Présentes en très grand nombre dans la Voie lactée, les naines ultra-froides restent toutefois mal connues, de par leur physique et leur atmosphère complexe avec la formation et sédimentation de poussières, et leur faible luminosité rendant leur observation difficile. La compréhension de ces astres est particulièrement intéressante puisqu'ils font le lien entre les étoiles et les naines brunes. Le satellite Gaia permet pour la première fois de détecter ces astres grâce à l'astrométrie et pas seulement la photométrie. Plusieurs dizaines de milliers ont ainsi été révélés, dont plus de 200 à moins de 30 pc. Le suivi spectroscopique des objets les plus proches a démarré dans le cadre d'une collaboration internationale, à l'aide de différents instruments de moyenne résolution spectrale sur des télescopes de la classe de 4 à 10m.

Grâce à l'outil SPLAT (The SpeX Prism Library Analysis Toolkit), une librairie python d'analyse, nous avons caractérisé 120 de ces naines ultra froides par comparaison avec des modèles d'atmosphères stellaires et substellaires afin de déterminer leurs paramètres atmosphériques comme la température effective, la gravité et le type spectral. Pour ce faire, nous avons utilisé plusieurs grilles de modèles récents d'atmosphère de naines froides BT-Settl et Drift incluant différentes physiques : différentes abondances solaires ; prenant en compte les résultats des simulations RHD sur la longueur de mélange. Nous avons ensuite étudié l'impact de ces différents modèles sur la détermination des paramètres stellaires, en particulier la température effective.

Dans un deuxième temps, en utilisant la mesure de la parallaxe faite avec Gaia, nous plaçons notre échantillon d'étoiles dans un diagramme M_G vs Teff en les comparant à des modèles d'évolution stellaire nous permettant de suivre l'évolution de ces étoiles de très petite masse jusqu'à la limite de combustion de l'hydrogène.

Ce travail est une première étape vers une analyse systématique d'un grand nombre de naines brunes révélées par le satellite Gaia permettant ainsi une meilleure compréhension de ces astres; et sera dans l'avenir essentiel lorsque de futures missions spatiales, telles que EUCLID, permettront l'observation de plus de milliers de naines brunes, jusqu'aux plus froides et dans les différentes populations de la Galaxie.

Space-based asteroseismology has revolutionised our understanding of stellar structure, evolution, mixing, and rotation. In particular, intermediate-mass, main-sequence g-mode pulsators like gamma-Dor and slowly pulsating B-type (SPB) stars allow us to probe rotation and mixing at their convective core/radiative envelope interface with a high precision. This constitutes a gold mine for our global understanding of stellar rotation and related mixing. To fully exploit the information that is provided by detected g-mode pulsations, it is crucial to improve our understanding of how stellar rotation influences g-modes in rapidly rotating stars for which the action of the Coriolis and the centrifugal accelerations have to be taken into account.

In this framework, the Traditional Approximation of Rotation (hereafter TAR) provides a flexible treatment of the adiabatic propagation of gravity modes modified by rotation (i.e. gravito-inertial modes including Rossby modes which propagate under the combined action of the buoyancy force and the Coriolis acceleration), that is extensively used for intensive seismic forward modelling. However, it has been built on the restrictive assumptions of spherical uniformly rotating stars. In this work, we generalise the TAR to take into account simultaneously the centrifugal deformation and differential rotation. We determine the validity domain of this generalised TAR using the state-of-the-art 2D stellar structure and evolution code ESTER. We then demonstrate how these new physics affect the pulsation-period spacings between consecutive g-mode pulsations, which are a common diagnostic that allow us to probe rotation and the chemicals mixing, for instance by convective overshoot or penetration, at the core boundary. We show that the effects induced by the centrifugal acceleration and the differential rotation are detectable using high-precision asteroseismic data. Finally, we discuss how this work can be generalised in a near future to include the effects of stellar magnetic fields and how it will lead to more realistic and accurate asteroseismic modelling of OBA-type stars.

Authors: Laurane Fréour, Coralie Neiner and Colin Folsom

Magnetic fields are known to exist in about 10% of hot stars on the main sequence. High-resolution data and the increase in computing power have enabled the development of new methods to map and characterize stars. Zeeman-Doppler-Imaging (ZDI) is one of them and allows to determine the magnetic topology at the stellar surface. Through a modelling of the intensity and polarized line profiles, ZDI aims at reconstructing the vectorial magnetic field at the stellar surface.
We have acquired high resolution spectropolarimetric observations for two hot stars: V352Peg, a main sequence Bp star, and iCar, a rather evolved early-B star. Through a Least-Square Deconvolution, we extracted the intensity and mean Zeeman signature of thousands of spectral lines. We have established the presence of a magnetic field in both stars. Using a python ZDI code, we mapped and characterized their magnetic field, revealing two completely different configurations. While V352Peg exhibits a strong, dipolar field with a polar field strength of ~5.3 kG, iCar shows a weaker field strength and a more complex field topology.
These results enable a better understanding of the magnetic configuration of hot stars at different stages of their stellar evolution.

    For solar-like oscillating stars, the determination of fundamental parameters can be subject to many biases such as the mass-helium degeneracy. An independent helium estimate is then needed and may be obtained by exploiting the so-called ionisation glitch. Caused by a local structural fluctuation, this deviation of the oscillation frequencies from an asymptotic pattern appears to be a promising indicator of the helium abundance in the outer layers of the star. However, although progressively becoming more sophisticated, current approaches for modeling glitches face problems such as the need for calibration by realistic stellar models. Accordingly, a physical model of the ionisation region is required in order to explicitly involve the parameters of interest such as the surface helium abundance Y_s.

    We present an analytical approximation for the adiabatic index Γ₁ obtained through a thermodynamic treatment of the ionisation region. The induced structure is found to depend on only 3 parameters including the surface helium abundance Y_s and the electron degeneracy ψ_CZ in the convective region. The model thus defined allows a wide variety of structures to be described and, in particular, is able to approximate a realistic model in the ionisation region.

    The modeling work conducted enables us to study the structural perturbations causing the glitch. More elaborate forms of perturbations than the ones usually assumed are found. It is also suggested that there is a stronger dependence of the structure on both the electron degeneracy in the convective zone and on the position of the ionisation region rather than on the helium amount itself.

We measure the Ca II H & K and H line equivalenth widths from13815 high resolution spectra
in 752 M dwarfs from spectral sub-types M0.5 to M8.3. We plot the surface fluxes of these lines
as a function of stellar radius (from Houdebine et al. 2019) and find that there is an important
decrease between 0.500R⊙ and 0.330R⊙. The amplitude of this decrease is a factor of 5.6 in the
Ca ii lines for our sub-sample of low activity M dwarfs. Similar patterns are observed for our
sub-sample of Me dwarfs, but with a somewhat smaller amplitude of 2.8. These radii correspond
approximately to the spectral types dM1.2 and dM3.4 respectively, i.e., from partially convective
early M dwarfs to approximately fully convective M dwarfs down to the Transition To Complete
Convection (TTCC, Jao et al. 2018). We believe that we are observing the spectral signature
of the progressive disapearance of the radiative core in M dwarfs, and therefore the progressive
transition from an  alpha-Omega type of dynamo to an alpha^2 type of dynamo.
We also investigate the Ca II/Halpha hardness ratio. We find that it is moderate in early type
dwarfs, then increases up to the TTCC and then decreases for late type M dwarfs. This is
consistent with the grid of models of Houdebine & Stempels (1997) that also predict hardness
ratios down to -12 in low activity M3-M4 dwarfs. This behaviour is consistent also with a
continuous decrease in activity levels from early M dwarfs to late M dwarfs.

Subject headings: Stars: late-type dwarfs - Stars: Activity - Stars: Dynamo Mechanisms - Stars: Fundamental parameters

Transiting Exoplanet Survey Satellite (TESS) is a NASA all-sky survey mission. The Full Frame Images (FFI) from TESS cover 85% of the sky with 10 and 30 minutes cadence. The light curve from the FFI can be used for studies of astrophysical phenomena such as exoplanets, binaries, supernovae etc. We developed a graphics-based python tool that combines various available pipelines (Eleanor and LightKurve) to generate a light curve from the FFI. With this tool, one can interactively inspect the target pixel files and select aperture pixels to generate a light curve, particularly useful from a crowded region. Aperture selection helps to identify and remove the contamination on the light curve due to neighbouring sources. The tool is also integrated with Ds9 and the Gaia catalogue, with which we can inspect the environment of the target source. 

As a first application of the tool, we analysed a crowded young open cluster, Collinder 69. We identified many variable sources in this cluster comprising binaries, intrinsic oscillations and rotation.

CUBES is a future UV spectrograph that will be mounted at ESO/VLT. Working in the UV band at a medium resolution of 20000, it will allow numerous science cases. I present here simulations to assess the determination of Be abundances in dwarf stars with atmospheric parameters and metallicity close to those of the Sun. I briefly highlight the importance of Be in stellar physics.

WEAVE is an ongoing mid- (5000) and high-resolution (20000) multi-fibres (1000) spectroscopic survey. Operated at WHT on La Palma, the galactic archaeology sub-survey aims at mapping the kinematics and the chemistry of the stellar components of the Milky Way. Observing with two possible grating configurations (Blue+Red or Green+Red), it gives access to numerous atomic and molecular absorption features. We present the pipeline AROpalines designed to measure abundances of light (C, N, O) and neutron-capture elements for FGK main-sequence and giant stars. As an example of the science case possible at such a resolution of 20000, we discuss some results obtained by the Gaia-ESO consortium.