In this paper we explore the consequences of the interaction between internal gravity waves, rotation-induced mixing and atomic diffusion on the rotation profile and internal element distribution of main sequence low-mass stars. We present the first fully hydrodynamical models that include self-consistently element and angular momentum redistribution by these three mechanisms. We show how internal gravity waves extract angular momentum from the deep stellar interior and shape the internal rotation profile of solar-type stars. We compare predictions of our models with lithium observations in open cluster stars.

Numerical simulations have become one of the main tools in the research on stellar convection zones and they can be expected to become very important for the study of the convection-diffusion interaction as well. In this review, I will first provide some historical background and then select a few sample problems where numerical simulations of stellar convection have provided useful results or are expected to do so in the near future: solar granulation, interacting convection zones, and semi-convection.

This paper discusses numerical simulations performed in order to better understand the dynamics of various physical processes present in the radiative regions of stars. Those processes include semi-convection, Eddington-Sweet meridional circulation and anisotropic turbulence. The impact of some of those results on evolutionary models is also discussed.

We consider the problem of trace elements diffusion in parts of radiative envelopes where radiative transfer is not negligible, i.e. viscosity is mainly due to an exchange of momentum between photons and ions and when one has local thermodynamical equilibrium. The flow is made anisotropic by an external shear that mimics the effect of meridional circulation. Although the photons are nearly thermalized, the radiation field is not isotropic because there is an external flux of radiation in the radial (here vertical) direction The tracer is then released into the flow and we study how the anisotropy of turbulence breaks the diffusion of a trace element.

We briefly report on an ongoing numerical project that aims at simulating, with the ASH code, the intricate magnetohydrodynamic processes present in the inner region of A-type stars. We mainly focus our attention, on the dynamics of the convective core, its associated differential rotation and meridional ciculation and the dynamo action than can occur in such tubulent MHD system. We indeed find that magnetic fields with amplitude greater than 10 kG are likely to be present in the core of A-type stars.

Abundances anomalies and helioseismology provide constraints on the turbulent transport of chemical species in stellar radiative zone. We use these constraints to derive upper bounds on the velocity and lengthscale of the vertical turbulent motions. We also present numerical simulations showing that a strong anisotropy between horizontal and vertical motions could help understand the observed transport.

In this paper I give an overview of the numerous ways in which rotation and magnetic fields can interact under stellar interior conditions. I first provide “tutorial” examples of how magnetic fields can (1) alter existing stellar internal flows, (2) generate internal flows, and of how rotation can (3) amplify or (4) destroy magnetic fields. The upshot of all this is that treating rotation or magnetic fields in isolation of one another, as intermediate steps towards the “full picture”, may yield a situation that can only be applied meaningfully under very limited and specific astrophysical circumstances, if any.

Magnetic fields play an important role in producing and modifying the photospheric chemical peculiarities of intermediate-mass main sequence stars. This article discusses the basic theory and methods of measurement used to detect and characterise stellar magnetic fields, and reviews our current knowledge of selected characteristics of magnetic fields in intermediate-mass stars.

Recent X-ray observations of unprecedented length (10 days) of the Orion Nebula Cluster with the Chandra satellite (the COUP project) have revealed X-ray flares on the light-curves of late-B and A stars, as well as two new cases of rotational modulation, in addition to the already known case of the O7 star θ1 OriC. We show that the X-ray emission of A stars (which have neither deep outer convective zones nor strong UV radiation) when detected must be due to the solar-like magnetic activity of unresolved late-type companions, and that most, if not all, of the O and early B stars of the Trapezium may be magnetized and emit X-rays by magnetically channelled wind shocks.

We address important mathematical properties of solutions of the Magnetic Doppler Imaging problem. We show that the underlying inverse problem based on all 4 Stokes parameters is well-posed and thus has a unique solution. The only purpose of regularization is to suppress instabilities induced by sparse sampling of phase and/or wavelength when dealing with practical data. Therefore, the final solution is stable with respect to the initial guess and is not affected by the selection of a specific type of regularization.

Abnormal line profiles and line intensities observed in Ap stars, and their deviations from those expected in homogeneous atmosphere provide a strong evidence for an existance of abundance stratification. For a sample of Ap stars in the effective temperature range 7000–9500 K abundance distributions of Mg, Si, Ca, Cr, Fe are derived from line profiles, and they are compared with the diffusion calculations. A special attention is paid to Ca distribution for which we found an evidence for the differential stratification of Ca isotopes. Temperature and magnetic field effects on the observed abundance distributions are discussed.

I review the chemical abundance measurements of blue horizontal branch stars within the theoretical framework of diffusion and radiative levitation. The present observational scenario reports normal abundances for stars cooler than Teff ~ 11 000 K, while largely anomalous abundances for hotter stars are found. In particular, large He depletions and overabundances of Fe, Ti, Cr, P and Mn are present. On the contrary, the abundances of Mg, Si and Ca are roughly normal, in the hot stars as well as in the cooler ones. Related physical properties as stellar rotation will be also described.

The current status of abundance determinations for A and F dwarfs in open clusters is reviewed. An observational programme was launched a few years ago to observe a large number of A/F and Am/Fm dwarfs in open clusters of various ages. The status of that particular programme is also reviewed. High resolution high S/N spectroscopy of a large number of stars well distributed in mass along the Main Sequence was secured. Abundances of 11 chemical elements have been derived by iteratively fitting synthetic spectra calculated in LTE to the observed spectra. The results obtained are reviewed for the Hyades and Coma Berenices. New abundance determinations for 24 A and F dwarfs in the Ursa Major moving group are presented. In all three clusters, a pronounced scatter of the abundances versus effective temperature for most chemical elements is found for the A dwarfs. In contrast, the abundances of F dwarfs are much less scattered around their mean value. These abundance determinations are useful to set constraints to self-consistent evolutionnary models of A and F dwarfs including transport processes. Directions for future work in particular to derive more accurate abundances are outlined.

I briefly review the role of atomic diffusion in various types of stars and describe some of the limits of current calculations. These suggest numerous directions for further work.

We explore the problems of determining the abundance of Si in a number of hot Ap stars.

Following the observations of Israelian et al. (2004), we compare different evolutionary models in order to study the lithium destruction processes and the planetary formation scenarii.

Thanks to the accurate determination of the baryon density of the Universe by the recent cosmic microwave background experiments, updated predictions of the standard model of Big Bang nucleosynthesis yield the initial abundances of the primordial light elements with an unprecedented precision (Bennet et al. 2003; Spergel et al. 2003; Coc et al. 2004; Cyburt 2004; Serpico et al. 2004). In the case of 7Li, the CMB+SBBN value is significantly higher than the generally reported abundances for Pop II stars along the Spite plateau. 
Here, we report on the very recent results we obtained by revisiting a large sample of literature Li data in halo stars that we assembled following some strict criteria on the quality of the original analyses published from the early 90 s onwards. We put a strong emphasis on the temperature scale and reddening issues, and on the determination of the evolutionary status of each of our sample stars. Using our “best" (i.e. most consistent) set of temperatures we discuss the resulting mean Li value along the plateau for the dwarf stars on one hand and for the turnoff and subgiant stars on the other hand. 


Four probable Ap members of the open cluster NGC 6475, were investigated through detailed spectrum synthesis. Cluster membership was confirmed for all four stars, however chemical abundance analysis shows that only three are truly Ap. Derived $v \sin i$  values and chemical abundances for 20 elements ranging form C to Eu are presented.

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, of magnetic fields and chemical peculiarities in the Herbig Ae/Be stars HD 72106 and HD 101412. These stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. At the same time, we fail to confirm claims by Hubrig et al. (2004) of the presence of magnetic fields in the Herbig Ae star HD 139614.

Abundances of Fe, Ti, Cr, Mg and He have been determined for 10 stars: 5 B, 3 A, 1 F dwarfs and 1 chemically peculiar giant B8IIIp with apparent velocities ranging from 15 to 130 km s-1, members of the Alpha Persei open cluster (log age = 7.40 yrs). The method consists in adjusting grids of synthetic spectra calculated with SYNSPEC48 (Hubeny & Lanz 1992) in the wavelength range of 4450–4600 Å. The input model atmospheres are LTE, RE, plane parallel line blanketed models calculated with ATLAS9 (Kurucz 1992a). A correlation may exist between [Fe/H] and the effective temperature. Further observations of a much larger sample of B, A and F stars in this cluster are foreseen in the near future.