We present new empirical Colour-Colour and Effective Temperature-Colour Gaia Red Clump calibrations. The selected sample takes into account high photometric quality, good spectrometric metallicity, homogeneous effective temperatures and low interstellar extinctions. From those calibrations we developed a method to derive the absolute magnitude, temperature and extinction of the Gaia RC. We tested our colour and extinction estimates on stars with measured spectroscopic effective temperatures and Diffuse Interstellar Band (DIB) constraints. Within the Gaia Validation team these calibrations are also being used, together with asteroseismic constraints, to check the parallax zero-point with Red Clump stars.

Red Clump (RC) stars are known to be good distance indicators. However the accuracy on these calculations strongly depends on the knowledge of their basic properties: a complete calibration is required.

With Gaia currently in nominal mission mode and sending data to earth, the challenge for the astronomical community is to prepare for the use of what will be at the time of release one of the largest and most complex astronomical catalogues ever produced. Use of parallax data is not straightforward due to the presence of many statistical biases and selection effects. We present an overview of a techniques for correct use of the Gaia parallax information, which relies on statistical modelling of the data in order to infer derived quantities such as distance and absolute magnitude in an unbiased way. The methods rely on a Bayesian methodology and have been applied to case studies on normal stars, variable stars, open clusters and the LMC.

Basic stellar parameters such as effective temperature, surface gravity, chemical composition, and projected rotational velocity, are important to classify stars and are crucial for successful asteroseismic modelling. However, the Kepler space data do not provide such information. Therefore, ground-based spectral and multi-colour observations of Kepler asteroseismic targets are necessary to complement the space data. For this purpose, in coordination with the KASC ground-based observational Working Groups, high-resolution spectroscopic data for more than 500 B, A, F and G-type stars were collected.

Gaia's five-year observation baseline might naively lead to the expectation that it will be possible to fit the parallax of any sufficiently nearby object with the default five-parameter model (position at a reference epoch, parallax and proper motion). However, simulated Gaia observations of a ‘model Universe’ composed of nearly 107 objects, 50% of which turn out to be multiple stars, show that the single-star hypothesis can severely affect parallax estimation and that more sophisticated models must be adopted. In principle, screening these spurious single-star solutions is rather straightforward, for example by evaluating the quality of the fits. However, the simulated Gaia observations also reveal that some seemingly acceptable single-star solutions can nonetheless lead to erroneous distances. These solutions turn out to be binaries with an orbital period close to one year. Without auxiliary (e.g., spectroscopic) data, they will remain unnoticed.

This is an overview of the processing of the dispersed images for the Blue and Red Photometers in the Gaia Satellite. The data are corrected for CCD related effects in the pre-processing, where also the sky background and the flux by neighbours are removed; the data are then internally calibrated to the same “mean instrument” and externally calibrated to obtain spectrum and flux in physical units, which will be stored in the final catalogue.

The Gaia pixel-level data simulator GIBIS (Gaia Instrument and Basic Image Simulator,Babusiaux (2005)) provides detailed artificial data for all three instruments on-board theGaia spacecraft. This data is used for the preparation of procedures required for theanalysis of real Gaia data to come during the mission. Among the effects that stronglyaffect all Gaia data, that therefore have to be modelled with GIBIS, is charge transferinefficiency (CTI). CTI, caused by radiation-induced microscopic defects in the CCDdetectors, becomes manifest in a distortion of the line spread functions of observedobjects, as well as in a loss of photo-generated charges inside the window allocated toeach observed source. It affects the astrometric, photometric, and spectroscopic accuracyof the data. The CTI effects on a particular observation depend on observations donebefore, on CCD operations such as gate activity and charge injections, and on physicaleffects such as the sky background brightness and cosmic ray events in the detectors. Inthis paper, an approach for the simulation of CTI with GIBIS is presented and theinfluence of the sky background brightness and cosmic ray events of CTI is discussed inmore detail.

In this paper we discuss a few aspects of the data volume to be generated by the ESA space astrometry mission Gaia. This volume is assessed first from the on-board point of view in the form of the instantaneous number of sources in the combined astrometric fields of view as a function of time. Then one focuses on the data flow for the data processing itself measured by the number of objects entering the pipeline every day. Finally the combination of the on-board acquisition scheme with the number of stars per day gives the actual telemetry volume to be transferred to the ground and its variation during the mission.

Since January 1st, 2010, the IAU (International Astronomical Union) fundamental celestialreference frame has been the 2nd International Celestial Reference Frame (ICRF2), which iscomposed of Very Long Baseline Interferometry (VLBI) positions for more than 3000extragalactic radio sources. This frame is constantly improving through joint efforts ofthe VLBI community. By surveying the whole sky up to magnitude 20, the European spaceastrometric mission Gaia will soon create its own celestial reference frame directly inthe optical domain and with many more sources. By 2015–2020, the two frames will thuscohabit and it will be important to align these to the highest accuracy for consistencybetween optical and radio positions. In this paper, we present the various observationalapproaches that are undertaken to improve the VLBI frame in the future. These includeextension to weaker sources for densification, extension to higher radio frequencies totake advantage of the more compact morphology of the sources at these frequencies, andfurther observations in the southern hemisphere for homogeneous sky coverage. We alsoelaborate on how such future radio frames should contribute to highly-precise alignmentbetween the VLBI and Gaia frames within the next decade.

We have chosen the name of GYES, one of the mythological giants with one hundred arms,offspring of Gaia and Uranus, for our instrument study of a multifibre spectrograph forthe prime focus of the Canada-France-Hawaii Telescope. Such an instrument could provide anexcellent ground-based complement for the Gaia mission and a northern complement to theHERMES project on the AAT. The CFHT is well known for providing a stable prime focusenvironment, with a large field of view, which has hosted several imaging instruments, buthas never hosted a multifibre spectrograph. Building upon the experience gained at GÉPIwith FLAMES-Giraffe and X-Shooter, we are investigating the feasibility of a highmultiplex spectrograph (about 500 fibres) over a field of view one degree in diameter. Weare investigating an instrument with resolution in the range 15 000 to 30 000, whichshould provide accurate chemical abundances for stars down to 16th magnitude and radialvelocities, accurate to 1 km s-1 for fainter stars. The study is led byGÉPI-Observatoire de Paris with a contribution from Oxford for the study of thepositioner. The financing for the study comes from INSU CSAA and Observatoire de Paris.The conceptual study will be delivered to CFHT for review by October 1st 2010.

This paper is a summary of a presentation done during the ELSA conference in Sèvres, onJune 7th 2010 to describe the actions of the French space agency for space astronomy.

It starts by remembering what was done for Hipparcos, then on more recent astronomyprograms. It describes the supporting role of CNES for the French astronomy laboratories,and the acting role in the DPAC consortium for the Gaia data processing: CNES isintegrating the scientific chains of object processing (CU4), spectroscopic processing(CU6) and astrophysical parameters (CU8) in one processing centre. It will be operated inCNES during the whole Gaia mission.

As data collection and computers expand their capabilities, modelling changes its characteristics. Not only the purpose of modelling becomes multiple, but thresholds are reached beyond which the best suited methods change. In view of the expected impact of the GAIA mission, the possibilities of modelling the Milky Way and spiral galaxy dynamics over the next decade are here discussed, taking the past development of galactic astronomy as a guide.

I present ideas of Gaia’s impact on the determination of the properties of starsprimarily connected to the study of their atmospheres. This mainly relates to effectivetemperatures, gravities and high-fidelity chemical abundances obtained by combiningenvisioned Gaia measurements with ground-based spectroscopy ranging from single objects towell-selected stellar populations. I further discuss the impact of Gaia on the study ofthe kinematics of atmospheric flows.

Gaia will also provide a comprehensive survey of Solar System objects. If compared toother large ground-based surveys, Gaia potential for discoveries of small asteroidsappears to be limited by its magnitude threshold for detection. However, it can providevaluable data on the physical properties of the observed objects. If compared to LSST,Pan-STARRS or other surveys Gaia is complementary and occupies a clearly definedniche on its own.

ELSA (European Leadership in Space Astrometry) is a four-year, EU-funded ResearchTraining Network ending in September 2010. It has employed 10 postgraduate students and 5postdocs for 2–3 years and financed a number of workshops, training events and topicalmeetings, culminating in the present symposium. The primary goal of ELSA is to train youngscientists in the context of the Gaia project while contributing to the scientificpreparations for the mission. The organization, aims and history of ELSA are outlined.

Pulsating variable stars are powerful tools to study the structure and evolution of galaxies. Among different types of pulsating variables the Classical Cepheids trace the young stellar component in galaxies, and are one of the most important primary stellar distance indicators in establishing the cosmic distance scale. Instead, the RR Lyrae stars, with ages comparable to the age of the Universe, eyewitnessed the processes occurring in the very early times of galaxy formation, and thus can provide hints on how galaxies have formed. The role played by the pulsating variable stars in our understanding of the galactic structure and evolution is briefly reviewed in light of the promises of the scientific exploitation of the Gaia mission.

The Gaia spectro-photometric data consist of white-light G magnitudes from unfilteredfluxes measured in the astrometric field CCDs, and low resolution prism spectra measuredin the blue (BP) and red (RP) CCDs. The integrated flux of these BP and RP spectra willyield GBP and GRP magnitudes astwo broad passbands. The internal flux calibration will correct for all possibleinstrumental effects and produce mean spectral energy distributions in an internallydefined flux scale. The purpose of the absolute flux calibration is to tie this flux scaleto physical units by means of spectro-photometric standard stars that rely ultimately onVega’s absolute flux scale. The characteristics of the Gaia spectrophotometric system willbe described, and the absolute calibration process will be outlined and compared withexamples of classical systems.

Gaia is the sixth cornerstone of the ESA Scientific Programme. Beginning 2006, the programme implementation phase was kicked off at Astrium Satellites. At summer 2010 all Critical Design Reviews of the modules have been passed successfully, which enables to give a good snapshot of the development progress. This presentation summarizes the history of the satellite programme. The status of the current development will be presented with a focus of the main challenging equipment, with a particular insight on the payload.

During the five years of the mission, the Gaia spectrograph, the Radial Velocity Spectrometer (RVS) will repeatedly survey the celestial sphere down to magnitude V ~ 17–18. This talk presents: (i) the system which is currently developed within the Gaia Data Processing and Analysis Consortium (DPAC) to reduce and calibrate the spectra and to derive the radial and rotational velocities, (ii) the RVS expected performances and (iii) scientific returns.