G. Alecian

INFLUENCE OF THERMOHALINE CONVECTION ON DIFFUSION-INDUCED IRON ACCUMULATION IN A 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.

Bi-dimensional element stratifications computed for magnetic Ap star atmospheres

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 Teff = 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 Teff = 8500 K model in the printed version, and compare it with stratifications derived from observed spectra.

Modelling element distributions in the atmospheres of magnetic Ap stars

Context. In recent papers convincing evidence has been presented for chemical stratification in Ap star atmospheres, and surface abundance maps have been shown to correlate with the magnetic field direction. Radiatively driven diffusion, which is known to be sensitive to the magnetic field strength and direction, is among the processes responsible for these inhomogeneities. Aims. Here we explore the hypothesis that equilibrium stratifications – such that the diffusive particle flux is close to zero throughout the atmosphere – can, in a number of cases, explain the observed abundance maps and vertical distributions of the various elements. Methods. An iterative scheme adjusts the abundances in such a way as to achieve either zero particle flux or zero effective acceleration throughout the atmosphere, taking strength and direction of the magnetic field into account. Results. The investigation of equilibrium stratifications in stellar atmospheres with temperatures from 8500 to 12 000 K and fields up to 10 kG reveals considerable variations in the vertical distribution of the 5 elements studied (Mg, Si, Ca, Ti, Fe), often with zones of large over- or under-abundances and with indications of other competing processes (such as mass loss). Horizontal magnetic fields can be very efficient in helping the accumulation of elements in higher layers. Conclusions. A comparison between our calculations and the vertical abundance profiles and surface maps derived by magnetic Doppler imaging reveals that equilibrium stratifications are in a number of cases consistent with the main trends inferred from observed spectra. However, it is not clear whether such equilibrium solutions will ever be reached during the evolution of an Ap star.

The new Toulouse-Geneva stellar evolution code including radiative accelerations of heavy elements

Context. Atomic diffusion has been recognized as an important process that has to be considered in any computations of stellar models. In solar-type and cooler stars, this process is dominated by gravitational settling, which is now included in most stellar evolution codes. In hotter stars, radiative accelerations compete with gravity and become the dominant ingredient in the diffusion flux for most heavy elements. Introducing radiative accelerations into the computations of stellar models modifies the internal element distribution and may have major consequences on the stellar structure. Coupling these processes with hydrodynamical stellar motions has important consequences that need to be investigated in detail. Aims. We aim to include the computations of radiative accelerations in a stellar evolution code (here the TGEC code) using a simplified method (SVP) so that it may be coupled with sophisticated macroscopic motions. We also compare the results with those of the Montreal code in specific cases for validation and study the consequences of these coupled processes on accurate models of A- and early-type stars. Methods. We implemented radiative accelerations computations into the Toulouse-Geneva stellar evolution code following the semianalytical prescription proposed by Alecian and LeBlanc. This allows more rapid computations than the full description used in the Montreal code. Results. We present results for A-type stellar models computed with this updated version of TGEC and compare them with similar published models obtained with the Montreal evolution code. We discuss the consequences for the coupling with macroscopic motions, including thermohaline convection.

Looking for pulsations in HgMn stars through CoRoT ⋆ lightcurves

Context. HgMn chemically peculiar stars are among the quietest stars of the main-sequence. However, according to theoretical predictions, these stars could have pulsations related to the high overabundance of iron peak elements, which are produced by atomic diffusion in upper layers. Such pulsations have never been detected from ground-based observations. Aims. Our aim is to search for signatures of pulsation in HgMn stars using the high quality lightcurves provided by the CoRoT satellite. Methods. We identified three faint stars (V > 12), from a VLT-GIRAFFE multiobject spectrographic survey in a field planned for observation by CoRoT. They present the typical characteristics of HgMn stars. The three stars were observed by the CoRoT satellite during the long run (131 days) which started on 24 October 2007, with the exoplanets CCDs (Additional Programme). In the present work, we present the analysis of the ground-based spectra of these three stars and of the corresponding CoRoT lightcurves. Results. Two of these three HgMn candidates show low amplitude (less than 1.6 mmag) periodic variations (4.3 and 2.53 days respectively, with harmonics) which are compatible with periods predicted by theoretical models.

Abundance distributions over the surfaces of magnetic ApBp stars: theoretical predictions

Recently published empirical abundance maps, obtained through (Zeeman) Doppler mapping (ZDM), do not currently agree with the abundance structures predicted by means of numerical models of atomic diffusion in magnetic atmospheres of ApBp stars. In a first step towards the resolution of these discrepancies, we present a state of the art grid of equilibrium abundance stratifications in the atmosphere of a magnetic Ap star with T_eff = 10000 K and log g = 4.0. A description of the behaviour of 16 chemical elements including predictions concerning the over- and/or under-abundances over the stellar surface is followed by a discussion of the possible influence of presently neglected physical processes.

A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy ⋆

The existence of pulsations in HgMn stars is still being debated. To provide the first unambiguous observational detecti on of pulsations in this class of chemically peculiar objects, the bright sta r HD 45975 was monitored for nearly two months by the CoRoT satellite. Independent analyses of the light curve provides evidence of monoperiodic variations with a frequency of 0.7572 d −1 and a peakto-peak amplitude of∼2800 ppm. Multisite, ground-based spectroscopic observations overlapping the CoRoT observations show the star to be a long-period, single-lined binary. Furthermore , with the notable exception of mercury, they reveal the same periodicity as in photometry in the line moments of chemical species exhibiting strong overabundances (e.g., Mn and Y). In contrast, lines of other elements do not show significant variations. As found i n other HgMn stars, the pattern of variability consists in an absorption bump moving redwards across the line profiles. We argue that t he photometric and spectroscopic changes are more consistent with an interpretation in terms of rotational modulation of spots a t the stellar surface. In this framework, the existence of pu lsations producing photometric variations above the∼50 ppm level is unlikely in HD 45975. This provides strong constraints on the excitation/damping of pulsation modes in this HgMn star.

Scandium in AmFm stars in the light of new atomic data

Context. The abundance anomalies observed in AmFm stars are believed to be caused by atomic diffusion below the superficial convection zone. The process by which these anomalies, which are produced in deep layers, propagate toward the stellar surface is well understood. However, it depends on quantities which are not directly observed: the mass-loss flux and the exact position of convections zones. Scandium, which is systematically underabundant at the surface of AmFm stars, is a key element in understanding the interplay between atomic diffusion and the stellar structure and how this structure evolves with time. Aims. In the light of a new extensive set of atomic data, we calculate much more accurate radiative accelerations of Sc than previously done in the aim to better understand the observed abundance anomalies of this element in AmFm stars. Methods. Two methods are employed to obtain these radiative accelerations based on the so-called SVP (single-valued parameters) parametric method and the GLAM method while employing this new atomic data. Results. The radiative accelerations, shown here in a typical Am stellar model, are discussed in the light of the observed anomalies of Sc. Our results are more compatible with the scenario where the scandium depletion is created below the hydrogen convection zone.

New catalogue of chemically peculiar stars, and statistical analysis

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 Spearmans 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.

Impacts of radiative accelerations on solar-like oscillating main-sequence stars

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. 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. We implemented the calculation of radiative accelerations 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 found that radiative accelerations may not be neglected for stellar masses larger than 1.1~M