GEPI

Subject: In recent years, numerical simulations have shown that dynamic processes, known as "radial migration",
could mix stars in the disc. Some of these simulations predicted that the mixture should be strong in
a galaxy such as the Milky Way, and that half of the stars observed in the solar neighborhood
could come from the inner regions of our Galaxy. The Sun itself would have migrated from about 2 to 4 kpc from internal regions to
Its present position. These studies are very controversial and have given rise to extensive literature.
At the same time, the study of the chemical compositions of the stars in the disc of the Milky Way showed that the Sun,
at 8 kpc from the Galactic center, is in a transition region: the inner disk (less than 7 kpc from the center
Galactic) had a chemical evolution different from that of the outer disk (more than 9 kpc from the galactic center).
These observations therefore tend to contradict these simulations: if the stars of the inner disk have a composition
different from those of the outer disk, this would mean that the two parts of the disk were not mixed, or in a very limited amount.
The aim of this internship is to understand and quantify this separation or the
two parts of the disc from the analysis of the most recent observations.
The APOGEE catalog contains chemical composition measurements of about 100,000 stars on the entire disc,
and provides unprecedented information on these mixing processes.
The first catalogue of the Gaia mission, made public in July 2016, contains the parallaxes and proper motions
of about 2 million objects, with precision 2 to 3 times better than was previously available.
The combination of the two catalogs makes it possible to have a very complete information on both the
chemical properties of stars but also on their orbits in the Galaxy.
The aim is to study this sample, to identify the stars that could have migrated significantly in
the disc of the Galaxy from their place of formation, to quantify their number and to deduce constraints on the
the strength of radial migration in the disc of our Galaxy.
It is the occasion also to familiarize with galactic stellar populations, the dynamic and chemical evolution of the Galaxy,
the analysis of kinematic and spectroscopic data, while working on a hot topic with very recent data.