During its mission Gaia will give access to a so far never reached astrometric accuracy of a large number of Solar System objects. Nevertheless some of these Gaia observations could be efficiently completed by ground-based observations. This will allow us to get a better understanding of the dynamical and physical properties of these objects. We present a general overview of these cases.

In the decade to come major improvements are expected in the field of stellar physics coming from all sides: theory, laboratory and numerical experiments, and observation. We illustrate some aspects of the expected progress on the basis of a few exemples concerning the input physics and parameters of the stellar models and the asteroseismic diagnostics.

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.

In this preliminary analysis we discuss on a statistical inversion method applied to themeasured stellar Vrot sin i. Our goal is toobtain the Vrot of the cluster stars which is a key parameterof stellar evolution computations. We estimate rotation impact on the age determination ofcluster stars using the Maeder-Zahn’s theory of stellar rotation in the Cesam2k code.

Surface brightness asymmetries (SBA) are a very common feature of stars and result in a difference between the projected barycentre and photocentre. The evolution of those surface features makes this difference time-dependent. In some cases, e.g. for supergiant stars, the displacement can be a non-negligible fraction of the star radius. We investigate the impact of SBA on the Gaia-based astrometric solution and on the data processing flow.

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 wealth of information in the Gaia catalogue of exoplanets will constitute afundamental contribution to several hot topics of the astrophysics of planetary systems. Ibriefly review the potential impact of Gaia micro-arsec astrometry in several areas ofexoplanet science, discuss what key follow-up observations might be required as acomplement to Gaia data, and shed some light on the role of next generation astrometricfacilities in the arena of planetary systems.

The advent of deep, wide, accurate, digital photometric surveys exemplified by the SloanDigital Sky Survey (SDSS) has had a profound impact on studies of the Milky Way. In thepast decade, we have transitioned from a scarcity to an (over)abundance of precise, wellcalibrated, observations of stars over a large fraction of the Galaxy. The avalanche ofdata will continue throughout this decade, culminating with Gaia and LSST. This newreality will necessitate changes in methodology, habits, and expectations both on the sideof the large survey projects as well as the astrophysics community at large. We argue,based on the experience with SDSS, that surveys should release data as early and often aspossible incorporating incremental improvements in each subsequent release, as opposed toholding off for a single, big, final release. The scientific community will need toreciprocate by performing analyses and (re-analyses) appropriate to the current fidelityof the released data, understanding that these are continually evolving and improvingproducts.

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.

Many are the proposed scenarios for the formation of the thick disk of the Milky Way,going from the merger with smaller galaxies to fast gas accretion in the early Universe.Crucial information on which was the dominant mechanism for the formation of the thickdisk and other Galactic components is encoded in the chemical properties of their stars.By comparing the chemical properties of bulge, thick and thin disk stars one can addressthe question of which, among the competing processes of dissipation, satellite accretionand radial migration, play the dominant role in the formation of the different Galacticcomponents. The main difficulty at present is that current data samples suffer fromselection biases. Gaia shall bring a quantum leap to the solution of this problem, byproviding fiducial samples of the different galactic components with both, chemical andfull kinematic information. To pave the road for Gaia, theoretical models need to beimproved by taking into account both chemistry and dynamical aspects. A work of this kindis underway in Geneva Observatory.

We have recently identified a previously unknown radial migration mechanism resulting from the overlap of spiral and bar resonances in galactic discs (Minchev & Famaey 2010; Minchev et al. 2010b). This new mechanism is much more efficient than mixing by transient spirals and its presence is unavoidable in all barred galaxies, such as our own Milky Way. The consequences of this are a strong flattening in the metallicity gradient in the disc, an extended disc profile, and the formation of a thick disc component, all taking place in only a couple of Gyr. This timescale is drastically shorter than previously expected and thus can put strong constraints on the longevity, strength and pattern speeds of the Galactic bar and Spiral Structure.

From the Gaia mission, kinematic and star count data, together with the physicalparameters of the stars - ages and metallicities-, will allow to characterise our Galaxypopulations and, from that, the overall Galactic gravitational potential. One of thepromising procedures to reach such goal will be to optimise the present PopulationSynthesis models by fitting, through robust statistical techniques, the large and smallscale structure and kinematics parameters that best will reproduce Gaia data. As a firststep we present here the comparison between the data from the Tycho-2 catalogue and theBesançon Galaxy Model simulations, as well as our present and future work on the modeloptimization.

The resonances of the bar and the spiral arms of the Milky Way (MW) can induce kinematicgroups in the local stellar velocity distribution. The Gaia mission is going to provide uswith accurate information on the velocity distribution for an extended region of theGalactic disc. Here we aim to evaluate the Gaia capabilities to constrain thenon-axisymmetric large-scale structure of the MW from the observed velocitydistributions.

Although the Milky Way bulge is our closest opportunity to study in detail such a complexchemo-dynamical system, its formation and evolution is still poorly understood. The highextinction, the crowding, and the superposition of multiple structures along the line ofsight make studies of the inner Galactic regions challenging. Two main scenarios have beeninvoked for the bulge formation: gravitational collapse or hierarchical merging ofsubclumps and secular evolution of the disc through a bar forming a pseudo-bulge. Recentresults suggest that the Galactic bulge could be the result of both formation processes.Gaia will be limited by the extinction and the crowding but will still observe accuratelymore than 20 million bulge stars (Robin et al. 2005). The Gaia parallaxes and proper motions will probe the structureof the Galactic inner regions, will allow to obtain a clean bulge sample and will solvethe different biases in distance currently present in the bulge surveys. Radial velocitieswill also be obtained for the brightest stars in the less crowded regions, providing 6Dconstrains to dynamical models. Completed by ground-based abundances, the Gaia bulgesurvey will provide unique constraints on the formation history of the Galactic innerregions.

I discuss recent progress in our understanding of the formation of the stellar halo of the Milky Way in the context of the concordance cosmological model. The Gaia mission will provide unique insights especially into the early assembly of the Galaxy and likely be key in unraveling its merger history.

Gaia is expected to resolve nearby galaxies in stars, improving greatly our knowledge of them. The main goal of our group is to obtain the spatial distribution of different stellar components in these galaxies and work towards producing a model of the Magellanic Clouds to be used, among others, in simulations by CU2 during the Gaia mission preparation. In this study the Magellanic Clouds are the main targets. The spatial distribution of various stellar populations in these galaxies is being investigated using isodensity maps and radial density profiles. We are now presenting the latest results from our work on modelling the Magellanic Clouds as expected to be seen by Gaia.

We aim to derive accurate, multi-epoch radial velocities and atmospheric parameters(Teff, log g and [M/H]) of a largesample of Red Clump stars. We acquired data of the target stars at high signal-to-noise(S/N) and medium resolution with the Asiago B&C spectrograph. Radial velocitieswere obtained by applying cross-correlation against observed IAU standard stars andatmospheric parameters via χ2 fit to a synthetic spectrallibrary. Extensive tests were carried out with the multi-epoch observations of theprogramme stars and re-observations of the Red Clump stars that are taken from differentcatalogues in the literature. A total of 305 Red Clump stars (60 of them with a secondepoch observation) were observed, and presented in an output catalogue that containsaccurate radial velocities(σ(RV) = 2.6 km s-1), atmospheric parameters(σ(Teff) = 68 K,σ(log g) = 0.38 dex and σ([M/H]) =0.17 dex), distances derived from spectrophotometry, space velocities (U,V,W) withadditional astrometric and photometric data taken from literature. We aim to use our finalcatalogue to study the substructure and kinematics of the Galaxy.

Classification and parametrization of unresolved galaxies to be detected by Gaia is thegoal of our contribution to DPAC/CU8. Gaia will observe more than several times106 unresolved galaxies seen almost as point sources. For the classificationand parametrization of these sources we have undertaken the following tasks: a) providinglibraries of synthetic galaxy spectra (produced under the PÉGASE.2 code of galaxyevolution) and a semi-empirical library using observed galaxy spectra from SDSS dataarchive. b) Developping an appropriate algorithm called Unresolved Galaxy Classifier(UGC). The algorithm is being developed implementing the Support Vector Machines (SVM), asupervised training technique. This algorithm utilizes Gaia simulated BP/RP galaxy spectraof our synthetic galaxy library.

The Gaia space astrometric mission will construct its own celestial reference frame based on thousands of QSO positions measured with an accuracy of a few tens of microarcseconds. For consistency with the current realization of the International Celestial Reference Frame, which is based on Very Long Baseline Interferometry (VLBI) positions for more than 3000 extragalactic radio sources, it is crucial that the two frames be aligned with the highest possible accuracy. This alignment requires a number of common objects with excellent optical and radio astrometric properties. Because of their physical properties, it is possible that the QSO optical and radio positions do not physically coincide at the 100 microarcsecond level. While such shifts may affect the accuracy in aligning the two frames, they may also help gaining insights into the physical conditions in the nuclear regions of these objects. This paper reviews the ongoing work to align the VLBI and Gaia frames along with the potential astrophysical outcome that may result from comparing/combining ultra-accurately these two frames.

Telescopes don’t make catalogues, they make intensity measurements; any preciseexperiment performed with a telescope ought to involve modelling those measurements.People make catalogues, but because a catalogue requires hard decisionsabout calibration and detection, no catalogue can contain all of the information in theraw pixels relevant to most scientific investigations. Here we advocate makingcatalogue-like data outputs that permit investigators to test hypotheses with almost thepower of the original image pixels. The key is to provide users approximations tolikelihood tests against the raw image pixels. We advocate three options, in order ofincreasing difficulty: The first is to define catalogue entries andassociated uncertainties such that the catalogue contains the parameters of an approximatedescription of the image-level likelihood function. The second is to produce aK-catalogue sampling in “catalogue space” that samplesa posterior probability distribution of catalogues given the data. The third is to exposea web service or equivalent that can compute the full image-level likelihood for anyuser-supplied catalogue.