Context: One of the groundbreaking results of asteroseismology with CoRoT and Kepler has been the detection of non-radial solar-like oscillations in red-giant stars. These observations have paved the way for detailed studies of the internal structure of giant stars, and hence provide unique and revolutionary tests to the current models of stellar evolution. On the other hand, the development of large spectroscopic surveys provide a detailed and accurate chemical properties of stars.
Results: I proposed to study one of the first sample of CoRoT giant stars for which the spectroscopic observations were available. This pioneer work, coupling the spectroscopic and asteroseismic constrains, reveals the very nice agreement between my stellar evolution models and observed chemical as well as seismic properties. It helps to a thinest understand of the stellar structure and evolution.
Fig. 1: Left panel: colour-coded Hertzsprung-Russell diagram for different stellar masses. The colour code represents the values of A(Li) at the stellar surface. Right panels: the evolution of surface lithium abundance (from the ZAMS to the end of the He-burning phase) as a function of effective temperature. Circles and diamonds denote, respectively, Li detections and upper limits for stars with [Fe/H] ≥ −0.125. Error bars are shown for all stars.
- “Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints - The open cluster NGC 6633 and field stars”
Lagarde, N, Miglio, A., Eggenberger, P., Morel, T., Montalbán, J., Mosser, B., et al.
2015, A&A, 580, A141
Context: Asteroseismology has also been widely used to estimate stellar properties (e.g. stellar mass, radius, and distance), providing a fundamental contribution to the characterization of planet hosting stars (e.g. Huber et al. 2013; Johnson et al. 2014) and stellar populations (Miglio et al. 2013). Rotation and convective-core overshoot are two examples of a number of key processes that change all the outputs of stellar models, with a significant impact on asteroseismic observables (e.g. Eggenberger et al. 2010; Lagarde et al. 2012; Montalban et al. 2013; Bossini et al. 2015; Constantino et al. 2015). Consequently, transport processes can have a significant impact on the determination of planet and star masses.
Results: In the context of the determination of stellar properties using asteroseismology, we study the influence of rotation and convective-core overshooting on the properties of red-giant stars. We used models in order to investigate the effects of these mechanisms on the asymptotic period spacing of gravity modes ( \Delta \Pi_{\ell=1} ) of red-giant stars that ignite He burning in degenerate conditions ( M\lesssim 2.0M_\odot). We also compare the predictions of these models with Kepler observations. For a given \Delta \nu , \Delta \Pi_{\ell=1} depends not only on the stellar mass, but also on mixing processes that can affect the structure of the core. We find that in the case of more evolved red-giant-branch (RGB) stars and regardless of the transport processes occurring in their interiors, the observed \Delta \Pi_{\ell=1} can provide information as to their stellar luminosity, within 10-20%. In general, the trends of \Delta \Pi_{\ell=1} with respect to mass and metallicity that are observed in Kepler red-giant stars are well reproduced by the models.
Fig. 2: \Delta \Pi_{\ell=1} at =13.5μHz (top panel) and at =7μHz (bottom panel) as a function of the initial stellar mass, for models that follow standard predictions (black circles), include overshooting on the main sequence (blue squares), or include the effects of rotation-induced mixing, with VZAMS=50 km.s−1 (red upwards triangles) and VZAMS=250 km.s−1 (red downwards triangles). Kepler observations of red giants from Vrard et al. (2016) are indicated using grey and green stars. We distinguish Kepler-observed red giants with metallicity determinations obtained by APOGEE (Pinsonneault et al. 2014, green stars) and Huber et al. (2014, grey stars).
- “Testing the cores of first ascent red-giant stars using the period spacing of g modes”
Lagarde, N., Bossini, D., Miglio, A., Vrard, M., Mosser, B.
2016, MNRAS, 457, L59
- “Mixed modes in red giants: a window on stellar evolution”
Mosser, B., Benomar, O., Belkacem, K., Goupil, M.J., Lagarde N., et al. 2014, A&A, 572, L5