In close two-body astrophysical systems, like binary stars or Hot-Jupiter systems, tidal interactions often drive the dynamical evolution of the system on secular timescales. Many host stars and presumably giant gaseous planets feature a convective envelope. Tidal flows that are generated there, due to the tidal potential of the companion, are dissipated through viscous friction mechanisms which lead to the redistribution and exchange of angular momentum in the convective shell and with the companion respectively. In the tightest systems, non-linear effects are likely to have a significant impact on the tidal dissipation and change the zonal flows triggering differential rotation as shown in a recent study. In this context, we also investigate how the addition of non-linearities affect the tidal flow properties, the energy and angular momentum balances, thanks to 3D non-linear simulations of an adiabatic and incompressible convective shell. In our study, we have chosen a  body forcing where the equilibrium tide (the quasi-hydrostatic tidal flow component) acts as an effective force to excite tidal inertial waves, while using stress-free boundary conditions. With this more realistic set-up, we show new results for the amplitude of the energy stored in zonal flows, angular momentum evolution, and its consequences on tidal dissipation in the envelope of low-mass stars or giant gaseous planets.

La moitié des exoplanètes de période orbitale supérieure à 100 jours et dont la masse est comparable à celle de Jupiter a une excentricité entre 0.1 et 0.4. Cela laisse à penser qu’il doit y avoir un ou des mécanismes relativement génériques permettant de croître copieusement l’excentricité des planètes géantes, qui se forment sur des orbites quasi-circulaires dans leur disque protoplanétaire. Une possibilité est que les planètes géantes acquièrent leur excentricité après avoir migré dans une cavité de gaz dans leur disque protoplanétaire. La présence d’une telle cavité, qui pourrait être induite par photo-évaporation ou par des vents magnétisés du disque, est indiquée dans de nombreuses observations de disques protoplanétaires. S’il existe effectivement des planètes géantes sur des orbites excentriques à l’intérieur des cavités des disques protoplanétaires, pourrions-nous les détecter par l’émission du gaz et des poussières des disques? C’est la question à laquelle je tâcherai de répondre au cours de cette présentation.

The observation of gravity modes is expected to give us unprecedented insights into the inner dynamics of the Sun. Nevertheless, there is currently no consensus on their detection. Within this framework, predicting their amplitudes is essential to guide future observational strategies and seismic studies. In complement to the current numerical simulations whose control parameters are still far from stellar regimes, semianalytical estimates remain very useful to obtain quantitative estimates. While previous analytical models considered convective turbulent eddies as the driving mechanism, we aim to predict the amplitude of low-frequency asymptotic gravity modes generated by the penetration of downward convective plumes at the top of the radiative zone.In this talk, we will present the model developed and compare its predictions with the prevous estimates. The implications of these findings for the detection of the gravity modes in the Sun will be finally discussed.

The formation and evolution of light elements in the Universe act as important cosmological constraints. The oldest stars of the Galaxy have long been assumed to display in their outer layers the primordial lithium abundance, although all studies of stellar physics proved that this abundance must have decreased with time. The primordial Li abundance deduced from the observations of the Cosmological Background is indeed larger than the maximum one observed in these stars. Recent observations gave evidence of a large Li abundance dispersion in very metal poor stars.

During this presentation, we address the general question of the lithium abundance dispersion obtained from observations of metal-poor stars, and how the interplay of atomic diffusion and accretion of matter modifies the element abundances in these metal-poor stars. In particular, we focus on the hydrodynamic processes that could take place after accretion. We consider initial metallicities from [Fe/H]=-2.31 down to [Fe/H]=-5.45.

We show that the observations of lithium dispersion, associated or not with carbon enrichment, are well accounted for in terms of accretion onto the metal-poor stars, with accreted masses smaller than a few Jupiter masses, when using a lithium initial abundance in accordance with the primordial lithium abundance obtained from latest BBN results.

The final fate of massive stars is largely set by their mass loss rates, which are highly unconstrained due to our limited knowledge of the wind micro-structure, or “clumpiness”, which intervenes in the currently available diagnostics we rely on. In high mass X-ray binaries, which are good progenitor candidates for merging compact objects, a blue supergiant is in close orbit with a young and highly magnetized neutron star or a black hole which captures a fraction of the stellar wind, leading to intense X-ray emission. The compact object can then be used as an orbiting X-ray probe to constrain the wind properties in general and its clumpiness in particular.

In this talk, I will present new results on the two main approaches at our disposal in order to deduce how inhomogeneous the wind is based on the variability of the X-ray flux. First, I will show how numerical simulations of wind accretion onto the compact object reveal that variability of the intrinsic emission is a poor tracer of the properties of the clumps. Alternatively, unaccreted clumps passing by the line-of-sight lead to variable absorption of the X-ray continuum. The stochastic properties of the absorbing column density grant us a direct access not only to the size but also to the mass of single clumps. Finally, I will discuss the implications for the associated mass loss rates and the representativity of the stellar wind properties in high-mass X-ray binaries compared to isolated massive stars.

Binary companions are known to open large cavities in their circumbinary discs. Resolving a binary companion, however, remains a difficult observational problem and there exists a number of discs with observed cavities and as yet no resolved binary. An indirect detection method is therefore needed to infer the presence of a binary companion in these systems. As such we investigate the effect of a binary companion on the circumbinary disc in an attempt to find some signatures detectable in observations.We perform radiative transfer calculations on a suite of SPH simulations to create synthetic observations. We investigate the asymmetries in the CO channel maps caused by the presence of a binary companion and develop a metric which aids in the inference of an otherwise undetected binary companion.

The nature of circumstellar envelopes (CSE) around Cepheids is still a matter of debate. The physical origin of their infrared (IR) excess could be either a shell of ionized gas, or a dust envelope, or both, and can potentially bias the measurement of distances. We show how the new interferometric instument MATISSE/VLTI allows to constrain the geometry and the IR excess of the environment of the long-period Cepheid l Car (P=35.5 days) at mid-IR wavelengths to understand its physical nature.

We report the first detection in the L band of a centro-symmetric extended emission around l Car, of about 1.7 Rstar in FWHM, producing an excess of about 7.0% in this band. In the N band, there is no clear evidence for dust emission from VLTI/MATISSE correlated flux and Spitzer data.  We also test the hypothesis of a shell of ionized gas to explain both the IR excess and the geometry of the envelope. While the compact CSE of l Car is probably of gaseous nature, the tested model of a shell of ionized gas is not able to simultaneously reproduce the IR excess and the interferometric observations. Further Galactic Cepheids observations with VLTI/MATISSE are necessary for determining the properties of CSEs,
 which may also depend on both the pulsation period and the evolutionary state of the stars.

Using Spitzer and Gaia data and applying a Maximisation Likelihood Estimation (MLE) approach, we investigate the distances and the depths of the NGC2264 Star Forming Region (SFR). Located at 720pc, this SFR is well known to exhibit   a gradient of star formation in the projected sky. Our preliminary results  outlines  the potential “multilayer” nature  of its spatial structure along the line of sight.

Biography

I am associate professor at university of Grenoble, in the ODYSSEY team at IPAG. My main research project focuses on young stars spatial structures detection and analysis in connection with gas structures of the natal molecular cloud to provide constraints on star formation process.

Brown dwarfs (BD) are sub-stellar objects (13.5<M<85 Mjup) that are only able to sustain deuterium burning in their inner core. A dearth in the detection of BD companions with orbital period less than 5 years, the so-called brown dwarf desert, has been long time observed in radial-velocity surveys of cool stars. It has been regarded as the mark of a strong transition between objects formed in protoplanetary discs (planets) and objects formed in molecular clouds (stars), BD standing in-between. Subsequent observations led however to revise the orbital period upper-bound of the BD desert below 100 days. The BD desert does not mark anymore a transition between different formation channels but rather of an orbital phase-space in which the dynamical interactions between a companion and its host star is in disadvantage of orbit survival. Conversely to a desert, the BD domain appears as a wealthy region where objects formed like stars and objects formed like planets may have comparable masses. It challenges and magnifies the task of differentiating the origin and formation tracks of BDs with respect to their mass, orbital and host-star properties. Up-to-now the BD region of RV-detected companions is still sparsely filled, while unpublished new BD companions could double the present statistics. It is crucial to characterise new BD companions in this mass range, with a well understood selection function. An ongoing PNPS observation program that I lead with SOPHIE, focus on completing an exhaustive and unbiased statistics of brown dwarfs. We are aiming on the 20-150 Mjup mass range not covered by the exoplanet surveys, in a volume-limited sample of FGK stars around the Sun. Moreover, a code that we recently developed, GASTON, allows using the astrometric excess noise from the Gaia DR1 to assess on the orbital inclination of the detected companions and on their true mass, thus helping in characterizing the distribution of brown dwarfs. In this talk I will show our most recent advances within this program. 

With the aim of characterizing the dynamical processes involved in the formation of young protostars, we present recent high angular-resolution ALMA dust polarization observations of Class 0 protostellar cores. Especially, the dust polarization has been found to be enhanced along the irradiated cavity walls of bipolar outflows, but also in region most likely linked with the infalling envelope, in the form of filamentary structure being potential magnetized accretion streamer, with magnetic field lines following precisely the orientation of these streamers. One needs to understand what physical mechanism would favor the development of streamers during the collapse of dense cores, and if they play a significant role in the evolution of the nascent circumstellar disk of Class 0 protostellar cores. Further work is needed to confirm their kinematic nature via observations of appropriate dense-gas tracers, which will allow us to quantify what is their accretion rate relative to the mass transported to the inner core via the isotropic collapse of the envelope. Moreover, the specific role of magnetic field in funnelling material via magnetized streamers is still unclear. Finally, the physical conditions (irradiation, density, grain size) of these streamers seem to be ideal to drive efficient dust grain alignment, which is required to reproduce the observed important level of dust polarization. We are currently working on the modelling of these structures via simulations and radiative transfer calculations of dust polarized emission in order to understand the dynamical role of streamers in core and young disk evolution, as well as the physics of dust grains that are populating accretion streamers.

The MOBSTER (Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties) collaboration aims at studying magnetic hot stars observed by the TESS satellite. On one hand, several known magnetic hot stars have been observed by TESS and these high-precision photometric data can inform us on the variability of the star. On the other hand, TESS data allow us to identify magnetic candidates through the rotational modulation visible in their lightcurve, and we can then study those new magnetic targets with spectropolarimetry to characterize their field strength and topology. In this talk, I will review the early results of the MOBSTER project, with particular focus on the characteristics of the light curves of magnetic massive stars and on our discoveries of new magnetic pulsating hot stars suitable for magneto-asteroseismology.

Convective cores are the fuel reservoir of stars that are more massive than about 1.2 Msun. Their size therefore has a substantial influence on stellar evolution. However, several physical processes that remain poorly understood by theory can extend those convective cores. Observations are therefore required to help constrain them. In this talk, I will speak about how we can use subgiant asteroseismology to indirectly constrain the main-sequence convective core extension. Indeed, subgiant stars exhibit mixed modes, whose dual nature as pressure and gravity modes allows us to probe the very core of the star. We therefore used the full Kepler data set to thoroughly model KIC10273246, using a method that we specifically tailored for subgiants. We obtained models that show a good statistical agreement with the observations, and found that adding overshooting significantly improves the quality of the seismic fit. We also found that having access to several g-dominated mixed modes provides a stronger constraint on the structure of the star, especially the Brunt-Väisälä frequency and the central density. This study paves the way of a more general study, which will include subgiants observed with Kepler and TESS.

During the evolution of T Tauri stars and the formation of their planetary systems, accretion processes play a key role. However, the more complex interaction at the rim of the inner region of the disk is still not well understood.  Some young stars exhibit recurrent, quite irregular flux dips in their photometry (dippers). These can be explained as extinction events by dusty material from the protoplanetary disk, which is finally accreted onto the star. In the magnetospheric accretion scenario, the magnetic field of a young star truncates the disk where the magnetic field pressure is equal to the ram pressure of the accreting material. If the temperature close to this distance is low enough to avoid dust sublimation, dust might be lifted above the disk plane and obscure the star.
The dataset for this study consists of K2  light curves of the Taurus region, which was observed continuously for $\sim$80 days. The periodicity of the eclipses delivers hints about a stable or unstable accretion regime. The stars classified as dippers have spectral types K4-M6, consistent with studies in other regions, and the mass range goes down to the brown dwarf limit. The co-rotation radii can be derived to a few stellar radii, with temperatures at co-rotation < 1600 K, that indicates that in most cases dust could survive at co-rotation. Temperatures close to 1600 K give some constraints about the dust composition. The range of derived inclinations suggests that, for some dippers, other phenomena than magnetospheric accretion might cause eclipses.

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 742095; SPIDI: Star-Planets-Inner Disk-Interactions).

The interaction between the star’s magnetic field and its accretion disc (star-disc interaction) is responsible for accretion and ejection processes leading to the formation of emission lines. These processes have an impact on the exchange of angular momentum, a key ingredient of the evolution of solar type stars.
Direct comparison between synthetic observables with observations is needed to unveil the complex dynamics of the star-disc interaction and, ultimately, characterise the evolution of young stars.

In this talk, I will present MCFOST-ART a code dedicated to the modelling of atomic lines using the “MALI” method.
I will place particular emphasis on the formation of hydrogen lines in the magnetosphere of classical T Tauri stars.