Gaia Data Release 2

The astrometric solution

¹ Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, 22100 Lund, Sweden
² ESA, European Space Astronomy Centre, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
³ HE Space Operations BV for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
⁴ Lohrmann-Observatorium, Technische Universität Dresden, Mommsenstrasse 13, 01062 Dresden, Germany
⁵ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 14, 69120 Heidelberg, Germany
⁶ Vitrociset Belgium for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
⁷ Telespazio Vega UK Ltd for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
⁸ Institut de Ciències del Cosmos, Universitat de Barcelona (ICCUB), Martí Franquès 1, 08028 Barcelona, Spain
⁹ Institute for Astronomy, School of Physics and Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁰ Gaia Project Office for DPAC/ESA, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
¹¹ Aurora Technology for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, Via Osservatorio 20, Pino Torinese, Torino 10025, Italy
¹⁴ Elecnor Deimos Space for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
¹⁵ SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, LNE, 61 avenue de l’Observatoire, 75014 Paris, France
¹⁶ GPA-Observatorio National/MCT, Rua Gal. Jose Cristino 77, CEP 20921-400 Rio de Janeiro, Brazil
¹⁷ Serco for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
¹⁸ INAF, Osservatorio Astrofisico di Catania, Catania, Italy
¹⁹ Astronomical Institute, Bern University, Sidlerstrasse 5, 3012 Bern, Switzerland
²⁰ EURIX SRL, Corso Vittorio Emanuele II, 61, 10128 Torino, Italy
²¹ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
²² University of Torino, Department of Computer Science, Torino, Italy
²³ ESA, European Space Research and Technology Centre, Keplerlaan 1, 2200 AG, Noordwijk, The Netherlands
²⁴ University of Padova, Via Marzolo 8, Padova 35131, Italy
²⁵ RHEA for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
²⁶ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Géo Azur, 250 rue Albert Einstein, CS 10269, 06905 Sophia Antipolis Cedex, France
²⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
²⁸ Observatoire Astronomique de l’Université de Genève, Sauverny, Chemin des Maillettes 51, 1290 Versoix, Switzerland
²⁹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Rd, 200030 Shanghai, PR China
³⁰ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
³¹ Las Cumbres Observatory, 6740 Cortona Dr. 102, Goleta, CA 93117, USA
³² Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
³³ ALTEC, Corso Marche 79, Torino 10146, Italy
³⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
³⁵ Università di Torino, Dipartimento di Fisica, via P. Giuria 1, 10125, Torino, Italy
³⁶ ATG for ESA/ESAC, Camino Bajo del Castillo s/n, 28691 Villanueva de la Cañada, Spain
³⁷ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands

Received: 29 January 2018
Accepted: 6 March 2018

Abstract

Context. Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase.

Aims. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these resultsperformed within the astrometry task.

Methods. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. These data were calculated in two steps. First, the satellite attitude and the astrometric calibration parameters of the CCDs were obtained in an astrometric global iterative solution for 16 million selected sources, using about 1% of the input data. This primary solution was tied to the extragalactic International Celestial Reference System (ICRS) by means of quasars. The resulting attitude and calibration were then used to calculate the astrometric parameters of all the sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion.

Results. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G < 14 mag) sources, 0.1 mas at G = 17 mag, and 0.7 masat G = 20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas yr⁻¹, respectively.The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas yr⁻¹. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr⁻¹ in proper motion are seen on small (< 1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.