Stellar population synthesis Model
BGM 2.0

Stellar studies Galactic studies

    Recently, the Gaia satellite, by providing high-precision astrometric data for 1.46 billion stars, including the trigonometric parallax, as well as the abundances of certain chemical elements for nearly 5.5 million stars, offers unique constraints for better understanding the physics of stars throughout their lifetime, and all the more so if coupled with complementary information from other large surveys. In particular, current spectroscopic surveys provide radial velocities and chemical abundances for hundreds of thousands of stars in different populations in our Galaxy. Finally, asteroseismology provides key properties about the structure of stars, as well as their age, evolutionary stage and mass. Samples of stars, where spectroscopic, astrometric and seismic data can be combined, provide essential constraints on the different scenarios for the formation and evolution of our Galaxy: relationships linking age to velocities and metallicities, their gradients and abundance ratios in different regions of the Galaxy, giving access to the chemodynamic evolution of the Galaxy.

    This field is expanding rapidly with current observations from the TESS satellite, future spectroscopic surveys such as WEAVE or 4MOST, the future PLATO space mission, and EUCLID's deep and near-infrared photometry. These new observations will open doors to our understanding of the physics at play in stars, and provide new clues for decoding the history of our Galaxy. This wealth of observational data is driving the development of statistical analysis of the stellar populations that make up our Galaxy, improving our understanding of them at the interface between stellar and galactic physics.

    In this rich observational context, I therefore took charge of developing a new version of the stellar population synthesis model for the Besançon Galaxy (BGM), with the aim of making it an essential tool for exploiting and interpreting these large datasets. The BGM version 2.0 can be used to simulate the global, chemical and seismic properties of stars belonging to the various stellar populations in our Galaxy. To this end, it takes into account the theory of stellar formation and evolution, integrating the transport mechanisms that occur in stellar interiors, models of stellar atmospheres, scenarios relating to the formation and evolution of the Galaxy, and constraints on Galactic dynamics. After implementing these numerical developments using both my skills in stellar evolution modelling and in the chemical evolution of the Galaxy, I used this innovative new tool in two separate studies, where it proved to be extremely useful.

Testing the efficiency of a transport process with the stellar properties

  • Lagarde N., et al. 2019, A&A, 621, A24
    “The Gaia-ESO Survey: impact of extra mixing on C and N abundances of giant stars”
  • Lagarde N., et al. 2017, A&A, 601, A27
    “Population synthesis to constrain Galactic and stellar physics. I. Determining age and mass of thin-disc red-giant stars ”

  • Context:
    Thermohaline instability, well known to oceanographers on Earth as an instability that induces large-scale circulation in the oceans, also occurs in the stars. It is induced by a molecular weight inversion due to the ^{3}He(^{3}He,2p)^{4}He reaction. This instability develops in red giant stars between the hydrogen burning shell and the convective envelope. It generates a transport of chemical species present in the convective envelope towards hotter regions, and vice versa; thus inducing changes in the surface abundances (e.g., Li, C, N) of the most evolved giant stars (e.g., Charbonnel & Lagarde 2010, A&A, 522, A10, Lagarde et al. 2012, A&A, 543, A108). In order to refine our understanding of the transport processes occurring in stars, it is crucial to be able to constrain their efficiency and therefore their effects as a function of stellar properties using observational constraints.

    In this first study, I investigated the efficiency of thermohaline mixing with stellar properties using the ground-based spectroscopic Gaia-ESO survey (e.g. Randich et al 2022 A&A 666 A121 & Gilmore et al. 2022 A&A 666 A120). Such a study has only been possible since the advent of large-scale high-resolution spectroscopic surveys, providing surface abundances for a large number of stars in our Galaxy and thus scanning a wide range of metallicities, ages and stellar masses. I have carried out simulations with the Besan\c con Galaxy Model of the fields observed by Gaia-ESO and have shown that the current model including thermohaline mixing reproduces the C and N abundances over the entire metallicity range studied by the Gaia-ESO survey. I have shown that the efficiency of thermohaline mixing increases as the star's metallicity decreases or its age increases (see Figure 1).
    These studies have highlighted the importance of this new tool BGM 2.0 for studying the effects of the transport processes occurring in stellar interiors, as well as the crucial role of these transport mechanisms in deducing the age (and mass) of stars from the observed surface abundances.

      Fig.1 - [C/N] as a function of stellar metallicity (left panel) and ages (right panel) for synthetic populations computed with the BGM with the effects of thermohaline instability (bottom panel) and without (Top panel). [C/N] for our sample of UVES giant stars members of open and globular clusters are also shown (each symbol represents a cluster). Orange circles show the location in these diagrams of the most important impact of the extra mixing. Figures from Lagarde et al 2019

Studying the dynamics of the Galaxy using different types of data: Gaia-APOGEE-Kepler

  • Lagarde N., et al. 2021 A&A, 654, A13
    “Deciphering the evolution of the Milky Way discs: Gaia APOGEE Kepler giant stars and the Besançon Galaxy Model ”

  • Context:
    Galactic archaeology aims to understand the formation and evolution of our Galaxy and its various components (thin and thick discs, halo, bulge, star clusters, interstellar matter and dark matter) in a cosmological context. The mechanisms of formation and evolution of our Galaxy are encoded in the kinematic and chemical properties of the different populations of stars, as well as in their age. Different types of magnetohydrodynamic transport processes occurring within stars affect both their spectroscopic characteristics and their lifetimes, to varying degrees depending on their type and evolutionary phase. Their study is therefore intimately linked to Galactic archaeology, and is facilitated by the observational constraints provided by the major spectroscopic, astrometric and asteroseismic surveys carried out in parallel with the Gaia mission.

    One of the revolutionary results of asteroseismology has been the detection of solar-like non-radial oscillations in red giant stars. These observations have opened the way to detailed studies of the internal structure of giant stars, and thus provide unique and revolutionary tests to current models of stellar evolution (e.g., Lagarde et al 2015, A&A, 580, A141, Lagarde et al. 2016, MNRAS, 457, L59). On the other hand, these observations make it possible to determine the ages but also the distance and surface gravity of stars, by determining their mass and radius (Chaplin & Miglio 2013). With increasingly precise ages combined with the observed chemical and kinematic properties of stars, we can characterise the stellar populations in the Galaxy more precisely.

    In this article, I have considered a sample of stars observed by both the Gaia and Kepler satellites and by the APOGEE high-resolution spectroscopic survey. By comparing these observations with simulations from the BGM 2.0, I was able to highlight some results: the thin disc exhibits a flat age-metallicity relation while [α/Fe] increases with stellar age. We confirm no correlation between radial and vertical velocities with [Fe/H], [α/Fe], and age for each stellar population. Considering both samples, Vφ decreases with age for the thin disc, while Vφ increases with age for the high-α metal-poor thick disc. We show that this difference is not due to sample selection. Although the age distribution of the high-α metal-rich thick disc is very close to that of the high-α metal-poor thick disc between 7 and 14 Gyr, its kinematics seems to follow that of the thin disc. This feature, not predicted by the hypotheses included in the Besançon Galaxy Model, suggests a different origin and history for this population. Finally, we show that there is a maximum dispersion of the vertical velocity, σZ, with age for the high-α metal-poor thick disc around 8 Gyr. The comparisons with the Besançon Galaxy Model simulations suggest a more complex chemo-dynamical scheme to explain this feature, most likely including mergers and radial

      Fig.2: The dispersion of the vertical velocity as a function of stellar ages from BGM (left panel) and from observations using the APOKASC sample (right panel). The whole thick disc (hαmp + hαmr thick discs) is also shown with the green dashed line. Figures from Lagarde et al. (2021)

      See also my poster at the Conference "Stellar evolution along the HR diagram with Gaia" Naples, 2022