Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Final Report Summary - SEISMOLOGY IN RGB (Asteroseismology to constrain physic of transport processes occurring in red giant stars)

Publishable summary

Stars are the building blocks of the Universe. Understanding their evolution is crucial to improve our knowledge from the chemical evolution of galaxies to the formation and evolution of planetary systems. Red giant stars are old, cool, highly luminous stars. They play a predominant role in stellar, galactic and extragalactic astrophysics; they serve as distance and age indicators for globular clusters and nearby galaxies. It is very important to understand the evolution of stellar structure and surface abundances in a broad astrophysical context. In their advanced evolution phases, low- and intermediate-mass stars undergo important changes of their structure and chemical composition. Indeed, stars can exchange matter with their surroundings, for example with planets or with interstellar medium, by ejection of their surface layers all along their evolution. In addition, transport processes inside the star, having a direct impact on stellar yields and stellar structure, have effects on chemical evolution of our Galaxy. Observations allow to constrain models of low- and intermediate- mass stars: interferometry, spectroscopy, and spectropolarimetry that are methods to get information about stellar mass, radius, rotation, magnetic activity, and surface chemical abundances.
Contrary to other methods, asteroseismology is a unique and very powerful tool to investigate the interior of stars. The CoRoT (Baglin et al. 2006), and Kepler (Borucki et al. 2010) space missions, have obtained a large number of asteroseismic data for different kinds of stars, which allowed the detection and characterization of solar like oscillations in a large number of red giants (e.g. De Ridder et al. 2009). The detection of rich oscillation spectra (including non-radial oscillation modes) paved the way for detailed studies of the internal structure of these stars, and hence allows unique and revolutionary tests to the current models of stellar evolution.
In particular, the comparison between asteroseismic constraints and models including a detailed description of transport processes (e.g. meridianal circulation, internal gravity waves), opens a new promising path for our understanding of the establishment of these transport processes. The internal rotation profile can also be accessed with seismology and it has been studied in some red giant and in subgiant stars (Beck et al. 2012, Deheuvels et al. 2012, Eggenberger et al 2012). These first observations show a disagreement with the behaviour of rotation predicted by stellar evolution theory.
The backbone of Dr Lagarde’s Marie Curie project was to utilise asteroseismology of red- giant stars to stress-test current theoretical predictions of the internal structure and evolution of stars, with the ultimate goal of improving upon our physical description of different hydrodynamic processes such as stellar rotation on the red giant branch.

Results of this 2-years project are divided into 3 parts.

(1) Coupling spectroscopic and asteroseismic observations to have a deeper knowledge of transport processes occurring in red-giant stars. In this first part of the project, Dr. Lagarde developed strong collaborations with researchers involved in spectroscopic and asteroseismic (CoRoT and Kepler) data analysis, to compare with her stellar evolution models. This provided two articles published in Astronomy & Astrophysics, three in preparation, and different presentations at international conferences. This project is a long-term one and will be updated with the large number of forthcoming complementary observations (Gaia, APOGEE, ESO-Gaia survey).
(2) Effect of rotation and other transport mechanisms on the asteroseismic properties. The period spacing of g-modes (ΔΠ) provides information of the evolutionary status of red giants (red-giant-branch stars, or core He burning stars) (Bedding et al 2011, Mosser et al 2011). Changes in ΔΠ could be explained by differences in the density profile and then in the Brunt-Väisälä frequency profile (Montalban et al. 2010) as well as by the presence of the convective core induced by the core Helium burning. In this context, we presented the measurement of the asymptotic period spacing for 1178 stars at various evolutionary stages observed by Kepler (Mosser et al 2014). Dr Lagarde quantified, in a second article (Lagarde et al. 2016), the impact of two processes (rotation and overshoot) on the inferred stellar properties. These results are useful for a wide range of applications in astrophysics, including the characterisation of planet-host stars.
(3) The core rotation rate of red giant stars measured by asteroseismology shows a significant disagreement with current predictions from stellar models. It is clear that the physics of red-giant models should be improved in the light of new constraints brought by asteroseismology. This implies an additional transport processes occurring on the red giant branch, which transport very efficiency the angular momentum from the core to the surface. For this project, Dr. N. Lagarde acquired the necessary expertise to compute the theoretical oscillations frequencies (LOSC and GYRE) from the outputs of the stellar evolution code STAREVOL. She proposed to quantify the efficiency of this additional mechanisms occurring on the red giant branch (Lagarde et 2016 in prep.), which is a first crucial step to to understand its physical nature.
Dr Lagarde is also interested in investigating the impact of predictions from her stellar evolution models have on our ability to learn about the formation and evolution of the Milky Way. In particular, she is currently quantifying how transport processes occurring in the stellar interior can change the chemical patterns observed at the surface of stellar populations in the Milky Way. On this topic she is collaborating with two groups: Dr A.Robin (Observatoire de Besançon, France) and Dr T. Masseron (Cambridge University, UK).
Dr Lagarde is now also fully involved in the activities of the CoRoT red giants collaboration, in the working groups AsteroSteP (asteroseismology of stellar populations) composed by more than 100 researchers in the world ; in the joint SDSS-APOGEE/Kepler (APOKASC)
consortium allowing her access to proprietary spectroscopic data and updated analyses of asteroseismic data of Kepler stars. Give her recognised expertise in modelling and seismology, Dr Lagarde is now also involved in three work packages for the preparation of the future space mission PLATO.

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