How can mixed modes constrain the mixing processes at the edge of the convective core? The case of the solar-like pulsator CoRoT-target HD 49385.
S. Deheuvels E. Michel
In solar-like pulsators massive enough to have a convective core, the mixing processes at the boundary between the core and the radiative region are still ill-described (overshooting, graviational settling, rotation, ...). This generates large uncertainties on the age and evolutionary stage of the observed stars. To improve the theoretical models, we need new observational constraints of these phenomena.
To this respect, the seismic study of evolved stars (end of the main sequence or beginning of the post main sequence) is very promising. The spectrum of these stars contain mixed modes which have a p-mode behavior at the surface and a g-mode behavior in the center, because of the coupling between the two cavities. It was already suggested that the frequency of these mixed modes is sensitive to the mixing processes at the edge of the core (Dziembowski and Pamyatnikh 1991). For modes of low degree l, the coupling between the cavities is strong, and we showed in a previous work (Deheuvels and Michel 2009) that it had an effect on more than two modes and induced a characteristic distortion of the ridge of degree l in the échelle diagram. This distortion brings complementary information on the profile of the gradient of chemical composition in the center, and thus on the transport of chemical elements.
Such a distortion was observed in the oscillation spectrum of the solar-like pulsator HD 49385, obtained from 137 days of CoRoT data collected for this star (Deheuvels et al. 2010). We show in this presentation how the frequency of the mixed mode and the distortion of the ridge allow to constrain the mixing at the edge of the convective core.
Geometry and kinematic of two Be stars with the VEGA/CHARA instrument
Delaa O. et al
The origin, the geometry and the kinematics of the circumstellar disk around Be stars, responsible for the observed near IR-excess and emission lines, is still debated even if recent interferometric observations with the VLTI /AMBER and VLTI/MIDI instruments seem to indicate that the presence of an equatorial disks and polar stellar wind around Be stars are not correlated. We propose to measure the size, orientation, shape and kinematics of 2 Be stars, namely 48 Per and psi Per.
Stellar rotation in the Hyades and Praesepe: gyrochronology
and braking timescale
P. Delorme A. Cameron
(University of St-Andrews)
We present the results of photometric surveys for stellar rotation in the Hyades and in Praesepe, using data obtained as part of the SuperWASP exoplanetary transit-search programme. We determined accurate rotation periods for more than 150 sources whose cluster membership was confirmed by common proper motion and colour-magnitude fit to the clusters' isochrones. This allowed us to determine the effect of magnetic braking on a wide range of spectral types for ages around 600Myr for the Hyades and Praesepe. Both clusters show a tight and nearly linear relation between colour and rotation period on the F,G and K spectral range. This confirms that loss of angular momentum was significant enough that stars with strongly different initial rotation rate did converge to the same rotation period for a given age and mass, by the age of Hyades and Praesepe. In the case of the Hyades our colour-period sequence extends well into the M dwarf regime and shows a clear-cut breakdown of the colour-period relation, with identification of numerous rapid rotators from 0.5SM down to the lowest masses probed by our survey (0.25SM). This provides crucial constraints on the rotational braking timescales and further clears the way to use gyrochronology as an accurate age measurment tool for main sequence stars.
Thermohaline instability and rotation-induced mixing in low- and intermediate-mass stars.
Lagarde Nadège & Charbonnel Corinne
(Observatoire de Genève)
Thermohaline mixing was recently identified as the dominating process that governs the photospheric composition of low-mass bright giant stars (Charbonnel & Zahn 2007a). Here we present the predictions of stellar models computed with the code STAREVOL that takes into account this mechanism together with rotational mixing. We compare the predictions with recent observations and discuss how the corresponding yields for 3He are compatible with the observed behaviour of this light element in our Galaxy.
Hydrodynamical simulations of Pinwheel nebula WR 104
Astrid Lamberts, Guillaume Dubus, Sebastien Fromang
Pinwheel nebulae are formed in colliding wind binaries composed of a Wolf-Rayet star and an early-type star. The interaction of the winds creates a shocked structure that turns into a spiral due to orbital motion. Those systems present infrared emission from dust, whose origin is still poorly understood. Up to now, all models have assumed ballistic motion along an Archimedean spiral. We performed hydrodynamical simulations of a particular system, WR 104. Adaptive mesh refinement enables us to resolve the initial wind interaction region within the binary and to follow the flow up to several steps of the spiral. This allows us to determine the density and velocity in the shock region at all scales, providing new constraints for dust formation in pinwheel nebulae.
The 3D structure of Rossby vortices in protoplanetary discs
H. Meheut, F. Casse, P. Varniere, M. Tagger
(APC - Université Paris Diderot)
Vortices are at the center of numerous studies in the context of planetesimal growth in protoplanetary discs. They have been invoked to accelerate the growth of grains (Barge & Sommeria, 1995) by concentrating them in their centre. However most of the studies concerned two dimensional vortex. Here we address the question of the vertical structure of a vortex formed by the Rossby wave instability and how the 3D structure change the evolution of the grains in the vortex.
How to use and publish with the free available code CESAM2K
In this very short presentation (5-10 mn), the free available code CESAM2k for stellar structure and evolution is presented with a special emphasis on the conditions of publishing results in journals (with referees). The chart of using CESAM2k is available on http://www.oca.eu/cesam/charte_CESAM.txt. Note that the code CESAM (prior to 2000) is obsolote and only CESAM2k (since 2001) is updated.