Oscillations of the Sun have been used to understand its interior structure. The extension of similar studies to more distant stars has raised many difficulties despite the strong efforts of the international community over the past decades. The CoRoT (Convection Rotation and Planetary Transits) satellite, launched in December 2006, has now measured oscillations and the stellar granulation signature in three main sequence stars that are noticeably hotter than the sun. The oscillation amplitudes are about 1.5 times as large as those in the Sun; the stellar granulation is up to three times as high. The stellar amplitudes are about 25% below the theoretic values, providing a measurement of the nonadiabaticity of the process ruling the oscillations in the outer layers of the stars.

Atomic diffusion may lead to heavy element accumulation inside stars in certain specific layers. Iron accumulation in the Z-bump opacity region has been invoked by several authors to quantitatively account for abundance anomalies observed in some stars, or to account for stellar oscillations through the induced κ-mechanism. These authors however never took into account the fact that such an accumulation creates an inverse µ-gradient, unstable for thermohaline convection. Here, we present results for A-F stars, where abundance variations are computed with and without this process. We show that iron accumulation is still present when thermohaline convection is taken into account, but much reduced compared to when this physical process is neglected. The consequences of thermohaline convection for A-type stars as well as for other types of stars are presented.

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Context. Theoretical modelling of abundance stratifications and surface distributions of chemical elements in Ap stars constitutes a major challenge. The atomic diffusion model provides the most appropriate framework in which to understand these abundance anomalies.Aims. We present theoretical 2D stratifications of 16 metals in upper main sequence chemically peculiar stars, with and without magnetic fields to provide a reference point for further theoretical and observational studies. Methods. We used our code CaratStrat to compute a large grid of stratifications (equilibrium solutions in LTE) for plane-parallel T eff = 8500, 10 000, 12 000, and 14 000 K stellar atmospheres. By interpolation, we constructed bi-dimensional cuts through these stellar atmospheres, which are permeated by a dipolar magnetic field of strength 20 kG at the magnetic pole. We also provide vertical (1D) stratifications of metals in non-magnetic stars (HgMn).Results. We present a large number of 2D and 1D stratifications, mostly as online material. We discuss in detail the case of Fe for the T eff = 8500 K model in the printed version, and compare it with stratifications derived from observed spectra.

Context. Chemical element transport processes are among the crucial physical processes needed for precise stellar modelling. Atomic diffusion by gravitational settling nowadays is usually taken into account, and is essential for helioseismic studies. On the other hand, radiative accelerations are rarely accounted for, act differently on the various chemical elements, and can strongly counteract gravity in some stellar mass domains. The resulting variations of the abundance profiles may significantly affect the structure of the star. Aims. In this study we aim at determining whether radiative accelerations impact the structure of solar-like oscillating main-sequence stars observed by asteroseismic space missions. Methods. We implemented the calculation of radiative accelerations operating on C, N, O, Ne, Na, Mg, Al, Si, S, Ca, and Fe in the CESTAM code using the Single-Valued Parameter method. We built and compared several grids of stellar models including gravitational settling, but some with and others without radiative accelerations. We considered masses in the range [0.9, 1.5] M and 3 values of the metallicity around the solar one. For each metallicity, we determined the range of mass where differences between models due to radiative accelerations exceed the uncertainties of global seismic parameters of the Kepler Legacy sample or expected for PLATO observations. Results. We found that radiative accelerations may not be neglected for stellar masses larger than 1.1 M at solar metallicity. The difference in age due to their inclusion in models can reach 9% for the more massive stars of our grids. We estimated that the percentage of the PLATO core program stars whose modelling would require radiative accelerations ranges between 33 and 58% depending on the precision of the seismic data. Conclusions. We conclude that, in the context of Kepler, TESS, and PLATO missions, which provide (or will provide) high quality seismic data, radiative accelerations can have a significant effect when inferring the properties of solar-like oscillators properly. This is particularly important for age inferences. However, the net effect for each individual star results from the competition between atomic diffusion including radiative accelerations and other internal transport processes. Rotationally induced transport processes for instance are believed to reduce the effects of atomic diffusion. This will be investigated in a forthcoming companion paper.A&A proofs: manuscript no. cestamI_v8 asteroseismology analysis(coupled with spectroscopic observations) which will provide precise masses, radii and more importantly ages of the host stars.

In this paper we present a new catalogue of Chemically Peculiar (CP) stars obtained by compiling publications in which abundances of metals are provided. Our catalogue includes 428 stars for which the data were obtained through spectroscopic observations. Most of them (416) are AmFm, HgMn and ApBp stars. We have used this compilation to proceed to a statistical overview of the abundance anomalies versus the physical parameters of the stars. The Spearman's rank correlation test has been applied, and a significant number of correlations of abundance peculiarities with respect to effective temperature, surface gravity and rotation velocity have been found. Four interesting cases are discussed in details: the Mn peculiarities in HgMn stars, the Ca correlation with respect to effective temperature in AmFm stars, the case of helium and iron in ApBp stars. Furthermore, we checked for ApBp stars using Anderson-Darling test wether the belonging to a multiple system is a determinant parameter or not for abundance peculiarities.

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The chemical peculiarities of Ap stars are due to abundance stratifications produced by atomic diffusion in their outer layers. Theoretical models can predict such stratifications, but so far only provide equilibrium solutions which correspond to the maximum depth‐dependent abundances for each element that can be supported by the radiation field. However, these stratifications are actually built up through a non‐linear, time‐dependent process which has never been modelled for realistic stellar atmospheres. Here, we present the first numerical simulations of time‐dependent diffusion. We solve the continuity equation after having computed, as accurately as possible, atomic diffusion velocities (with and without a magnetic field) for a simplified fictitious – but still realistic – chemical element: cloudium. The direct comparison with existing observations is not the immediate aim of this work but rather a general understanding of how the stratification build‐up proceeds in time and space. Our results raise serious questions as to the relevance of equilibrium solutions and reinforce the suspicion that certain accumulations of chemical elements might prove unstable.