Stellar Evolution

Thermohaline & Rotation Magnetic Field Hydrodynamics

Effects of thermohaline mixing and rotation-induced mixing

  • Lagarde N., et al., 2012b, 2012b, A&A 543, A108, citations count: 226 on 20/07/2023
    “Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to AGB for low- and intermediatemass stars at various metallicities”
  • Charbonnel C. & Lagarde N., 2010, A&A, 522, A10. citations count: 236 on 20/07/2023
    “Thermohaline instability and rotation-induced mixing in low and intermediate-mass stars. I - Solar metallicity”

  • Context:
    Numerous spectroscopic observations have provided compelling evidences for a non-canonical process that modifies the surface abundances of low- and intermediate-mass stars beyond the predictions of standard stellar theory. With my grid of stellar models including different transport processes, I studied effects of these transport processes on stellar structure, evolution and nucleosynthesis.
    During my thesis, I computed a grid of stellar models including, for the first time, the effects of the thermohaline instability, induced by the molecular weight inversion due to the reaction 3He(3He,2p)4He reaction, as well as mixing induced by rotation at different masses and metallicities. These models, computed with the stellar evolution code STAREVOL, from the pre-main sequence to the AGB phase, have enabled me to study the impact of these mixings on the structure and surface abundances of low- and intermediate-mass stars (Charbonnel & Lagarde 2010). This work confirms the importance of these two processes at different stages of the stellar evolution, and allows previously unexplained observations to be successfully reproduced. In addition to these studies, I have provided the community with a stellar evolution model grid that includes the effects of the first physical transport process capable of explaining the chemical properties of low-mass stars. In addition, I have also shown the crucial impact of thermohaline mixing and rotation-induced mixing on the global and asteroseismic properties of stars. These models of stellar evolution have been widely used in the literature, being crucial for different fields of astrophysics: stellar physics including spectroscopy and asteroseismology; Galactic physics; and for characterising the host star in planetary systems.

    • Magrini L., Lagarde N., et al. 2021, 651, A24
      “The Gaia-ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars”
    • Sanna, N et al. 2020, A&A, 639, L2
      “The Gaia-ESO Survey: an extremely Li-rich giant in globular cluster NGC 1261”
    • Szigeti L., et al. 2018, MNRAS, 474, 4810
      “12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791”
    • Smiljanic R., Pasquini P., Charbonnel C., and Lagarde N., 2010, A&A, 510, A50. citations count: 29 on 17/10//2018
      “Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models”
    • Fig.1 - Evolution of the surface ^{12}C/^{13}C value as a function of stellar luminosity for the 1.25 M_{\odot} models including thermohaline instability and rotation-induced mixing (for initial rotation velocities of 50, 80, and 110 km.s^{-1} shown as solid red, dashed green, and dotted blue lines respectively). The non rotating case is also shown (black solid line). Observations along the evolutionary sequence of the open cluster M67 are from Gilroy & Brown (1991). The triangle is for a subgiant star for which only a lower value could be obtained, while black squares and red circles correspond respectively to RGB and clump stars

Magnetic fields at the surface of giant stars

      This model grid is proving to be a very important tool for understanding the magnetic field. In two papers in which I contributed, it was used to study the generation of magnetism in evolved stars using spectropolarimetric data obtained with the NARVAL and ESPaDOnS instruments.

    • Charbonnel, C, Decressin, T, Lagarde, N et al. 2017, A&A, 605, A102
      “The magnetic strip(s) in the advanced phases of stellar evolution. Theoretical convective turnover timescale and Rossby number for low- and intermediate-mass stars up to the AGB at various metallicities”
    • Aurière, M. et al. 2015, A&A, 572, L5
      “The Magnetic Fields at the Surface of Active Single G-K Giants”

To improve the physics of transport processes

      Although my models including different transport processes reproduce the surface abundances of evolved stars very well over a wide range of masses and metallicity, there are still uncertainties and approximations concerning the treatment of these processes in stellar interiors. Thanks to the development of (magneto-)hydrodynamic simulations (in 2D or 3D), the modelling and study of these processes have become increasingly efficient, and new prescriptions can be included in 1D stellar evolution codes. We are working in collaboration with (magneto-)hydrodynamicists to include the most relevant prescriptions. These two articles, in which I contributed, reflect the latest work done in this direction within the STAREVOL collaboration.

    • Mathis S. et al. 2018 A&A 620 A22
      “Anisotropic turbulent transport in stably stratified rotating stellar radiation zones”
    • Amard et al. 2019 A&A, 631, A77
      “First grids of low-mass stellar models and isochrones with self-consistent treatment of rotation. From 0.2 to 1.5 M_{\odot} at seven metallicities from PMS to TAMS”