The Gaia satellite was launched on December 19th 2013 : its objective was to observe over a billion stars in the Milky Way (i.e. roughly one per cent of the objects within it) and to measure their positions, distances, proper motions and physical characteristics with a heretofore unequaled accuracy.
This has now been done, thanks to the coordinated work of the scientists from 20 countries, grouped within the Consortium in order to treat and analyze ESA’s Gaia date (DPAC).
Gaia makes 500 million measurements per day. For each celestial body observed, it stores on board the information. This data is then transmitted to the ESA ground stations in Spain, Australia and Argentina, to be subsequently redistributed within the consortium which, organized in Coordination Units (CU), is responsible for their conversion into scientifically significant information.
Within this structure, France has played a major role involving about a hundred CNRS, university and observatory scientists and engineers,who have ensured 40% of the data processing, as well as the National Space Center (CNES), which has set up a center for the handling of “big data”.
The first Gaia catalogue was published on September 14th 2016. It contained the positions and brightnesses of 1 100 million (1,1 billion) stars, but the distances and the proper motions of only two million of the brightest stars.
The Gaia catalogue DR2 was made available on April 25th , after only 22 months of observations (July 2014-May 2016).
Gaia DR2 contains :
• 1,7 billion positions and brightnesses of stars ;
• 1,3 billion stellar distances and proper motions ;
• 7,2 million radial velocities ;
• 500 000 light curves of variable stars ;
• 500 000 quasars, which enables one to fix, for the first time, the celestial reference frame on the basis of optical observations of extragalactic bodies
• 160 million surface temperatures and 77 million radii as well as the stellar luminosities. With the help of these parameters, it has been possible, for example to identify 120 000 solar twins ;
• 14 000 asteroids.
Eagerly awaited by astronomers world wide, this catalogue open a new chapter in astronomical research. By combining astrometric, photometric and spectroscopic data, , Gaia will furnish an unprecedented harvest of information about the Galaxy, leading to a detailed study of its three dimensional structure, its cinematic, its origin and its evolution.
At the same time as this data is being made available, starting April 25th 2018 a series of scientific papers will be published in the journal Astronomy and Astrophysics. This work was done in order to validate the quality of the new data set and highlight the potential of Gaia.
Spectroscopic analysis :
The Gaia DR2 data contains the very first catalogue of radial stellar velocities, obtained using the inboard spectrograph Radial Velocity Spectrometer (RVS), with a much better accuracy than foreseen. This catalogue of the radial velocities of 7,2 millions stars is the largest in the world and the only one to cover the whole celestial sphere. The radial velocity measured along the stellar line of sight is the indispensable addition to astrometric measurements of motions tangential to the celestial sphere, in order to study the three dimensional dynamics of stellar populations and thereby access the history of the formation of our galaxy.
Paola Sartoretti is the leading author of the paper :*
*« Gaia Data Release 2 : Processing, validation and performance of the spectroscopic data »
Hertzsprung–Russell diagram :
The Gaia data furnish the most detailed and complete Hertzsprung–Russell (H-R) available to date. These diagrams represent stellar luminosities as a function of their color or temperature. They enable one to classify stars and determine their evolutionary state. Eagerly awaited by all, the Gaia H-R diagrams will have fundamental applications in numerous studies concerning stellar evolution and galactic populations.
Carine Babusiaux, leading author of the paper :*
*« Gaia Data Release 2 : Observational Hertzsprung-Russell diagrams »
Galactic structure :
The outstanding data obtained by Gaia will enable one to make the most accurate three dimensional map of the positions and motions and chemical compositions of stars in our galaxy.
David Katz, leading author of two papers :*
*• « Gaia Data Release 2 : Mapping the Milky Way disk kinematics »*
*• « Gaia Data Release 2 : Properties and validation of the radial*
*velocities »
The solar system :
The Gaia data for the positions of solar system bodies are a factor of 100 more accurate than before. These results suggest that one could use the Gaia observations to determine in great detail the motions asteroids, thereby enabling one to study their evolution and origin.
Paolo Tanga, second author of the paper :*
*« Gaia Data Release 2 : Observations of Solar System objects »
Gaia is a mission foreseen to last a nominal five years. ESA has already approved an indicative extension till the end of 2020, which should be confirmed towards the end of 2018.
List of the laboratories involved in Gaia
The main contributors to the data analysis are:T
• The GEPI (Observatoire de Paris - PSL / CNRS)
• The IMCCE (Observatoire de Paris - PSL / CNRS / UPMC / Université de Lille 1)
• The SYRTE (Observatoire de Paris - PSL / CNRS / Sorbonne Université / LNE)
• The LERMA (Observatoire de Paris - PSL / CNRS / Sorbonne Université /ENS -
PSL / Université de Cergy-Pontoise)
• The Lagrange laboratory (Observatoire de la Côte d’Azur / CNRS / Université
Côte d’Azur) at Nice
• The OSU THETA Franche-Comté-Bourgogne (CNRS / Franche-
Comté University) at Besançon
• The Bordeaux astrophysics laboratory (CNRS / Bordeaux University)
• The Strasbourg Observatory (CNRS / Strasbour University)
• The Laboratoire Univers et Particules at Montpellier (CNRS /
Montpellier 2 University)
• The Paris Institute for Astrophysics (CNRS / Sorbonne University)
• The Grenoble Instittute for Planetology and Astrophysics (IPAG - CNRS/UGA)
laboratoire de l’Observatoire des Sciences de l’Univers de Grenoble (OSUG)
33 scientists from the Paris Observatory - PSL are involved in the DPAC (all departments)
Context of the Gaia mission :
The measurement of distances in the Galaxy and beyond is of capital importance in astronomy. This is particularly difficult. Seen from the Earth, the stars are scattered over the celestial sphere, but they are in fact distributed over a huge volume. Taking for instance the stars in the constellation of the Lion, its stars are in fact dispersed over many distances – from less than 10 to over 700 light years. Only the measurement of distances enables us to situate the stars in spaced.
Distance measurement
How does one measure stellar distances ? A particular phenomenon enables us to understand how this measurement is done : seen from the Earth, in orbit around the Sun, a given so-called neighboring star does not appear in the same place with respect to « distant » stars, depending on where the Earth is in its orbit (for example, in winter and in summer). The problem is that these minute changes in position are extremely difficult to make from the surface of the Earth. The first ones were made 170 years ago. But only since the dawn of the space era has it been possible to improve their precision significantly, as it became possible to eliminate the effects of atmospheric turbulence.
A French idea
As early as 1965, Pierre Lacroute, at that dime Director of the Strasbourg Observatory, had the original idea at the heart of Hipparcos, the first astrometric satellite of the European Space Agency. In operation from 1989 to 1993, Hipparcos measured the distances and proper motions of 118 218 stars to an accuracy 50 time better than ground based measurements. The Hipparcos catalogue, published in 1997 is still a reference in this domain.
The birth of Gaia
With this success behind them, European astronomers suggested as early as 1992 a new project for ESA, with the ambition to measure a billion bodies with a precision 50 times better than that of Hipparcos. « Proposing a satellite with two telescopes having much larger mirrors than that of Hipparcos, and using a much more detector technology, we hope to rise to the challenge of extreme precision and reach a microsecond of arc », recalls Catherine Turon, astronomer at the Paris Observatory and a pioneer in space astrometry. Gaia will revolutionize the domain of celestial cartography at the dawn of the XXIst century, by including the 3^rd dimension, and will enable one to combine astrometeric , photometric and spectroscopic data.
Data reduction
The quantity of data to handle is to date unprecedented in astronomy. The treatment of the whole set of scientific data was under the responsibility of an international consortium, following a call for tender launched by ESA in 2006, and to which responded a group of European scientists making up a consortium referred to as the DPAC (Data Processing and Analysis Consortium). Today, the DPAC is constituted of 422 European scientists and 10 others from the rest of the world, and who are responsible for the design and development of data analysis.
French research has a major role in this work
The French astronomical community is in the forefront of the mission. With about a hundred scientists involved in the mission, France is the principal European actor, followed by Italy and Great Britain. The project management for the construction of the satellite is the responsibility of the French aerospace group Airbus Defense & Space (latterly EADS/Astrium) in Toulouse. The CNES, the French space agency contribute in an important way to the Gaia mission via the development, housing and operation the French data processing center (DPCC, Data Processing Center CNES), an essential element in the data processing chain.
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