GEPI

From instrumental design
to scientific exploitation

Research topics

GALACTIC PHYSICS

Reconstructing the Milky Way formation and evolution through the study of its Galactic stellar populations

The knowledge of all Galactic components, from the halo to the disc(s) and bulge, is undergoing a profound revolution thanks to the arrival of data from Gaia, the European astrometric mission, and complementary spectroscopic surveys such as APOGEE, GES, GALAH, RAVE, and, in a near future, WEAVE, 4MOST, MOONS. In particular, the second Gaia catalogue, published in 2018, has provided the astrometric solution for most of the sky, together with radial velocities for more than 7 millions stars. We are now now able to reconstruct the orbits of several millions stars in the Galaxy, to have detailed chemical abundances for some hundred thousands and ages for several thousands, and it is making use of this unique set of data to build an unprecedented cartography of our Galaxy, from the thinnest stellar streams to the large scale structure of the stellar halo.

Our team is giving fundamental contributions to this field of research in many aspects:
on the one hand, in tracing and understanding the relationships between the ages, chemical abundances and kinematics of the stars in the various galactic structures (bulge, discs, stellar halo) and, on the other hand, in developing models that can explain the existence of these relationships and help to understand what they reveal about the formation and evolution of these components. This work is leading to review the association of the various Galactic structures with specific stellar populations: the stellar halo is not only formed by the oldest stars in the Galaxy, but is also made up of stars of the Galactic disc kinematically heated over time; the bulge is mostly made up of stars with properties typical of disc stars, but with morphology, kinematics and distribution of chemical abundances typical of galaxies having a central bar and a boxy/peanut-shaped bulge; the most metal-poor stars in our Galaxy, which until recently were sought only in the halo of the Milky Way, are also partly confined in the disc, and seem to show a continuity of chemical and kinematic properties with the “canonical” disc.

Our implication and commitment in Gaia, and in complementary spectroscopic surveys such as GES, WEAVE and MOONS, give us the opportunity to have a profound knowledge of the data these missions are providing or will provide in the near future.
Our expertise in dynamical and chemical modeling gives us the opportunity to propose new scenarios for the evolution of the Milky Way, by quantifying the role that secular processes, as well as satellites, gas accretion and in-situ star formation, had been having in shaping our Galaxy over time.

Cartographie des bras spiraux et des résonances de la barre de la Voie lactée avec la Gaia Data Release 2
Credits : Khoperskov et al 2020, A&A, 634, L8. Cette nouvelle cartographie des bras spiraux de la Voie lactée a été choisie par l’ESA comme l’une de 5 découvertes fascinantes faites à ce jour avec Gaia

Thanks to the huge amount of data that have been made available to the community in the last few years, and to their extraordinary quality, we are discovering a new Galaxy in some ways: links between the different main Galactic components, unsuspected until few years ago, are now becoming evident and reveal us that all regions of the Galaxy, from the innermost few kpc to the outer disc, from the bulge to the halo, are indeed connected.

To learn more: (1) Gaia Data Release 2. Mapping the Milky Way disc kinematics; (2) In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2; (3) The Milky Way has no in-situ halo other than the heated thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE; (4) Reviving old controversies: is the early Galaxy flat or round? Investigations into the early phases of the Milky Way’s formation through stellar kinematics and chemical abundances; (5) High-speed stars: Galactic hitchhikers; (6)Disk origin of the Milky Way bulge: the necessity of the thick disk; (7) Revisiting long-standing puzzles of the Milky Way: the Sun and its vicinity as typical outer disk chemical evolution; (8) The echo of the bar buckling: Phase-space spirals in Gaia Data Release 2; (9) Hic sunt dracones: Cartography of the Milky Way spiral arms and bar resonances with Gaia Data Release; (10) TOPoS. V. Abundance ratios in a sample of very metal-poor turn-off stars

STELLAR PHYSICS

Binary stars

Stellar evolution, and consequently galactic structure, is very dependent on stellar multiplicity. Our team has been strongly involved since Hipparcos in the observational study of binary stars: from the elaboration of the Hipparcos Double and Multiple Systems Annex to its scientific exploitation; determination of the masses and luminosities of SB2 and of a TTL binary; confirmation of the desert of brown dwarfs with Hipparcos; determination of orbits for the binarity statistics of solar-type stars. 
"Binary" is to be understood in the broadest sense, up to the exoplanets: the oldest detection of the transit of HD 209458; but also the proof that Hipparcos could not determine their orbital inclinations.
Thanks to the Gaia satellite, this study is continuing. Firstly, concerning the simulation of these objects and a long-term programme for the determination of SB2 masses. 
Without even waiting for Gaia DR3, the combination of Hipparcos and Gaia DR2 made it possible to obtain the mass of the exoplanet Proxima c, and the multiplicity of Cepheids and RR Lyrae.
To learn more: (1) Screening the Hipparcos-based astrometric orbits of sub-stellar objects; (2) Simulating multiple stars in preparation for Gaia; (3) Stellar and substellar companions of nearby stars from Gaia DR2. Binarity from proper motion anomaly; (4) Orbital inclination and mass of the exoplanet candidate Proxima c

Stellar rotation

Stellar rotation, if rapid, creates a thermal disequilibrium in the star. This results into large-scale motions, meridional circulation, and geometric deformation of the star causing latitude-dependent surface gravity and a non-uniform temperature distribution. From the study of stellar spectra, only the so-called ’apparent’ fundamental parameters can be obtained, which must be corrected for the effects mentioned by using appropriate models and codes. The ’true’ stellar parameters are then deduced, including mass, luminosity, equatorial radius and Vsini.


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