[ABRIDGED] CONTEXT: Gaia DR2 has opened a trove of astrometric and photometric data for Galactic clusters close to the Sun. Lucky imaging has been an operational technique to measure the positions of visual binary systems for a decade and a half, enough to apply its results to the calculation of orbits of some massive multiple systems within ~1 kpc of the Sun. AIMS: We are measuring distances to Galactic stellar groups containing O stars and I start with two of them: Collinder 419 in Cygnus and NGC 2264 in Monoceros. I also aim to derive new astrometric orbits for the Aa,Ab components for the main ionizing sources for both clusters: HD 193 322 and 15 Mon, respectively. METHODS: First, I present a method that uses Gaia DR2 photometry, positions, proper motions, and parallaxes to obtain the membership and distance of a stellar group and apply it to Collinder 419 and NGC 2264. Second, I present a new code that calculates astrometric orbits by searching the whole 7-parameter orbit space and apply it to HD 193 322 Aa,Ab and 15 Mon Aa,Ab using as input literature data from the Washington Double Star Catalog (WDS) and the AstraLux measurements recently presented by Maíz Apellániz et al. (2019). RESULTS: I obtain Gaia DR2 distances of 1006+37-34 pc for Collinder 419 and 719+-16 pc for NGC 2264, with the main contribution to the uncertainties coming from the spatial covariance of the parallaxes. The two NGC 2264 subclusters are at the same distance (within the uncertainties) and they show a significant relative proper motion. The distances are shown to be robust. HD 193 322 Aa,Ab follows an eccentric (e = 0.58+0.03-0.04) orbit with a period of 44+-1 a and the three stars it contains have a total mass of 76.1+9.9-7.4 M_Sol. The orbit of 15 Mon Aa,Ab is even more eccentric (e = 0.770+0.023-0.030), with a period of 108+-12 a and a total mass of 45.1+3.6-3.3 M_Sol for its two stars.
We examine the capacity to identify binary systems from astrometric errors and deviations alone. Until the release of the fourth Gaia data release we lack the full astrometric time series that the satellite records, but as we show can still infer the presence of binaries from the best fit models, and their error, already available. We generate a broad catalog of simulated binary systems within 100 pc, and examine synthetic observations matching the Gaia survey's scanning law and astrometric data processing routine. We show how the Unit Weight Error (UWE) and Proper Motion Anomaly (PMA) vary as…
We report on the first results from a large-scale observing campaign aiming to use astrometric microlensing to detect and place limits on the mass of single objects, including stellar remnants. We used the Hubble Space Telescope to monitor stars near the Galactic Center for 3 years, and we measured the brightness and positions of $\sim$2 million stars at each observing epoch. In addition to this, we monitored the same pointings using the VIMOS imager on the Very Large Telescope. The stars we monitored include several bright microlensing events observed from the ground by the OGLE collaboration…
Gaia DR3 astrometric orbit determination with Markov Chain Monte Carlo and Genetic Algorithms. Systems with stellar, substellar, and planetary mass companions
Context. Astrometric gravitational microlensing can be used to determine the mass of a single star (the lens) with an accuracy of a few percent. To do so, precise measurements of the angular separations between lens and background star with an accuracy below 1 milli-arcsecond at different epochs are needed. Hence only the most accurate instruments can be used. However, since the timescale is in the order of months to years, the astrometric deflection might be detected by Gaia, even though each star is only observed on a low cadence. Aims. We want to show how accurately Gaia can determine the m…
Context. The first release of astrometric data from Gaia will contain the mean stellar positions and magnitudes from the first year of observations, and proper motions from the combination of Gaia data with Hipparcos prior information (HTPM). Aims. We study the potential of using the positions from the Tycho-2 Catalogue as additional information for a joint solution with early Gaia data. We call this the Tycho-Gaia astrometric solution (TGAS). Methods. We adapt Gaia's Astrometric Global Iterative Solution (AGIS) to incorporate Tycho information, and use simulated Gaia observations to demonstra…
Near-infrared spectroscopic surveys have uncovered a population of short-period, blended-light spectral binaries composed of low-mass stars and brown dwarfs. These systems are amenable to orbit determination and individual mass measurements via astrometric monitoring. Here we present first results of a multi-year campaign to obtain high-precision absolute astrometry for spectral binaries using the Gemini-South and Gemini-North GMOS imagers. We measure the complete astrometric orbits for two systems: 2M0805$+$48 and 2M1059$-$21. Our astrometric orbit of 2M0805$+$48 is consistent with its 2-year…
We have developed a new, fast method of estimating the orbital properties of a binary or triple system using as few as two epochs of astrometric data. FOBOS (Few Observation Binary Orbit Solver) uses a flat prior brute force Monte Carlo method to produce probability density functions of the likely orbital parameters. We test the code on fake observations and show that it can (fairly often) constrain the semi-major axis to within a factor of 2-3, and the inclination to within $\sim$20$^{\circ}$ from only two astrometric observations. We also show that the 68 and 95 per cent confidence intervals…
The Gemini Multi-conjugate adaptive optics System (GeMS) is a facility instrument for the Gemini-South telescope. It delivers uniform, near-diffraction-limited image quality at near-infrared wavelengths over a 2 arcminute field of view. Together with the Gemini South Adaptive Optics Imager (GSAOI), a near-infrared wide field camera, GeMS/GSAOI's combination of high spatial resolution and a large field of view will make it a premier facility for precision astrometry. Potential astrometric science cases cover a broad range of topics including exo-planets, star formation, stellar evolution, star …
We have measured an annual parallax of the Mira variable R~Ursae~Majoris (R~UMa) with the VLBI exploration for Radio Astronomy (VERA). From the monitoring VLBI observations spanning about two years, we detected H$_2$O maser spots in the LSR velocities ranges from 37 to 42 km\,s$^{-1}$. We derived an annual parallax of 1.97$\pm$0.05\,mas, and it gives a corresponding distance of 508$\pm$13\,pc. The VLBI maps revealed 72 maser spots distributed in $\sim$110 au area around an expected stellar position. Circumstellar kinematics of the maser spots were also revealed by subtracting a systemic motion…
Stellar activity is a potential important limitation to the detection of low mass extrasolar planets with indirect methods (RV, photometry, astrometry). In previous papers, using the Sun as a proxy, we investigated the impact of stellar activity (spots, plages, convection) on the detectability of an Earth-mass planet in the habitable zone (HZ) of solar-type stars with RV techniques. We extend here the detectability study to the case of astrometry. We used the sunspot and plages properties recorded over one solar cycle to infer the astrometric variations that a Sun-like star seen edge-on, 10 pc…
We apply a PCA-based pre-whitening method to the entire collection of main Kepler mission long-cadence data for KIC 8462852 spanning four years. This technique removes the correlated variations of instrumental origin in both the detected light curves and astrometry, resolving intrinsic changes in flux and image position of less than 100 ppm and 1 mas, respectively. Beside the major dips in the light curve during mission quarters 8 and 16, when the flux dropped by up to 20%, we confirm multiple smaller dips across the time span of observation with amplitudes ranging from 0.1% to 7%. A variation…
Extrasolar planet searches targeting very low-mass stars and brown dwarfs are hampered by intrinsic or instrumental limitations. Time series of astrometric measurements with precisions better than one milli-arcsecond can yield new evidence on the planet occurrence around these objects. We present first results of an astrometric search for planets around 20 nearby dwarf stars with spectral types M8-L2. Over a timespan of two years, we obtained I-band images of the target fields with the FORS2 camera at the Very Large Telescope. Using background stars as references, we monitored the targets' ast…
Astrometric data from the recent Gaia Data Release 1 has been matched against the sample of stars from Kepler with known rotation periods. A total of 1,299 bright rotating stars were recovered from the subset of Gaia sources with good astrometric solutions, most with temperatures hotter than 5000 K. From these sources, 894 were selected as lying near the main sequence using their absolute G-band magnitudes. These main sequence stars show a bimodality in their rotation period distribution, centered roughly around a 600 Myr rotation-isochrone. This feature matches the bimodal period distribution…
One of the key phases of stellar evolution that remains poorly understood is stellar death. We lack a predictive model for how a star of a given mass explodes and what kind of remnant it leaves behind (i.e. the initial-final mass relation, IFMR). Progress has been limited due to the difficulty in finding and weighing black holes and neutron stars in large numbers. Technological advances that allow for sub-milliarcsecond astrometry in crowded fields have opened a new window for finding black holes and neutron stars: astrometric gravitational lensing. Finding and weighing a sample of compact obj…
Extrapolation of the astrometric motion of the nearby low-mass star VB 10 indicates that sometime in late December 2011 or during the first 2-3 months of 2012, the star will make a close approach to a background point source. Based on astrometric uncertainties, we estimate a 1 in 2 chance that the distance of closest approach ρ_{min} will be less than 100mas, a 1 in 5 chance that ρ_{min}<50mas, and a 1 in 10 chance that ρ_{min}<20mas. The latter would result in a microlensing event with a 6% magnification in the light from the background source, and an astrometric shift of 3.3 mas. The lensing…
We present orbital elements and mass sums for eighteen visual binary stars of spectral types B to K (five of which are new orbits) with periods ranging from 20 to more than 500 yr. For two double-line spectroscopic binaries with no previous orbits, the individual component masses, using combined astrometric and radial velocity data, have a formal uncertainty of ~0.1 MSun. Adopting published photometry, and trigonometric parallaxes, plus our own measurements, we place these objects on an H-R diagram, and discuss their evolutionary status. These objects are part of a survey to characterize the b…
The GAPS programme at TNG LXXV. Validating and confirming Gaia substellar astrometric candidates with HARPS-N
INAF – Osservatorio Astrofisico di Torino（意大利国家天体物理研究所都灵天文台）；Department of Physics, University of Rome Tor Vergata（罗马第二大学物理系）；INAF – Osservatorio Astronomico di Palermo（意大利国家天体物理研究所巴勒莫天文台）；INAF – Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；INAF – Osservatorio Astrofisico di Catania（意大利国家天体物理研究所卡塔尼亚天文台）；INAF – Osservatorio Astronomico di Trieste（意大利国家天体物理研究所里雅斯特天文台）；Dipartimento di Fisica, Università degli Studi di Torino（都灵大学物理系）；INAF – Osservatorio Astronomico di Brera（意大利国家天体物理研究所布雷拉天文台）
The ESA global astrometry space mission Gaia has been monitoring the position of a billion stars since 2014. The analysis of such a massive dataset is challenging in terms of the data processing involved. In particular, the blind detection and characterization of single or multiple companions to stars (planets, brown dwarfs, or stars) using Gaia astrometry requires highly efficient algorithms. In this article, we present a set of analytical methods to detect and characterize companions in scanning space astrometric time series as well as via a combination of astrometric and radial velocity tim…
In this paper we develop a differential astrometric framework that is appropriate for a scanning space satellite such as Gaia. We apply it to the first of the GAREQ fields, which is the fruit of dedicated efforts within the Gaia project focused on measuring the relativistic deflection of light close to Jupiter's limb. This provides a preliminary assessment of: a) the observability of the relativistic deflection of light close to Jupiter, and, b) Gaia's astrometric capabilities under extremely difficult conditions such as those around a bright extended object. Inputs to our differential astrome…
A Scalable Path to Astrometric Exomoon Discoveries with the Nautilus Space Observatory
University of Arizona（亚利桑那大学）；Seoul National University（首尔国立大学）；Massachusetts Institute of Technology（麻省理工学院）；University of Maryland, College Park（马里兰大学帕克分校）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；Center for Research and Exploration in Space Science and Technology II (CRESST II)（空间科学与技术研究探索中心二期）；NASA Ames Research Center（美国宇航局艾姆斯研究中心）；Max Planck Institute for Solar System Research（马克斯·普朗克太阳系研究所）；University of Graz（格拉茨大学）；Space Telescope Science Institute（太空望远镜科学研究所）
The formation of massive planetary or brown dwarf companions at large projected separations from their host star is not yet well understood. In order to put constraints on formation scenarios we search for signatures in the orbit dynamics of the systems. We are specifically interested in the eccentricities and inclinations since those parameters might tell us about the dynamic history of the systems and where to look for additional low-mass sub-stellar companions. For this purpose we utilized VLT/NACO to take several well calibrated high resolution images of 6 target systems and analyze them t…
The Hipparcos catalog and its Double and Multiple System Annex (DMSA) lists 4099 components with individual proper motions and coordinates on the epoch 1991.25. Many of these long-period binary stars are also present in the Gaia Data Release 2 (DR2). Using the available relative positions and proper motions separated by 25.25 years, the equations of relative orbital motion can be solved for the two epoch eccentric anomalies, orbital period, and eccentricity. The method employs elimination of the linear Thiele-Innes unknowns and nonlinear optimization of the remaining condition equations. The q…
Mass-dependent multiplicity fraction in low mass stars revealed by Gaia astrometry
Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）；Fakultät für Physik und Astronomie, Universität Heidelberg（海德堡大学物理与天文学院）；Department of Astronomy, California Institute of Technology（加州理工学院天文学系）
Partial measurements of relative position are a relatively common event during the observation of visual binary stars. However, these observations are typically discarded when estimating the orbit of a visual pair. In this article we present a novel framework to characterize the orbits from a Bayesian standpoint, including partial observations of relative position as an input for the estimation of orbital parameters. Our aim is to formally incorporate the information contained in those partial measurements in a systematic way into the final inference. In the statistical literature, an imputati…
We present orbital elements, orbital parallaxes and individual component masses, for fourteen spatially resolved double-line spectroscopic binaries derived doing a simultaneous fit of their visual orbit and radial velocity curve. This was done by means of a Markov Chain Monte Carlo code developed by our group, which produces posterior distribution functions and error estimates for all the parameters. Of this sample, six systems had high quality previous studies and were included as benchmarks to test our procedures, but even in these cases we could improve the previous orbits by adding recent …
The pre-main sequence star CHXR74 (M4.25) in ChaI was detected a few years ago to be a very low-mass spectroscopic binary. Determination of its mass would provide a valuable dynamical mass measurement at young ages in the poorly constrained mass regime of <0.3 Msun. We carried out follow-up radial velocity (RV) monitoring with UVES/VLT between 2008 and 2011 and high-resolution adaptive optic assisted imaging with NACO/VLT in 2008 with the aim to constrain the binary orbit. We present an orbital solution of the system based on the combined RV data set which spans more than 11 years of UVES moni…
Context. It has been shown that convection in red supergiant stars gives rise to large granules causing surface inhomogeneities together with shock waves in the photosphere. The resulting motion of the photocenter (on time scales ranging from months to years) could possibly have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the granulation on the photocentric and photometric variability. We quantify these effects in order to better characterize the error possibly altering the parallax. Methods. We use 3D radiative-hydrodynamics simulations of convectio…
