’s first flagship survey telescope. One of its major surveys will be the WFIRST exoplanet microlensing survey that will find planets down to the mass of Mars in orbits wider than 0.5 AU. When combined with the results from Kepler, WFIRST will provide a statistical of planets down to low masses at all orbital separations. The microlensing light curves provided by WFIRST will yield direct measurements of the planet-star mass ratio, but WFIRST’s high angular resolution will allow astrometric measurements of the unresolved host and source stars that can determine the masses of the host star and planet. I present new results that demonstrate this method with high angular resolution follow-up observations from the Hubble Space Telescope and adaptive optics observations with the Keck-2 telescope. I also discuss astrometric microlensing observations that can be made with WFIRST that can measure the masses of planets orbiting stellar remnants. Finally, I discuss a novel method for WFIRST’s wide field camera to obtain astrometry of nearby stars that may allow the detection of habitable zone terrestrial planets orbiting a handful of nearby stars.

## About planets around the pulsars B0329+54 and B0525+21

Starovoit, Elena

At the moment, several thousands exoplanets are known and pulsar planets are one of the rarest types of exoplanets. Our study is devoted to the investigation of two prospective owners of the planetary system: these are PSR B0329+54 and PSR B0525+21. For research of the pulsar B0329+54 we used the measurements obtained during 1968–2012 at the Pushchino Radio Astronomy Observatory (at 102 and 111 MHz), the Jet Propulsion Laboratory (2388 MHz) and the Kalyazin Radio Astronomy Observatory (610 MHz). For study of the PSR B0525+21 we used the data obtained at the Jet Propulsion Laboratory (2388 MHz) and the Jodrell Bank Observatory (408, 610, 910, 1410, 1630 MHz). The span of the pulsar's B0525+21 measurements cover 36 years. The barycentric post-fit timing residuals of the pulsars B0329+54 and B0525+21 have harmonic variations which can be explained by the presence of planets orbiting these pulsars. The motion models and parameters of the proposed pulsar planets have been determined. The planet of the PSR B0329+54 has orbital period P1 = 27.8 yr, orbital semi-major axis a1 = 10.26 AU, eccentricity e1 = 0.23, and mass m1sini = 2M⊕. In addition, there are short-period variations in the timing residuals of that pulsar, but the existence of the second planet of the PSR B0329+54 predicted by previous researchers with the period about 3 yrs has not been confirmed. The planet of the pulsar B0525+21 has orbital period P2 = 20.2 yr, orbital semi-major axis a2 = 8.4 AU, eccentricity e2 = 0.96, and mass m2sini = 0.5M⊕. Additionally, the precession of the planet's line of apses has been measured, and it is dφ = -0.77 rad/period. A high eccentricity may indicate that this planet has survived a supernova explosion.

## A systematic search for hypervelocity stars from the LAMOST spectroscopic surveys

Huang, Yang

We have carried out a systematic search for Hypervelocity stars (HVSs) in nearly 6.5 million qualified (SNRs > 10) stellar spectra of 4.4 unique stars collected upto June 2016 by LAMOST spectroscopic surveys, and have discovered three HVSs, including the one found earlier by Zheng et al. (2014). The two newly discovered HVSs are respectively a B2V type star of ∼ 7 solar mass with a Galactic rest- frame radial velocity of 502 km/s at a Galactocentric radius of ∼21kpc and a B7V type star of ∼ 4 solar mass with a Galactic rest-frame radial velocity of 408 km/s at a Galactocentric radius of ∼ 30 kpc. More importantly, in the era of Gaia, accurate measurements of three dimensional motions become possible for the three bright HVSs. The measurements provide constraints on the shape of the dark matter halo of the Galaxy.

## ABUNDANCES OF SUPERHEAVY ELEMENTS IN EXPLOSIVE ASTROPHYSICAL ENVIRONMENT

BARUAH, RULEE

Dynamical astrophysical event like Supernova is believed to be one of the most probable site for the production of heavy elements by r-process nucleosynthesis. Here at extremely high temperature and density, the neutron capture is much faster than beta decays and heavy and superheavy neutron rich elements are formed which are otherwise not possible by fusion reactions . The particular model we have used is the beta delayed explosion of massive stars powered by neutrino energy deposition in a hot-bubble. Astrophysical parameters needed for our analysis are temperature (> 109 degrees K) and neutron number density which we take to be greater than 1020 cm-3. In the later expansion stages after SN explosion where the neutron density supposedly falls, the r-process nucleosynthesis produces the heavy elements which subsequently beta decays and the r-process path forms. The experimental data of observed elements are found to be in agreement with our calculated ones along the path. It is found that the superheavy elements' (Z > 105) formation along the r-process path is highly favored with the increase in temperature. Later ejecta are neutron-rich (Ye < 0.5) and leaves behind a compact neutron star . We note that the element 98Cf 254 shown by the SN light curves is found in our classical astrophysical condition of T = 1.9× 109 K and nn = 1020 cm-3. Also we note an element of mass 273 corresponding to atomic number 115, at temperature 3.0 × 109 K and neutron density 1020 cm−3. It is found that the dynamical timescale of the final collapse is dominated by electron capture on nuclei and not on free protons.

## Detection of Exoplanets in Multisystem Stars Using Quantization of Orbits

Detection of the exoplanets in multisystem stars represents one of the important tasks in the coming interval of time. Thousands of multisystem in particular triple systems have been observed. The detection of the nature of the third body as a planet represents a hard question to be answered in triple systems. In a previous theoretical trial, Morcos in 2013, derived a simple relation, in the framework of scale relativity (Nottale (1997)), to find the orbital period of solar system plants by using a quantized form. That formula has been adapted to calculate the quantum orbital number of some exoplanets (Morcos, et al (2016)). In this work, the previously mentioned relation has been reformed using Hill radius of Margot (2015) to detect exoplanets in multisystem stars. The results may through a light on detection of the third body as an exoplanet by using the quantization of the body's orbit.

## Numerical integration of gravitational field for general three-dimensional objects and its application to gravitational study of grand design spiral arm structure

Fukushima, Toshio

In order to construct a realistic rotation curve of Milky Way theoretically, we present a novel method to integrate the gravitational field for general three-dimensional objects (Fukushima 2016, MNRAS, 463:1500). By adopting the spherical polar coordinates centered at the evaluation point as the integration variables, we numerically compute the volume integral representation of the gravitational potential and of the acceleration vector. The variable transformation completely removes the algebraic singularities of the original integrals. The comparison with exact solutions reveals around 15 digits accuracy of the new method. Meanwhile, the 6 digit accuracy of the integrated gravitational field is realized by around 10^6 evaluations of the integrand per evaluation point, which costs at most a few seconds at a PC with Intel Core i7-4600U CPU running at 2.10 GHz clock. By using the new method, we show the gravitational field of a grand design spiral arm structure as an example. The computed gravitational field shows not only spiral shaped details but also a global feature composed of a thick oblate spheroid and a thin disc. The developed method is directly applicable to the electromagnetic field computation by means of Coulomb's law, the Biot-Savart law, and their retarded extensions.

## A novel approach to detect periodicity in two-dimensional astrometry

Zucker, Shay

I hereby propose a novel approach to detect periodicities in two-dimensional astrometric data. Periodic astrometric signals are the telltale signs of orbital motion in binary stars and exoplanets. The conventional approach to detect such periodic signals is by first applying the one-dimensional (Generalized) Lomb-Scargle periodogram, separately to each coordinate (right ascension and declination), and then adding the two periodograms to obtain a joint periodogram. This approach has two limitations. First, it is sensitive to the inherent arbitrariness in the definition of the two coordinates: a different coordinate definition will change the obtained periodogram. Second, it is biased towards the detection of circular orbits, which induce pure sinusoidal variability on the two axes. The novel approach I present here is much more suitable and natural to two-dimensional data, and is sensitive to signals which are not necessarily circular. It is based on the newly introduced statistical concept of 'distance correlation', I have recently adapted this concept in order to construct a periodicity search technique for one-dimensional signals - the 'phase distance correlation (PDC) periodogram'. The work I present here is an adaptation of the PDC periodogram to two-dimensional data. The future planned astrometric instruments and space missions will benefit considerably from such a technique, when they address the issue of astrometrically detecting exoplanets, and specifically those with 'exotic' eccentric orbits.

## Small-JASMINE mission

Gouda, Naoteru

Small-JASMINE is an infrared space astrometry mission which is underway to be launched with a primary mirror aperture of 30-cm, in around 2024. Small-JASMINE will provide pallaxes accurate to ~20 micro-as for stars, brighter than Hw=12.5 mag (Hw-band: 1.1 ~ 1.7 micron) with proper motion precision of ~20 micro-as/yr. This satellite will engage in observations of a limited area around the Galactic nuclear bulge and certain specific astronomical objects. Main scientific significance of Small-JASMINE is clarification of the Galactic nuclear bulge. The Galactic nuclear bulge has a lot of interesting astronomical phenomena and objectives which have never been resolved until now, given as follows: (1) Peculiar kinematics by the inner bar or nuclear stellar disk? (2) Infall of supermassive black holes (>~0.1M solar mass) to the Galactic center? (3) Gas transformation mechanism from the outer bulge to the central region? (3)Formations of young stellar clusters? (4)Existence of higher velocity stars? (5) Amount of massive black holes (~30 solar mass), and so on. Next to this primary objectives, Small-JASMINE will have many other scientific targets selected among proposals submitted for open call program. Small-JASMINE can measure the same target every 100 minutes, so it is useful to resolve phenomena with short periods such as X-ray binaries, extrasolar planetary systems and gravitational lens effects.We have been aiming at the realization of the Small-JASMINE mission as a Medium Class mission executed by JAXA. We should have multiple steps of reviews by JAXA. We 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 in international collaboration with other missions such as Gaia, Theia, GaiaNIR, APOGEE etc.

## Discovery of new white dwarfs using Gaia accurate astrometry

Mickaelian, Areg

The discovery of new White Dwarfs (WDs) is extremely important for understanding the kinematics and dynamics of the local stellar population. They are evolutionary signatures of the Milky Way, as most of stars pass through this evolutionary stage. Gaia’s astrometric accuracy allows more detailed studies of WDs and many other stars. Gaia DR1 contains astrometric results for > 1 billion stars with < 20.7. Our method of combined calculation of proper motions (PM) and estimation of stellar distances (Mickaelian & Sinamyan 2010, MNRAS 407, 681) resulted in accuracy of 3.9 mas/yr for PM in each coordinate and revelation of 640 First Byurakan Survey UV-excess stars with PM >10 mas/yr. Adopting 50 km/s upper limit for tangential velocities, we calculated maximum distances and absolute magnitudes and estimated luminosity types for these objects, revealing 185 probable (M > 8), 69 possible WDs (6 < M < 8) and 42 candidate subdwarfs/WDs. The Digitized First Byurakan Survey (DFBS) is more efficient for discovery of new WDs by spectral energy distribution. Using Gaia data for accurate PM in combination with DFBS low-dispersion spectra and additional multiwavelength data, during the pilot survey we have revealed thousands of new WDs and candidate subdwarf/WDs. Many of them are subject for studies on binarity and variability. Gaia DR2 will provide huge amount of data to confirm these WDs and reveal many more such objects. Our method is useful for statistical studies; however, it may be also used for individual studies with an accuracy of 70% when adopting 50 km/s as upper limit for tangential velocities and with an accuracy of 90% when adopting 100 km/s as such.

## Discovery of two new hot Jupiters: WASP-163 b and WASP-170 b

Barkaoui, Khalid

We present two new hot Jupiters, WASP-163 b and WASP-170 b, discovered by the WASP-South survey [Pollacco et al. 2006]. We performed a combined analysis of radial velocity data obtained with the CORALIE spectrograph mounted on the 1.2 m Euler-Swiss telescope [D. Queloz et al. 2001] and follow-up transit observations with the TRAPPIST-North [M. Gillon et al. 2017], TRAPPIST-South [M. Gillet et al. 2011; E. Jehin et al. 2011], NITES [McCormac et al. 2014] and Euler telescopes, to determinate the system parameters (K. Barkaoui et al. in prep). WASP-163 b and WASP-170 b are slightly larger than Jupiter while being about twice more massive, and are in short orbits (<3 days) around bright (V<13) solar-type stars. The radii of WASP-163b and WASP-170b are well reproduced by classical models of irradiated giant planets [J. J. Fortney et al. 2017]. This discovery made use of a new 60 cm robotic telescope, TRAPPIST-North, installed in spring 2016 at Oukaimeden Observatory in Morocco. It is an instrumental project led by the University of Liege (Belgium), in collaboration with the Cadi Ayyad University of Marrakech (Morocco), that is, like its southern twin TRAPPIST-South, totally dedicated to the observations of exoplanet transits and small bodies of the solar system.

## Biosignature detection and statistics of nearby bio-habitable planets

Wandel, Amri

We argue that habitable-zone planets of M-dwarf stars may have temperatures supporting liquid water and complex organic molecules on at least part of their surface, for a wide range of atmospheric properties (Wandel 2018, ApJ, in press). We apply these results to Proxima Centauri b and to the Trappist-1 system and discuss the implications to searching oxygen and other biosignatures in transiting habitable planets of nearby M dwarfs. From the Kepler data, we estimate that within 30pc TESS may find ~10-40 transiting candidates.  We suggest that by detecting a few planets with atmospheric oxygen it may be possible to estimate the abundance of photosynthesis and biotic planets.

## Astrometric Data Reduction (JASMINE case)

Astrometric data reduction is very tough work with complex system identification. We Japanese team plan Nano-JASMINE and Small-JASMINE missions.  Observational strategyof Nano-JASMINE is very similar to that of Gaia, and data reduction software development is being done as collaboration with Gaia AGIS team.  On the other hand, Small-JASMINE will observe bulge region with step-stare strategy.  We are now start software develpment.  International Collaboration(s) will be also planned.Astrometric Data Reduction will be done by iterative least square fit, and need to very large matrix inversion.  Stellar motion of primary stars are simple, but calibration model will become complex. Matrix inversion part will be hard computational work but relatively simple as a software enginnering problem. But matrix generation part will need to be flexible because the systems model will change during the mission. It may be challenge in software development. We will discuss how the software development will be done in JASMINE case, and which point can be common with other astrometric missions like Theia.

## The USNO Robotic Astrometric Telescope (URAT) Program

ZACHARIAS, NORBERT

An overview of this astrometric, ground-based program is given. After completing the northern hemisphere observations the USNO astrograph began operations at CTIO in October 2015. New trigonometric parallaxes of nearby stars were obtained and published in 2018. Observations of bright stars continued until May 2018 and reductions are in progress to supplement the Gaia data.

## Using mHz QPOs to put constraints on neutron star size and equation of state

Stiele, Holger

4U 1636-536 is a well-studied LMXB, consisting of a neutron star (NS) in a 3.8 hr orbit with a companion star of about 0.4 solar masses. Showing the full range of rapid time variability, 4U 1636–53 is an ideal source for studying nuclear burning on the surface of a NS. We performed a variability study of archival XMM-Newton data of 4U 1636-536 and investigated the energy dependence of its low frequency variability. Here we present the results of our waveform analysis and phase resolved spectral investigations of the mHz quasi-periodic oscillations (QPOs). Our study showed that the oscillations are not caused by variations in the blackbody temperature of the NS, but revealed a correlation between the change of the count rate during the mHz QPO pulse and the spatial extent of a region emitting blackbody emission. The maximum size of the emission area allowed us to obtain a lower limit on the size of the NS that rules out equations of state that prefer small NS radii. Up-coming NICER data will allow us to reduce the statistical uncertainty in the lower limit on the NS radius, which together with better estimates of the hardening factor and distance, will allow for improved discrimination between different equations of state and compact star models.

## Astrometric studies of exoplanets with Gaia and GaiaNIR

Ranalli, Piero

Most of the exoplanets known today have been discovered by observingradial velocity motions and/or transits.  The astrometric shifts ofstars due to planetary orbits have been detected so far only in veryfew cases, despite a long history of interest and claims. However, itis estimated that ~20,000 planets will be detected at the end ofGaia's 5-year mission. The newly discovered planets will notnecessarily have the same orbital properties of those currently known,because of the different efficiency of the various detection techniques(astrometry, transits, radial velocities, ...). Gaia in particular hasa potential to detect long-period planets and to obtain solidmeasurements of the orbit inclination.We have recently reviewed Gaia's capabilities for exoplanet detectionusing an extensive set of simulations. In this talk, I will presentthe efficiency with which Gaia will be able to detect planets at theend of the 5 yr mission and after a 10 yr extension.  Focusing on theperiod, eccentricity, and inclination, I will show the accuracy withwhich parameters will be recovered. I will show how long-periodplanets (periods up to ~20 yr) can be detected, and uncertainties oneccentricity greatly reduced, if the mission is extended to a total of10 yr. I will consider multiple-planet systems and show Gaia'spotential in disentangling periods, eccentricities, and inclinations.Finally, I will consider the proposed infrared astrometric missionGaiaNIR, and show it potential for detecting planets, and how its datacould be merged with Gaia's to detect planets with periods of severaldecades.

## Analysis of data modern astrometric catalogues in GAIA era.

Akhmetov, Volodymyr

The new big catalogues of proper motion were created using of GaiaDR1 data. The names of these catalogues are HSOY, UCAC5, GPS1 and PMA. Using three or more catalogues in Wielen method it is easy to estimate the external accuracy of each of them. The random errors of stellar proper motions had been calculated based on the common objects from HSOY, UCAC5, GPS1 and PMA catalogues. From a comparison of PMA data with same stars from the UCAC5, GPS1 and HSOY catalogues, the equatorial components of the mutual rotation vector of these coordinate systems were determined. The angular velocities of mutual rotation of the PMA and UCAC5, HSOY, GPS1 reference frames change within range from 0.2 to 2.0 mas yr-1. The estimation of systematic and random errors of the stellar proper motions of the PMA, HSOY and GPS1 catalogues in the faintest magnitude range had been done via an analysis of formal proper motions of extragalactic objects are containing in these catalogues. Also, comparisons of stellar proper motions of these catalogues with Gaia DR2 data was planned to carry out, when it will be released.

## Geodetic VLBI, Earth rotation and the Sagnac effect

Titov, Oleg

Geodetic Very Long Baseline Interferometry (VLBI) measures the group delay and phase delay rate in the barycentric reference frame. These both observables are sensitive to the geocentric velocities of the telescopes. This results in a non-zero closure delay and closure delay rate (as a sum of the three values around the closed triangle of baselines). The group delay equation includes a term consistent to the Sagnac effect (usually applied to the ring laser interferometer technique or GNSS modeling) and, therefore, could be used to estimate the instantaneous vector of the Earth angular velocity. However, the delay rate suggests more effective way to do this job because it manifests as the Sagnac effect in the primary term of the relativistic model. The instantaneous vector of the Earth angular velocity is estimated with accuracy of  10-12 - 10-13 1/sec with a small set of modern geodetic VLBI data. This provides an opportunity to detect the Thomas (geodetic) precession, the Lense-Thirring effect (frame-dragging) and even the angular rotation of the Galaxy using the full 40-year set of geodetic VLBI observations.

## Dwarf Galaxy Dynamics in Standard and Modified Gravity

Hodson, Alistair

I will present ongoing research into dwarf galaxy dynamics in standardgravity with dark matter and in modified gravity. I investigatetheoretically and numerically the internal dynamics of dwarf spheroidalgalaxies, like the core-cusp problem, with the aim to determine thefeasibility of using future observational astrometric space missions to distinguish between dark matter models, including warm, self-interacting or superfluid dark matter, and models of modified gravity, where the presence of dark matter is unnecessary, as supported by the stringent upper limits on the gamma-ray fluxes from dark matter annihilation provided by the Fermi mission.

## GaiaNIR - Progress towards a future all-sky astrometry mission

Hobbs, David

With Gaia's second data release in April of this year Europe entered a new era of space astrometry. A weakness of Gaia is that it only operates at optical wavelengths as much of the Galactic centre and the spiral arm regions are obscured by interstellar extinction. In 2017 our proposal to ESA’s call for “New Science Ideas” was successful. Since then work began to define science requirements and a dedicated study was performed by ESA in the Autumn of 2017 to determine how best to implement the mission. A NIR mission requires new types of detectors to scan the entire sky and measure global absolute parallaxes. The spacecraft must have a constant rotation resulting in a moving image that must be compensated for by, for example, operating the detectors in Time Delayed Integration (TDI) mode. During the ESA study two approaches were considered, 1) by adding a descan optical mechanism to hold the image constant on conventional NIR detectors and 2) to develop new TDI detectors in NIR and maintain the classical Gaia like approach. The study found that the descan mechanism resulted in astrometric accuracies that were an order of magnitude worse at the faint end compared to using a TDI like approach. Clearly, such a degradation of accuracy would make a descan mechanism a difficult sell to the science community where improved accuracy at the faint end is a clear goal. Nevertheless, developing TDI in NIR detectors is a difficult and expensive challenge. If this challenge can be solved a new Gaia-like mission separated by a 20 year interval would give; 1) NIR astrometry and photometry to penetrate the obscured regions and to observe intrinsically red objects; 2) improved proper motions with 14-20 times smaller errors than from Gaia alone opening up new science cases; 3) allow the slowly degrading accuracy of the Gaia optical reference frame, which will be the basis for future astronomical measurements, to be maintained.

## The Dark Matter Distribution in HI Dominated Disc Galaxies and its Role in Star Formation

Das, Mousumi

Along the Hubble Sequence the late type spirals, low luminosity dwarfs and irregular galaxies are the most HI rich galaxies and their gas disks can be several times the size of their stellar disks. They are usually dark matter dominated systems and have HI rotation curves that extend into regions with hardly any visible stellar disk. Hence the question arises as to what helps to support the vertical equilibrium of the HI gas layer in the absence of a stellar disk? To explain this, we propose that there is dark matter in the disks of HI dominated galaxies and not just in their halos. The disk dark matter  provides the gravitational support for the HI gas layer. The disk dark matter can also contribute to the disk surface density required for local instabilities to form in the gas leading to the onset of star formation. This is important for explaining outer disk star formation as observed in XUV galaxies and the inside out growth of galaxy disks. We present a simple method for determining the radial distribution of disk dark matter using the HI velocity dispersion of face-on disk galaxies derived from their moment2 maps. We apply our method to the nearby dwarf spiral galaxy NGC4701 and show that the disk dark matter is important for lowering the Toomre Q factor in the halo dominated region of the galaxy, where there is NUV emission from star formation, but no visible stellar disk.

## Recovering and discovering clusters and moving groups with Gaia

Anders, Friedrich

Gaia DR2 will likely increase the known Galactic star cluster population by orders of magnitude. Using Gaia DR2, I will demonstrate the usefulness of a combined dimensionality reduction/clustering approach to detecting groups of comoving stars, and estimate the number of new clusters that may be discovered with DR2 using different statistical methods. I will also highlight the prospects of using combining spectroscopic survey data with Gaia DR2 for chemical and chemo-kinematical tagging in the Galactic disc.

## Mixture Model for Kinematics and Memberships of Open Cluster

Shao, Zhengyi

The Mixture-model approach is improved to measure the kinematics of open clusters, while all astrometric data, such as the coordinate position, proper motion and radial velocity, are used simultaneously to separate the motion of the cluster from that of the field stars. A Bayesian inference approach is employed to deal with the case of missing data of proper motion or radial velocity, and the Bayesian evidence is calculated to estimate the necessity of the mixture-model,since it is a quantitative identification of the existence of a cluster. The involving of multiple kinds of observational data, especially the high-precision radial velocity, significantly decreases the contamination degree of the cluster and field stars, and leads to the much more accurate value of the mean proper motion of the cluster. Based on the UCAC5 and radial velocities from RAVE and APOGEE, about 2000 clusters are determined. Meanwhile, the kinematic membership probabilities of individual stars are calculated. This method is expected to apply to the homogenous data from GAIA-DR2 to establish the cluster sample with full kinematic parameters. Furthermore, the dynamics of the Milky way disk will be analyzed by tracing of the open clusters.

## Machine Learning for RR Lyrae Stars

Zhang, Yanxia

We explore machine learning to target RR Lyrae stars from large survey databases (SDSS and GALEX). Classification of RR Lyrae stars from other types of stars belongs to the imbalance learming issue. Therefore the usual machine learing algrithms are not fit for this kind of issue. In this paper,cost-sensitive support vector machine,cost-sensitive random forest and fast boxes are applied to select RR Lyrae stars from other types of stars. Experimental results show that fast boxes is the best performer and the infomation added from ultraviolet band is helpful to seperate RR Lyrae stars.

## Discovery of new white dwarfs using Gaia accurate astrometry

Mickaelian, Areg

The discovery of new White Dwarfs (WDs) is extremely important for understanding the kinematics and dynamics of the local stellar population. They are evolutionary signatures of the Milky Way, as most of stars pass through this evolutionary stage. Gaia’s astrometric accuracy allows more detailed studies of WDs and many other stars. Gaia DR1 contains astrometric results for > 1 billion stars with < 20.7. Our method of combined calculation of proper motions (PM) and estimation of stellar distances (Mickaelian & Sinamyan 2010, MNRAS 407, 681) resulted in accuracy of 3.9 mas/yr for PM in each coordinate and revelation of 640 First Byurakan Survey UV-excess stars with PM >10 mas/yr. Adopting 50 km/s upper limit for tangential velocities, we calculated maximum distances and absolute magnitudes and estimated luminosity types for these objects, revealing 185 probable (M > 8), 69 possible WDs (6 < M < 8) and 42 candidate subdwarfs/WDs. The Digitized First Byurakan Survey (DFBS) is more efficient for discovery of new WDs by spectral energy distribution. Using Gaia data for accurate PM in combination with DFBS low-dispersion spectra and additional multiwavelength data, during the pilot survey we have revealed thousands of new WDs and candidate subdwarf/WDs. Many of them are subject for studies on binarity and variability. Gaia DR2 will provide huge amount of data to confirm these WDs and reveal many more such objects. Our method is useful for statistical studies; however, it may be also used for individual studies with an accuracy of 70% when adopting 50 km/s as upper limit for tangential velocities and with an accuracy of 90% when adopting 100 km/s as such.

## Observing in the past thanks to the Gaia astrometric reference star catalogue

Arlot, Jean-Eudes

The arrival of the Gaia astrometric reference star catalogue is not only a giant step in the astrometric accuracy allowing to measure new unknown small motions of solar system objects but also to observe in the past with today accuracy. In fact, accurate astrometry is performed since about one century after the use of photographic plates. The measures made at the time of the old observations used old reference catalogues, the accuracy of which being between 500 mas and one arcsec. We will show how we are making new measures and new reductions of photographic plates thanks to the Gaia reference catalogue allowing to provide valuable data and pre-discoveries of recently observed objects.

## Mapping the Milky Way's dark matter with high-precision astrometry

Bovy, Jo

Stellar tidal streams originating from disrupting globular clusters in the Milky Way’s halo hold enormous promise as probes of both the large-scale structure of the Milky Way halo’s density distribution and its small-scale structure. As such, the observed density, spatial, and kinematic structure of stellar streams can provide important new constraints on the interactions and small-scale structure of dark matter. I will discuss recent progress on modeling the dynamics of stream formation and its application to fitting observed stellar stream data from photometric, spectroscopic, and astrometric observations. I will in particular focus on the crucial role of high-precision astrometry in mapping the small-scale structure of streams and how this will help answer the fundamental question: Is dark matter cold?

## Detecting Isolated, Stellar-Mass Black Holes through Astrometric Microlensing

Sahu, Kailash

All stars with initial masses of larger than 20 solar mass are expected to end their lives as black holes (BHs). Theoretical studies suggest that there should be about 100 million stellar-mass BHs in the Galaxy. A large fraction of these BHs are expected to be single, either primordially or due to disruption of binaries by supernova (SN) explosions. Such isolated BHs are extremely difficult to detect directly, as they accrete from the ISM at very low rates, and thus they emit essentially no radiation. In fact, no isolated BH has ever been unambiguously found within our Galaxy. The only technique available to detect such isolated BHs is astrometric microlensing---the relativistic deflection of light from background stars. I will first discuss how astrometric microlensing provides a powerful tool not only to detect isolated stellar-mass black holes but also to determine their masses, distances and velocities. I will then discuss the first results from our two HST programs specifically aimed at the first detection of solitary BHs through astrometric microlensing.

## Phase-space correlations among satellites in the Local Group

Pawlowski, Marcel

Different types of phase-space correlations have been proposed or found among the satellite systems in the Local Group. Some of these appear to be in tension with cosmological simulations, most prominently the observed planes of satellite galaxies, but also a possible overabundance of satellite galaxy pairs. Other correlations, such as candidate cases for the infall of several satellites in a common group like the Crater-Leo objects, or the lopsidedness of the satellite system around Andromeda, might be more in line with cosmological expectations. I will give an overview of these correlations and discuss how precision astrometry can help us understand their origins, study their possible interdependences, and potentially resolve the challenges they pose for cosmological models.

## Ground-based astrometric project VERA : Overview, Science highlights and Synergy with Gaia

Japanese VLBI array VERA has been conducting astrometric observations toward star-forming regions and late-type stars in the Milky Way (e.g. Honma et al. 2018 in press). As of August 2017, astrometric observations of about 200 maser sources were completed, and parallaxes were derived for 97 maser sources (75 star-forming regions and 22 late-type stars). In this talk, we briefly introduce overview and future prospect of VERA project as well as recent science results on several topics, which are (i) Fundamental parameters of the Milky Way (e.g. Honma et al. 2018), (ii) Galactic (spiral) structure and dynamics (e.g. Yamauchi et al. 2016), (iii) Star formation (e.g. Sakai et al. 2017) and (iV) Period-Luminosity relation of Mira variable (e.g. Nakagawa et al. 2016). Also, we discuss synergy between VERA (radio) and Gaia (optical) astrometric projects in the Gaia era.

## Polaris: Live and -Loud-

S. Elgueta, Scarlet

Polaris (a Ursae Minoris) , the nearest and one of the brightest Cepheid, is indisputably the most famous and useful star after our Sun. Its fame is not only due to its brightness and proximity but because of its yet -not well understood- pulsation. Besides, its distance is still a matter of dispute, with a recent estimate for a parallax of~10 mas, contrasting the Hipparcos measurement of 7.54 +/- 0.11 mas which allowed Feast & Catchpole (1997) to propose that Polaris (P ~4 days) is pulsating in the first overtone, which is upheld by the recent reevaluation of the Hipparcos data (van Leeuwen et al. 2007).Usenko et al. 2015, 2016, 2017 has shown that a UMi is showing an increase of the pulsation period up to 8.6 min compared to the analysis performed in 2007, they also derived the radial velocity curve, finding that the amplitude was reduced to 3.43 km/s in 2016 and to 3.31km/s in the beginning of 2017 in comparison with 4.16 km/s in 2015. The group also derived effective temperature using the line depth method for spectroscopic data proposed by Kovtyukh in 2007, obtaining an average effective temperature Teff = 6021 K. This abrupt decrease in the amplitude is very unusual and demands verification as well as more analysis in other bands, such as infrared.In order to provide a new perspective on the problem described above, I would like to present our analysis of Polaris using NIR high resolution spectroscopy provided by our instrument WINERED (Ikeda et al. 2016), our dataset was collected during 2016 and careful analysis has been performed to determine effective temperatures and constrain the set of parameters which will allow us to estimate the distance. Having an agreement on the distance will allow us to apply the inverse Baade-Wesselink method which will help us to constrain the stellar parameters (and p factor) as well as test the forthcoming GAIA parallaxes.

## Polaris: Live and -Loud-

S. Elgueta, Scarlet

Polaris (a Ursae Minoris) , the nearest and one of the brightest Cepheid, is indisputably the most famous and useful star after our Sun. Its fame is not only due to its brightness and proximity but because of its yet -not well understood- pulsation. Besides, its distance is still a matter of dispute, with a recent estimate for a parallax of~10 mas, contrasting the Hipparcos measurement of 7.54 +/- 0.11 mas which allowed Feast & Catchpole (1997) to propose that Polaris (P ~4 days) is pulsating in the first overtone, which is upheld by the recent reevaluation of the Hipparcos data (van Leeuwen et al. 2007).Usenko et al. 2015, 2016, 2017 has shown that a UMi is showing an increase of the pulsation period up to 8.6 min compared to the analysis performed in 2007, they also derived the radial velocity curve, finding that the amplitude was reduced to 3.43 km/s in 2016 and to 3.31km/s in the beginning of 2017 in comparison with 4.16 km/s in 2015. The group also derived effective temperature using the line depth method for spectroscopic data proposed by Kovtyukh in 2007, obtaining an average effective temperature Teff = 6021 K. This abrupt decrease in the amplitude is very unusual and demands verification as well as more analysis in other bands, such as infrared.In order to provide a new perspective on the problem described above, I would like to present our analysis of Polaris using NIR high resolution spectroscopy provided by our instrument WINERED (Ikeda et al. 2016), our dataset was collected during 2016 and careful analysis has been performed to determine effective temperatures and constrain the set of parameters which will allow us to estimate the distance. Having an agreement on the distance will allow us to apply the inverse Baade-Wesselink method which will help us to constrain the stellar parameters (and p factor) as well as test the forthcoming GAIA parallaxes.

## The automated morphological classification of galaxies from the SDSS at z<1.0 by the machine learning technique

Vavilova, Iryna

We evaluated a new approach for the automated morphological classification of big samples of galaxies which is based on the combination of three methods: visual classification for training samples; diagrams «color indices – concentration index»; machine learning technique (Random forest).  This approach was tested and applied for the SDSS samples of 317,018 galaxies at z<0.1 (Dobrycheva, Vavilova, Melnyk, Elyiv, 2017, arXiv:1712.08955).  Namely, we used a well-known fact that galaxy morphological type is correlated with the color indices, luminosity, de Vaucouleurs radius, inverse concentration index etc. We plotted the diagrams of color indices g-i and one of these parameters and discovered that these parameters may be used for galaxy classification into three classes: E -- elliptical and lenticular, S -- types Sa-Scd, and L -- types Sd-Sdm and irregulars. The accuracy is 98% for E, 88% for S, and 57% for L types. The combinations of "color indices g-i and inverse concentration index R50/R90' and "color indices g-i and absolute magnitude M_r" gave the best result:  143263 E type, 112 578 S type, 61177 L type (Dobrycheva, Melnyk, Vavilova, Elyiv, 2015, Astrophysics).  After this, we undergone a training sample of 5,000 galaxies classified visually into early E (E, S0, S0a) and late L (Sa to Irr) types using a machine learning technique. To define an accuracy of classifiers we applied the 5-folds validation and found that Random Forest provides the highest accuracy. Applying it to the SDSS DR9 sample of 60,561 galaxies at z<0.1 we obtained that 47% E and 53% L types are among these galaxies.To be sure in this approach, we prepared a sample of about 1,800,000 galaxies at z<1.0 and provided the automated morphological classification. The effectiveness of this approach for galaxies at different cosmological scales, details of creation of this sample and accuracy of applied methods, as well as the obtained results will be discussed in this report.

## Masses from Astrometry of Unresolved Stars for Exoplanets Found By Microlensing

Bennett, David

Gravitational microlensing is sensitive to planets down to the mass of Mars in orbits that are beyond the reach of Kepler and other transit missions, and the microlensing survey of NASA's WFIRST mission will provide a statistical census of exoplanets at orbital separations of ~1 AU and beyond. Microlensing light curves usually provide planet-star mass ratios, but not host star and planet masses. I show how precise astrometry of the unresolved lens and source stars can yield planet and host star masses, as well as the distance to the planetary systems. For planets discovered by ground-based microlensing surveys, this can be accomplished with follow-up observations by the Hubble Space Telescope, ground-based adaptive optics systems, or the James Webb Space Telescope. No follow-up observations will be required for exoplanets discovered by the WFIRST exoplanet microlensing survey, because WFIRST's high angular resolution will allow these measurements to be made with the WFIRST images themselves. I discuss the details of this mass measurement method along with some early results, and I also discuss how our current follow-up observing program will test the predictions of the core accretion theory.

## Very wide stellar binaries as probes of gravity

Chanamé, Julio

Our present standard model of gravity, General Relativity (and Newtonian gravity, in the appropriate limit), has been experimentally tested with success at various scales and regimes, from our daily life to phenomena ranging from the Solar System to double massive black holes that merge and produce gravitational waves. Yet in the limit of very low accelerations, tests have been harder to come up with. This is an unsatisfactory situation, given that the rotation curves of disk galaxies fall precisely within that regime, and application of the standard model leads inevitably to requiring thepresence of dark matter. Even though the case for dark matter is not restricted to the rotation curves of galaxies, still they are arguably one of the most important pieces of evidence for it, and thus all assumptions leading to it must be tested by all means possible. The orbital accelerations experienced by the components of very wide binaries with semimajor axes of a few thousand AUs and larger fall within this special regime of gravity. Not only that, but they do sowithout the ambiguities associated to the potential presence of any dark matter, thus offering us this important and rare opportunity. Wide binaries have been explored for these purposes in the past, but never before with the exquisite astrometric quality that current facilities offer. I will present our latest results, based on Gaia (both TGAS as well as DR2) and state-of-the-art ground-based instrumentation, on what the widest genuine binaries are telling us about the behavior of gravity at the low acceleration regime.

## Revised estimates of the frequency of Earth-like planets in the Kepler field

Barbato, Domenico

The search for Earth-like planets around Sun-like stars and the evaluation of their occurrence rate is a major topic of research for the exoplanetary community. Two key characteristics in defining a planet as 'Earth-like' are having a radius between 1 and 1.75 times the Earth's radius and orbiting inside the host star's habitable zone; the measurement of the planet's radius and related error is however possible only via transit observations and is highly dependent on the precision of the host star's radius. A major improvement in the determination of stellar radius is represented by the Gaia astrometry satellite, which promises to provide unprecedented precision on stellar parameters.We present a new estimate of the frequency of Earth-sized planets orbiting inside the host stars's habitable zones, obtained using Gaia measurements of parallax for solar-type stars hosting validated planets in the Kepler field as input for reassessing the values of planetary radius and incident stellar flux. This updated occurrence rate can be an important element in coordinating future observational efforts searching for Earth-like system in the Sun backyard using next-generation astrometric missions.

## The Star Formation History of the Galaxy from Gaia Astrometry of White Dwarfs

von Hippel, Ted

What is the star-formation history of the Milky Way?  How old are Galactic halo and thick disk stars?  Traditional age-dating of stars relies on clusters or the white dwarf luminosity function, both of which offer only a limited view of these stellar populations.  We demonstrate that white dwarf stars with precision parallaxes offer a dramatic way forward.  Specifically, we show how optical and near-IR photometry, Gaia astrometry, and a Bayesian modeling approach allows us to characterize multimodal solutions in the mass-age plane and frequently determine precise ages (within 2%-5%) for individual white dwarfs.  These individual white dwarf ages are then pooled hierarchically to derive population age distributions.

## Differential Astrometry with Gaia simulated observations

Abbas, Ummi

We will describe the method of differential astrometry in defining a small field inertial differential astrometric reference frame at the micro-arcsecond level of precision using simulated observations of stars by Gaia.Different systematic effects due to physical effects such as those relativistic in nature, those due to the star's proper motion and the scanning motion of the satellite, along with instrumental effects due to the geometric instrument model, all affect the accuracy of such a definition. Over short intervals of time spanning several spin periods of Gaia we will show the reliability in modeling and estimating such systematics and in establishing the stability of the differential reference frame down to the micro-arcsecond level.This method has far-reaching applications in the study of key Gaia science cases over different characteristic time scales such as relativistic deflection of light experiments, extrasolar planets, and microlensing events. We will provide illustrative examples of these scientific applications.

## Classical Cepheids Near-Infrared periodo-luminosity relations based on Vista Magellanic Cloud (VMC) survey data and Gaia parallaxes

Ripepi, Vincenzo

We present new accurate Period-Luminosity (PL) and Period-Wesenheit (PW) relations in the J,Ks bands, based on a sample of more than 4000 Classical Cepheids in the Large Magellanic Cloud (LMC) in the context of the VISTA Magellanic Cloud (VMC) Survey. The extreme precision of these data allows us to study the geometry of the LMC and to establish a firm baseline for extragalactic Cepheid distance scale studies. The zero points of the PL and PW relations will be calibrated with great accuracy by using the parallaxes that will be released on April in the context of the Gaia Data Release 2. These relationships will be of fundamental importance in the next future, when JWST and ELT will progressively enter in operation.

## The theoretical scenario to interpret Gaia results for Cepheids and RR Lyrae

Marconi, Marcella

An updated theoretical scenario fro both Classical Cepheids and RR Lyrae has been developed on the basis of nonlinear convective pulsation models as a function of chemical composition. The predicted bolometric light curves have been transformed in a variety of photometric filters including the Gaia ones. The direct comparison between observed and predicted light curves is a powerful technique to constrain the intrinsic stellar parameters of the investigated pulsating stars. The application to a number of Gaia Cepheids and RR Lyrae is discussed. The theoretical Period-Luminosity and Period-Luminosity-Color relations in the Gaia filters are also presented and compared with the observations.

## Semi-analytical theory of the axially symmetrical Moon's rotation

iVANOVA, TAMARA

In the present paper the trigonometric theory of the rigid-body axially symmetrical Moon's rotation is constructed in the framework of the general planetary theory avoiding the non-physical secular terms and involving the separation of the long-period and short-period angular variables, both for planetary-lunar motions and Moon's rotation. The combined system of the equations of motion for the principal planets and the Moon and the equations of the Moon's rotation is reduced to the autonomous secular system describing the evolution of the planetary and lunar orbits independent of the Moon's rotation and the evolution of the Moon's rotation depending on the orbital planetary and lunar evolution. The secular system is solved in the trigonometric form. As a result, the theory of the axially symmetrical Moon's rotation is presented by means of the power series in evolutionary variables withquasi-periodic coefficients in mean longitudes of the planets and the Moon. All analytical calculations were performed by means of the Poisson series processor.

## Galactic tomography via information field theory

Enßlin, Torsten

Charting the 3D structure of the Milky Way from astrometric and astrophysical measurements is an information theoretical problem. The finite data provided by instruments is never able to constraint the infinite degrees of freedom of the 3D fields of the various physical quantities, like gravitational potential, gas, dust, dark matter densities, and others. Additional information like physical laws and empirical correlations has to be folded into the field inference. Information field theory (IFT) is a generic Bayesian framework for field reconstruction that enables the consistent fusion of measurement data and theoretical concepts. I will review the progress on IFT-based methods to reconstruct the 3D structure of the Milky Way in various quantities and discuss requirements for instrument descriptions that permit such analysis. Finally, I will present the concept of an Universal Bayesian Imaging toolKit (UBIK) to fascilate multi-dimensional field reconstructions.

## A new calibration of the cosmic distance ladder based on Gaia parallaxes of RR Lyrae stars

Muraveva, Tatiana

Parallaxes, proper motions, positions, and multi-band time series data for hundreds of thousands variable stars will become public as a part of the Gaia Data Release 2 (DR2) on 25 April, 2018. Precise Gaia DR2 parallaxes for a large sample of MW RR Lyrae stars will be used in order to calibrate the zero point of the RR Lyrae near-infrared period-luminosity (PL) and PL-metallicity (PLZ) relations. These relations are crucially important to set the basis of the extragalactic distance ladder with RR Lyrae stars. The new Gaia parallax-calibrated relations are used to study the distances to the Large and Small Magellanic Clouds.

## Using proper motions to constrain the origin of galaxies

Fritz, Tobias

Due to their proximity,  the globular clusters and dwarf galaxiessatellites of the Milky Way can be observed in great detail from theirresolved stars. Proper motions can provide crucial information on theorigin of several of these systems. In this talk we present results usingground based images and Gaia DR2 data to measure the proper motion ofglobular clusters and dwarf galaxies in the halo of the Milky Way. Due toits orbit and metallicity, we find that the Pyxis globular cluster did notform in-situ, but it was likely accreted from an unknown, likely destroyeddwarf galaxy more luminous than M_V=-10. Furthermore, we find that theultra faint Segue~1 was not a satellite of known dwarf galaxies moreluminous than M_V=-8.8 and therefore likely formed on its own without being"a satellite of a satellite". In case of Gaia DR2 data, we concentrate onthe recent discovered ultra-faint galaxies to explore their possibleassociated to the Magellanic clouds and use them to make inferences ontotal mass of the Magellanic clouds.

## The Gaia Archive. Status and challenges for Data Release 3+

Mora, Alcione

The data produced by the ESA Gaia mission, and analysed by the DPAC consortium, are provided to the Astronomical community via the Gaia Archive located at ESAC, and its partner and affiliated data centres.The bulk of Data Release 2 is a big catalogue (gaia_source), distributed using the Virtual Observatory TAP protocol, complemented with some ad-hoc extensions. The main extra features include a private space for each registered user (persistent uploads), and the possibility to share these resources among other researchers for a collaborative data analysis. These capabilities make the Archive a key component in the pre-release validation, when the data are ingested but only accessible to the dedicated DPAC team.Additional resources are also available at the Archive. Notably, detailed information on a sample of variable stars, together with their associated light curves, and epoch data for a selection of known asteroids. A set of major survey and their pre-computed cross-matches against Gaia positions is also provided for convenience.A number of technical improvements have been introduced, like the use of arrays, which is a novelty for TAP based archives. As opposed to the first data release, the light curves are not stored as a plain table, but in a separate infrastructure specifically designed to support the massive downloads needed to cope with Data Release 3+, when e.g. epoch data (trillions of photometric and astrometric points), average and epoch spectra (billions) and astrophysical parameter distribution functions (billions) will be progressively available. The additional data products will be indexed using the DataLink Virtual Observatory protocol.Additional features have also been incorporated, such as the possibility to propagate the Gaia positions, proper motions, parallaxes and their uncertainties to any epoch for easier cross-catalogue comparisonFinally, a summary of the system behaviour and users' feedback after the release will be presented.

## Astrophysics with Gaia Wide Binaries

Andrews, Jeff

With orbital periods in excess of thousands of years, wide binaries have traditionally been identified by finding common proper motion stars within astrometric catalogs. Using the additional inclusion of parallax measurements, sophisticated Bayesian algorithms designed to mine the first Gaia data release have robustly identified thousands of new wide binaries. Already, we are using this sample to place new constraints on stellar multiplicity, the strength of gravity in the weak acceleration regime, and the potential for chemical tagging to identify unique Galactic subpopulations. The much larger second Gaia data release provides precise proper motions and parallaxes for over 1 billion stars, allowing for the identification of wide binary samples of an unprecedented scale; however coupled with this opportunity is a challenging data science problem requiring modern statistical tools, efficient algorithms, and large computing clusters to accurately classify stellar pairs as wide binaries rather than the chance coincidence of unassociated stars. I will discuss the status of our search for wide binaries within the second Gaia data release, as well as how our resulting samples can help solve several open stellar and galactic astrophysical problems, particularly when our sample is cross-matched with large scale spectroscopic surveys such as APOGEE and GALAH.

## On the astrometric bias for objects with overlapping image profiles

Ivantsov, Anatoliy

Astrometric measurements of celestial objects that lie close in the imaging plane are subject to a small systematic error due to an overlap in their image profiles. The latter causes a displacement of the maxima of measured intensities with respect to their true positions. In fact, any astrometric measurement is affected by this bias, if the position of one object is not corrected for the displacement induced by light pollution from other nearby objects.In this contribution, we present an exact analytical expression for this displacement error assuming symmetric Gaussian point-spread functions. The astrometry of fast moving objects in the Solar system is particularly negatively affected by this issue. In order to assess the significance of this bias, we have analyzed past ground-based astrometric measurements of asteroids available through the IAU Minor Planet Center that had asteroids within a distance of 3 arcsec to Gaia catalogue stars. Regions of high correlation densities were detected between the proposed corrections in both right ascension and declination and (O—C) in positions calculated using the JPL HORIZONS system. Our results suggest that the above-described bias is indeed present and can be detected in past ground-based astrometric measurements of asteroids.As past astrometric measurements prolong the observation arc they tend to have more leverage on asteroid orbit solutions than present ones although the latter may be more precise. Correcting for such biases can be, therefore, crucial for asteroid impact monitoring.To better quantify and control the here described biases during orbit fitting we suggest requesting the measured image width (FWHM) from asteroid observers when submitting observations to the IAU Minor Planet Center. We emphasize, furthermore, that this type of bias is not exclusive to ground-based observations but is affecting space-borne astrometry, such as Gaia measurements of the closely imaged objects, as well.

## Accurate Parallaxes of Classical Cepheids: Opportunities and Challenges for the Distance Scale

Anderson, Richard I.

The cosmic distance scale (CDS) is currently undergoing significant renovation thanks to Cepheid parallaxes being measured with 30-100 µas accuracy by Gaia and the Hubble Space Telescope (HST). These extremely accurate parallaxes are expected to significantly improve the calibration of the Leavitt law (period-luminosity relation) and thus play a crucial role for elucidating the nature of dark energy and understanding recently reported tension between the Hubble constant (H0) measured via the CDS and H0 inferred using the Cosmic Microwave Background assuming standard cosmology. H0 tension may point to missing physics in the cosmological model; however, careful analyses must address possible systematics involved in the calibration of the CDS before such a claim can be made. Here we present recent research aimed at unveiling CDS systematics related to classical Cepheids, their companion stars, and their occurrence in clusters. Part one presents a detailed vetting process of high-quality Galactic Leavitt Law calibrators based on an unprecedented catalog comprising of >15,000 precise radial velocity measurements of more than 300 Galactic classical Cepheids. We systematically search for (spectroscopic) binaries that may bias or increase the uncertainty of parallaxes measured using sparse data (HST) or short temporal baselines (Gaia DR2, 22 months). We have thus discovered more than a dozen new binaries and determine upper limits on non-detections in support of the measured parallax accuracy.In part two, we use deep HST imaging of M31 to directly measure the mean photometric bias due to cluster companions on Cepheid-based distances. We quantify the combined H0 bias from close associations (binaries and clusters) to be approximately 0.3% (0.20 km s-1 Mpc-1) for the passbands commonly used. We thus demonstrate that stellar association bias does not explain H0 tension and does not prevent achieving the community goal of measuring H0 with an accuracy of 1%.

## Maser stars tracing the Milky Way

van Langevelde, Huib

Astrometric studies of Galactic radio sources using VLBI provide unique measurements of the Galactic distribution of both very young and evolved stars. Because radio observations are not affected by interstellar extinction, such measurements complement the results tat are obtained by Gaia. In this presentation the synergies are explored between VLBI and Gaia to study stellar populations in the Milky Way. For example we use te new Gaia output to find optical cluster companions to high mass star forming regions that harbour methanol and water masers. But we also use Gaia to characterise the AGB star population that we defined in te infrared. Using the SiO masers emission of these stars will allow for a study of the dynamics in the inner Galaxy.

## Can we recovering our sensitivity to Earth-like planets around Sun-like stars? Extending the Kepler baseline with PLATO

Christiansen, Jessie

Although the Kepler mission was designed to measure the occurrence rate of Earth-twins, a confluence of factors resulted in a data set that fell just short of the required sensitivity. There are several dozen habitable-zone, Earth-size candidates that are just above the significance threshold, and more which are just below. Here we investigate the possibility of improving our detections of these candidates with the addition of data from the ESA PLATO mission. PLATO is planning to re-observe the Kepler field during one of its two long stare surveys.

## LEGOLAS: the evolution

Riva, Alberto

The study of gravitational waves, and the associated information on crucial events throughout the Universe, is rapidly progressing with the definition and implementation of sensitive, large scale experiments either on ground (such as VIRGO, LIGO etc.) or in space. The goal of our investigation is to address the feasibility of an approach based on strictly astronomical, and in particular astrometric, techniques, for pointed observations of selected astronomical objects in our Galaxy, like compact binary systems, neutron stars and compact white dwarf binaries, which are expected to be sources of gravitational waves in the Very Low Frequency range, i.e 10-4 Hz < fg < 10-1 Hz. In those systems the source light positions evolve periodically with respect to the observer, inducing a modulation of the apparent orbital motion by the gravitational action on the photon paths. The experiment is based on indirect high precision astrometric observations, monitoring the astrometric light deflection of the photons crossing the buffer zone of the gravitational source. Microarcsecond level accuracy astrometry may allow separation from the actual orbital motion of the additional term associated to the gravitational waves. Following the idea of Legolas (Riva et al 2010), we propose an evolution of the instrument concept with a novel opto-mechanical configuration in order to provide a robust instrument. It is designed with an innovative approach, using astrometric techniques in order to have a well-defined and stable PSF. The design is focused on the suppression of the PSF wings through a smart annular displacement of the apodized-aperture. We describe the proposed experiment profile and the expected performance.

## Galactic rotation curve beyond the solar circle out to R0+10 kpc

Dambis, Andrey

We use the sim 12000 O-A-type star sample in the Galactic anticenter direction from the catalog of Monguio et al. (2013, 2014) with photometric distances (zero point calibrated via TGAS trigonometric parallaxes) and other parameters determined from Stroemgren photometry and proper motions from UCAC5 and GPS1 catalogs to analyze the run of the Galactic rotation curve and periodic variations of rotation velocity due to spiral density waves in the Galactocentric distance interval from R0 to R0+10 kpc. We find the smoothed rotation curve to be almost flat in the Galactocentric distance interval considered with a small depression of circular velocity at R0+3 kpc. When Gaia DR2 data becomes available, the analysis will be repeated including the trigonometric-parallax based recalibration of the photometric distance scale for the objects considered.

## The Astrometric Gravitation Probe Mission Concept

Busonero, Deborah

The Astrometric Gravitation Probe (AGP) is the concept of a space mission for Fundamental Physics tests in the Solar system, based on high precision differential astrometry, in a modern rendition of the 1919 Dyson-Eddington-Davidson experiment.The key innovation is a space-borne telescope with built-in permanent eclipse provided by a coronagraphic system.The precision goal on the "$\gamma$" and "$\beta$" parameters of the Parametrised Post-Newtonian formulations of General Relativity and competing models is respectively in the $10^{-8}$ and $10^{-7}$ range, improving by one or two orders of magnitude with respect to the expectations on current or near future experiments.Such precision is suitable to detect possible deviations from the unity value (associated to generalised Einstein models for gravitation) in the weak field limit applicable to the Sun neighbourhood, with potentially huge impacts on our understanding of the cosmological distribution of dark matter and dark energy from a Solar system scale experiment.The measurement principle is based on the differential astrometric signature on the stellar positions, i.e. based on the spatial component of the effect rather than the temporal component as in recent experiments using radio link delay timing variation (Cassini).The instrument concept is based on multiple field, multiple aperture Fizeau interferometry, observing simultaneously regions close to the Solar limb, embedding coronagraphic techniques, and other fields in opposition to the Sun.The design is focused on systematic error control through multiple field simultaneous observation and calibration.We describe the science motivation, the proposed mission profile, the instrument concept and the expected performance.Furthermore, we describe the potential for upgrade to larger scale experiments throughout the current century.

## Detection of giant planets combining astrometry and radial velocities: the GAIA potential.

Giacobbe, Paolo

The global impact of Gaia micro-arcsecond level astrometric measurements in the astrophysics of planetary systems has been addressed in the past (e.g. Sozzetti 2011). However, those studies only provided general metrics for gauging detectability thresholds. We revisit the topics of planet detection and characterization with Gaia focusing on the sample of nearby low-mass stars. The main thrust of this work is three-fold. First, we gauge the Gaia potential precision astrometry of exoplanets orbiting an actual sample of thousands of known dM stars. We then express Gaia sensitivity thresholds as a function of system parameters and in view of the latest mission profile. The analysis of the simulations results will also provide insight on the capability of high precision astrometry to reconstruct the underlying orbital element distributions and occurrence rates of the planetary companions. These results will help in evaluating the expected Gaia recovery rate of actual planet populations around late-type stars. Second, we investigate some elements of the synergy between the Gaia data on nearby M dwarfs and other ground-based and space-borne programs for planet detection and characterization, with a particular focus on: a) the potential for Gaia to precisely determine the orbital inclination, which might indicate the existence of transiting long period planets; b) the ability of Gaia to accurately predict the ephemerides of (transiting and non-transiting) planets around M stars, and c) its potential to help in the precise determination of the emergent flux, for direct imaging and systematic spectroscopic characterization of their atmospheres with dedicated observatories from the ground and in space. Third, we test a Hybrid Markov Chain Monte Carlo Dierential Evolution approach with the aim of tackling the challenge of combining astrometry and radial velocities of single planetary systems.

## GRAL: Search for Gravitational Lenses in GAIA DR2

Ducourant, Christine

The 'Gaia Gravitational Lenses' group (GraL) has dedicated efforts to systematically identify and characterize in current and future data releases of the ESA/Gaia satellite multiply imaged quasars by gravitational lensing. Owing to its exceptional angular resolution and very accurate photometry, Gaia is expected to observe about 3000 gravitational lenses among which 250 should have three or more lensed images. For the first time a survey of such phenomena over the entire sky is possible, thanks to Gaia, allowing to derive relevant constraints and invaluable  dataset  particularly  well-suited for extragalactic and cosmological studies.Here we present the activities of the GraL working group, we describe the instrumental and observational characteristics that turn Gaia into a powerful tool for detecting these rare extragalactic phenomena. We detail the method we employed for identifying them and present the most promising candidates we extracted based on the second data release of Gaia. Finally, we see how the uncertainties on the parameters of the lens models can be dramatically reduced by comparing the modeling of already known lenses using Gaia observations, on one hand, and observations from other ground-based and space observatory on the other hand.

## The GAIA Revolution in Pre-Main Sequence Stars and Star Clusters

Hillenbrand, Lynne

The Gaia mission, most recently through its DR2, has provided the long soughtthird dimension" in studies of stars and star clusters, includingstar forming regions.  Membership lists can now be refined and used to improvemeasurements of e.g. stellar IMF's, cluster ages and age spreads,disk evolution, and stellar rotation evolution.  Several case studies will beprovided, including in Sco Cen and lambda Ori, demonstrating the improvements enabled by nonmember rejection and individual star parallax information.  In addition, cluster kinematics can be probed for the first time at the sub-km/s level.  We have recently found (Kuhn et al. 2018, submitted) that among a sample of several tens of clusters and associations less than a few Myr old, that about 75% show expansion, with typical outward velocities of 0.25-1.5 km/s.  There is little evidence for cluster rotation.The 1-dimensional velocity dispersions range from 1 to 3 km/s, with morepopulous (massive) clusters having larger velocity dispersions, as expected.We also examine the motions of subgroups of stars within those regions containingsubclusters, and find that there is no evidence for ongoing coalescence of these groups. The kinematic results can provide constraints on the formationand dispersal of young clusters and useful inputs for simulations ofstar and star-cluster formation.

## Polaris: Live and -Loud-

S. Elgueta, Scarlet

Polaris (a Ursae Minoris) , the nearest and one of the brightest Cepheid, is indisputably the most famous and useful star after our Sun. Its fame is not only due to its brightness and proximity but because of its yet -not well understood- pulsation. Besides, its distance is still a matter of dispute, with a recent estimate for a parallax of~10 mas, contrasting the Hipparcos measurement of 7.54 +/- 0.11 mas which allowed Feast & Catchpole (1997) to propose that Polaris (P ~4 days) is pulsating in the first overtone, which is upheld by the recent reevaluation of the Hipparcos data (van Leeuwen et al. 2007).Usenko et al. 2015, 2016, 2017 has shown that a UMi is showing an increase of the pulsation period up to 8.6 min compared to the analysis performed in 2007, they also derived the radial velocity curve, finding that the amplitude was reduced to 3.43 km/s in 2016 and to 3.31km/s in the beginning of 2017 in comparison with 4.16 km/s in 2015. The group also derived effective temperature using the line depth method for spectroscopic data proposed by Kovtyukh in 2007, obtaining an average effective temperature Teff = 6021 K. This abrupt decrease in the amplitude is very unusual and demands verification as well as more analysis in other bands, such as infrared.In order to provide a new perspective on the problem described above, I would like to present our analysis of Polaris using NIR high resolution spectroscopy provided by our instrument WINERED (Ikeda et al. 2016), our dataset was collected during 2016 and careful analysis has been performed to determine effective temperatures and constrain the set of parameters which will allow us to estimate the distance. Having an agreement on the distance will allow us to apply the inverse Baade-Wesselink method which will help us to constrain the stellar parameters (and p factor) as well as test the forthcoming GAIA parallaxes.

## The USNO Robotic Astrometric Telescope (URAT) Program

Zacharias, Norbert

An overview of this astrometric, ground-based program is given. After completing the northern hemisphere observations the USNO astrograph began operations at CTIO in October 2015. New trigonometric parallaxes of nearby stars were obtained and published in 2018. Observations of bright stars continued until May 2018 and reductions are in progress to supplement the Gaia data.

## LEGOLAS: The Evolution

Riva, Alberto

The study of gravitational waves, and the associated information on crucial events throughout the Universe, is rapidly progressing with the definition and implementation of sensitive, large scale experiments either on ground (such as VIRGO, LIGO etc.) or in space. The goal of our investigation is to address the feasibility of an approach based on strictly astronomical, and in particular astrometric, techniques, for pointed observations of selected astronomical objects in our Galaxy, like compact binary systems, neutron stars and compact white dwarf binaries, which are expected to be sources of gravitational waves in the Very Low Frequency range, i.e 10-4 Hz < fg < 10-1 Hz. In those systems the source light positions evolve periodically with respect to the observer, inducing a modulation of the apparent orbital motion by the gravitational action on the photon paths. The experiment is based on indirect high precision astrometric observations, monitoring the astrometric light deflection of the photons crossing the buffer zone of the gravitational source. Microarcsecond level accuracy astrometry may allow separation from the actual orbital motion of the additional term associated to the gravitational waves. Following the idea of Legolas (Riva et al 2010), we propose an evolution of the instrument concept with a novel opto-mechanical configuration in order to provide a robust instrument. It is designed with an innovative approach, using astrometric techniques in order to have a well-defined and stable PSF. The design is focused on the suppression of the PSF wings through a smart annular displacement of the apodized-aperture. We describe the proposed experiment profile and the expected performance.

## The Gaia Mission and its Extension

Brown, Anthony

I will summarise Gaia DR2 and highlight some of the science that has come out since April 25, and briefly discuss the plans for data releases 3 and 4. A status update of Gaia will be provided, showing that observations can in principle continue for another 5 years beyond the nominal five year mission which ends in mid-2019. Doubling the mission lifetime of Gaia will benefit all the elements in the data releases through the factor 1.4 gain in signal-to-noise. The major impact however, will be on the proper motions which improve by a factor of almost 3, while more complex source motions, such as due to planetary companions, can improve by factors up to 20. A number of science cases will be discussed that can only be achieved through the extended mission.

## Near Field Cosmology and Astrometry

Bland-Hawthorn, Joss

Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars, from white dwarfs to supergiants. The oldest components provide unique insight into how galaxies form and evolve over billions of years. This is a veritable golden age for galactic archaeology with many large surveys now under way to map both chemistry and kinematics across the Galaxy. Detailed 6D phase space information combined with chemistry for millions of stars heralds a new era in how we slice up the Galactic disc. This has already enabled the most remarkable discovery to emerge from Gaia DR2 — the "phase spiral" — as we discuss. We review the main science goals of galactic archaeology, and look to what the future may hold. These studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.

## Astrometric Measurements for the Determination of H0

Casertano, Stefano

The value of the Hubble constant, as determined from measurements in the local universe, is significantly larger than the value inferred from CMB measurements combined with a Lambda-CDM cosmology. Recent astrometric results from both HST and Gaia have improved the local measurements, to the point that the discrepancy is now approaching 4-sigma significance. We review the key elements of the local determination of H0, and briefly discuss its potential implications.

## The Equation of State of Compact Objects and the Importance of Astrometric Measurements

Oertel, Micaela

Modelling compact stars is a complex task which depends on many ingredients, among others the properties of matter at densities exceeding that in the center of atomic nuclei. Understanding matter in the interior of compact stars thus probes the nature of the strong interaction under conditions dramatically different from those in terrestrial experiments. In this talk, I will present models for ultra-dense matter and existing experimental, theoretical and observational constraints. Future prospects to improve our understanding of ultra-dense matter, in particular with constraints from neutron star observations and gravitational wave detections will be discussed, too.

XXX IAU General Assembly | ACV - Austria Center Vienna  | Bruno-Kreisky-Platz 1  | 1220 Vienna