(Last update September 2016)
Our group is world-leading in the domain of detailed stellar evolution calculations including previously overlooked but important magneto-hydrodynamical processes. We are studying in a systematic way so-called “non-standard” stellar processes that transport angular momentum and chemicals in stellar interiors. These complex physical mechanisms turn out to be crucial to explain key features observed in stars all over the Hertzsprung-Russell diagram. Thanks to our previous studies we now have a very good overview of the impact and of the relative importance of mechanisms induced by rotation, internal gravity waves, thermohaline mixing, and atomic diffusion, as well as of their interplay in stars of various masses and metallicities. Some of these mechanisms play also a very important role on Earth (e.g., thermohaline circulation in the oceans and gravity waves in the atmosphere are crucial phenomena for the climate of our planet), making stars unique physics laboratories and revealing once more the importance of interdisciplinary studies. Our comprehensive theoretical and observational investigations bring new light on some long-standing problems plaguing our understanding of the evolution and properties of stars, as well as of their contribution to the evolution of chemical elements in our Galaxy.
The early evolution of stellar rotation
Understanding the angular momentum evolution of stars is one of the greatest challenges of modern stellar physics.
We study the predicted rotational evolution of solar-type stars from the pre-main sequence to the solar age with 1D rotating evolutionary models including state-of-the-art physical ingredients.
Amard, Palacios, Charbonnel, Gallet, Bouvier, 2016, A&A 587, A105 “Rotating models of young solar-type stars. Exploring braking laws and angular momentum transport processes
Rotation-induced mixing and thermohaline instability
We have studied the impact of rotation-induced mixing and thermohaline instability on the structure, evolution, lifetime, nucleosynthesis, and surface abundances of low- and intermediate-mass stars over a large range in metallicity. We showed that thermohaline mixing is the main physical mechanism that governs the photospheric composition of low-mass red giant stars. The predictions of our models were compared with great success to observational data for lithium, beryllium, carbon, nitrogen, and sodium abundances, as well as for carbon and oxygen isotopic ratios as observed in Galactic open clusters and in field stars, as well as in planetary nebulae.
Smiljanic, Pasquini, Charbonnel & Lagarde, 2010, A&A 510, A50, « Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 – A new test for hydrodynamical stellar models ».
Charbonnel & Lagarde, 2010, A&A 522, A10, « Thermohaline instability and rotation-induced mixing. I. – Low- and intermediate-mass solar metallicity stars up to the end of the AGB ».
We discussed for the first time in the literature the impact of thermohaline and rotation-induced mixings on the global asteroseismic quantities all along the evolution of low- and intermediate-mass stars, from the pre-main sequence up to the end of the TP-AGB phase, at various metallicities.
Lagarde, Decressin, Charbonnel, Eggenberger, Ekström & Palacios, 2012, A&A, 543, A108, « Thermohaline instability and rotation-induced mixing. III. – Grids of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars of various metallicities ».
Solving the 3He long-standing problem
We have identified thermohaline mixing to be the dominant process that reduces the net 3He stellar yields, as requested by the abundances derived for this light element in the Sun and the solar system, in the local interstellar cloud, and in several Galactic HII regions. Our results have thus provided a very elegant solution on a Galactic scale to the long-standing 3He problem that was originally identified with the solar wind composition experiment of Prof. J.Geiss (Berne) during Apollo missions on the Moon. The resulting evolution of the primordial light elements D, 3He, and 4He was successfully compared with their primordial values inferred from the Wilkinson Microwave Anisotropy Probe data for Big Bang nucleosynthesis, and from observations in various environments.
Charbonnel & Zahn, 2007a, A&A, 467, L15 (Highlighted paper) « Thermohaline mixing: A physical mechanism governing the photospheric composition of low-mass giants».
Charbonnel & Zahn, 2007b, A&A, 476, L29 «Inhibition of thermohaline mixing by a magnetic field in Ap star descendants: Implications for the Galactic evolution of 3He ».
Lagarde, Charbonnel, Decressin & Hagelberg, 2011, A&A 536, A28, « Thermohaline instability and rotation-induced mixing. II. – Yields of 3He for low- and intermediate-mass stars ».
Lagarde, Romano, Charbonnel, Tosi, Chiappini & Matteucci, 2012, A&A 542, 62, « Effects of thermohaline instability and rotation-induced mixing on the evolution of light elements in the Galaxy : D, 3He and 4He ».
The importance of Internal Gravity Waves in stellar interiors
We studied for the first time the impact of internal gravity waves (IGW), meridional circulation, shear turbulence, and stellar contraction on the rotation profile inside pre-main sequence low-mass stars. Over the whole mass range considered, IGW were found to be the major agent for angular momentum redistribution as they efficiently spin down the stellar core in the early phases of stellar evolution. The corresponding predictions for the surface rotation velocity are in excellent agreement with the observed rotational properties of young cluster stars. As we showed in 2005, IGW are one of the best candidates to explain the rotation profile inside the Sun as revealed by helioseismology.
C.Charbonnel & S.Talon, 2005, Science, Vol.309, 2189 “Influence of Gravity Waves on the Internal Rotation and Li Abundance of Solar-Type Stars”
C.Charbonnel & S.Talon, 2007, Science (Solicited Paper), Vol.318, 922-923 “Mixing a stellar cocktail”
Charbonnel, Decressin, Amard, Palacios, Talon, 2013, A&A, 554, A40, «Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars».
Charbonnel & Talon (1999, 2005)
Talon & Charbonnel (1998, 2003, 2004, 2005, 2008), Pantillon, Talon & Charbonnel (2007)
The origin of stellar magnetic fields
We use the new generation of spectropolarimeters (ESPaDOnS@CFHT and NARVAL@TBL) to perform a pilot study which goal is to infer the nature of magnetic fields at the surface of late-type giants with well determined evolution status by measuring directly their topology and properties. This pioneer work is crucial to understand the generation and the influence of magnetic fields all along stellar evolution. It opens key questions on the origin and impact of magnetic fields in evolved stars. The main results obtained so far are the following. i) All our sample stars observed with the highest precision possible are Zeeman detected; the majority of these objects are at the base of the red giant branch, or in the clump, and are both fast and slow rotators. ii) The fast rotators (including the AGB star EKBoo) host magnetic fields of several ten Gauss. iii) We detected magnetic fields in slowing-rotating G and K giants, some of them being as strong as in fast rotators. iv) We showed that several stars with outstanding magnetic fields (of ~ 100G) were certainly Ap-star descendants.
Charbonnel 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”
Borisova et al., 2016, A&A, 591, A57 “The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae”
Aurière et al., 2015, A&A, 574, A90 “The magnetic fields at the surface of active single G-K giants”
Tsvetkova et al. 2013, A&A, 556, A43 “Magnetic field structure in single late-type giants: β Ceti in 2010-2012″
Aurière et al. (2008, 2009, 2011, 2012, 2013), Konstantinova-Antova et al. (2010, 2011, 2013)
The formation of massive stars
Understanding the formation of massive stars is a great challenge for astrophysics. Here we revisit massive star formation by accretion.
Haemmerlé et al. 2016, A&A, 585, A65 “Massive star formation by accretion. I. Disc accretion”
Current projects and prospective
We pursue our pioneer study on the impact of internal gravity waves all over the Hertzsprung-Russell diagram, encouraged by our first estimates of the transport of angular momentum and of the chemicals inside the sun and stars of various masses and evolution stages.
An important part of our work is devoted to the study of Population II field halo and globular cluster stars that formed very early in the Galactic history. These objects contain a memory of the unique nucleosynthesis in the first stars offering a local benchmark to cosmology. Our approach is based on the computation of grids of stellar models over a large range in mass and metallicity that take into account the coupled effects of rotation, internal gravity waves, atomic diffusion, thermohaline mixing, and magnetic fields, that modified the chemical appearance of these long-lived stars all over their life.
We continue our systematic search for magnetic signatures at the surface of giant stars, using new generation spectropolarimeters (NARVAL at TBL/OHP, and ESPaDOnS at CFHT). Our aim is to infer the nature of magnetic fields in evolved stars, by measuring directly the magnetic field topology and its properties. Such measurements will provide crucial clues on stellar magnetic phenomena, and stellar models will be computed to model magnetic field generation in stellar interiors.
Our new generation stellar models are particularly important now that asteroseismic probes are blooming all across the Hertzsprung-Russel diagram thanks to the dedicated space missions CoRoT (ESA/CNES) and Kepler (NASA), and latter PLATO (ESA). In particular the very recent and extraordinary detection and characterization of solar-like oscillations in a large number of red giants promises to add invaluable and independent constraints to current stellar models. In this context, we shall be able to validate the current theoretical prescriptions for non-standard physics in stars through detailed seismic dissection of individual stars, as expected in a very near future.
This is paving the road for detailed and timely studies of stellar populations in the Galaxy in light of the sophisticated stellar models as those developed by the Stellar Physics team at Geneva Observatory. This will have particularly important implications for the interpretation of large upcoming surveys and space missions such as Pan-Stars, APOGEE, and ESA’s GAIA satellite, which will soon provide invaluable measurements (astrometry, photometry, and others) for millions of stars.