When a layer of heavy matter is above lighter matter in a star, the inverse $\mu $-gradient may lead to thermohaline (or double-diffusive) convection. This has been studied in the past for helium-rich atmospheres, but it may also occur for metal-rich layers. It has recently been studied for the accretion of hydrogen poor material onto the host stars of exoplanets. These stars present a metallicity excess compared to stars in which no planets have been detected. However, the reason for this excess is still a subject of debate. It may be primordial or the result of accretion, or both. In this last case, thermohaline convection may lead to “metallic fingers” which partially dilute the accreted matter inside the star. Such an effect can also be important in the chemically peculiar A stars in which metals accumulate in the atmospheric layers.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

KUBÁT: What are the differences between both codes for convection simulations without magnetic fields which we have just seen?

FREYTAG: Both codes solve the same set of basic equations and rely on very similar fundamental assumptions. However, the algorithms used to solve the hydrodynamics or the radiation transport equations differ. The codes have no routines in common. There is an ongoing project to perform a simulation of solar granulation with both codes, relying on the very same settings (grid, model extension, equation of state, opacities, ray system, etc.). The remaining differences are tiny, for example, much smaller than the difference between the 2D and the 3D models. Both codes have some extensions (for instance dust or magnetic field) not (yet) found in the other.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Atomic diffusion may play a significant role for the Sun and Population I Main Sequence stars up to some 25000 K, Population II turnoff stars and cluster age determinations, horizontal branch stars (including sdOs and sdBs), white dwarfs and neutron stars. In all these cases, radiative accelerations play a significant role. A stars are, however, arguably those that show most prominently the effects of atomic diffusion. In so far as the effects of accretion, mass loss, turbulence and meridional circulation may be neglected in the evolutionary models of A stars, the effects of atomic diffusion in them have now been calculated from first principles and are presented using complete evolutionary models of 1.7 and 2.5 $M_{\odot}$ stars. Their abundance anomalies are not only superficial, but extend over a significant fraction of the stellar radius. Iron convection zones appear at a temperature of about 200000 K. Abundance anomalies similar to those observed in Am stars are produced. However the comparison with the observations requires linking atmospheres to interior evolution. Models that have been proposed to take into account atomic diffusion in atmospheric regions to explain observations are critically reviewed. They depend on a number of parameters. Unfortunately the atmospheric regions are imperfectly modeled, the magnetic field is not taken into account, and important hydrodynamic processes currently require arbitrary parameters for their description.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Diffusion of elements in a stellar plasma is strongly modified by the presence of magnetic fields for two primary reasons. The first is that the average motions of ions in outer atmospheres are, because of their charge, substantially constrained by the magnetic field. Both its intensity and orientation play a role. The second is the Zeeman desaturation of absorption lines that often produces amplifications of the radiative accelerations. These effects are important and must lead to the building of complex surface abundance structures. I will present how these two effects are generally modeled and what results have, up to now, been obtained. Future developments will also be considered.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Several observational anomalies seem to confirm the presence of abundance gradients as a function of depth in different types of chemically peculiar stars. Results emanating from the construction of model atmospheres that take into account the abundance gradients caused by radiative diffusion will be presented. The atmospheric structure, which is calculated self-consistently along with the abundance gradients, will be compared to models with homogeneous abundances. Recent improvements brought to these models will be discussed, along with the intricacies of these calculations and the remaining uncertainties. Several possible applications of such models will also be presented.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We calculate multicomponent radiatively driven stellar wind models suitable for A stars. We discuss the possible decoupling of individual elements from the stellar wind and its influence on the chemical peculiarity of these stars. We obtain a range of stellar parameters for different types of multicomponent flow.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We describe the current status of a programme we started a few years ago to observe a large number of A/F and Am/Fm stars in open clusters of various ages. Spectra were obtained with the AURELIE and ELODIE spectrographs at a resolving power of about 40000 and $S/N$ ratios from 100 up to 500. Abundances of 11 chemical elements have been derived by using Takeda's (1995) procedure. A short review on previous abundance determinations of A and F dwarfs in open clusters and a progress report on the current status of this project are presented. New abundance determinations for 24 A and F dwarfs in the Coma Berenices cluster are presented. These abundance determinations serve to set constraints to self-consistent evolutionary models of A and F stars including transport processes.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Abundance determinations obtained from spectroscopic observations of Am stars provide information concerning the transport processes present in these stars. In this paper we have used models of Am stars which include gravitational settling, thermal diffusion, and radiative accelerations for 24 elements. We used a specific model of rotation induced mixing which has reproduced anomalies in other types of stars. For this preliminary study, models of $1.7 M_{\odot}$ and $1.9 M_{\odot}$ have been computed. A comparison of the predicted abundances to the observed ones for the Praesepe star HD 73045 sets constraints on rotational mixing.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

NOELS: What are the uncertainties in the di.usion velocities? What can we do if we would like to multiply or divide it by something? How can we play with that? What would be your best guess?To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The current status of our knowledge of magnetic fields in A-type stars is reviewed with special emphasis on the progress achieved for “classical” Ap stars over the last four years. For those stars, the distribution of the strength of the field, its three-dimensional structure, and its relation with rotation and evolution are discussed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

This article reviews the methods of measurement which allow us to infer the presence of magnetic fields in (A) stars. Beginning with the basic observational consequences of the Zeeman effect, we describe various modern spectroscopic and polarimetric techniques which allow us to directly detect and characterise magnetic fields in stellar photospheres. Sometimes, nature conspires to make such detections difficult, forcing us to rely on indirect (proxy) indicators of magnetism. This talk will also briefly discuss a number of these indirect field indicators, some of which demand space-based observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Some theoretical problems associated with the presence of large-scale magnetic fields in A stars are reviewed. Possible implications of some recent theoretical and observational results are discussed, with particular attention to the survival of fields from the ISM and their long-term stability. A more coherent picture of the origin of the strong large-scale fields seen in the magnetic CP stars may be beginning to emerge.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

High-resolution spectroscopic and spectropolarimetric data of the rapidly oscillating Ap star HD 24712 (HR 1217, DO Eri) has been analysed including modelling the vertical elemental abundance structures. We study the interaction and the relation of the vertical (stratification) and the horizontal (spots) abundance characteristics of Fe and the stellar magnetic field. By this synopsis and the relation of our results to the analysis of high resolution and high time resolved observations (Sachkov et al. 2005) we are likely to gain new insights about the atmospheric structure and the geometry, the origin, and the evolution of the magnetic fields of roAp stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

BAGNULO: I would like to ask Mathys and Moss to comment about the possible detection of the radial gradients of the magnetic .elds in Ap stars. What would be the consequence of such a feature on the stellar atmosphere? I refer in particular to that the length scale for horizontal variability of the field is not less than 105 km, whereas the depth of the photosphere may be 100 times less.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We briefly review the traditional classifications of CP stars. The current availability of large numbers of abundances now make it possible to use multivariate techniques, both to supplement traditional classification methods and probe the abundance patterns. We discuss cluster analysis and correlation matrices for sample material. We review the historical resistance to the notion that CP stars were indeed chemically peculiar. Modern work shows that while these objects do indeed have atmospheric anomalies, they are nevertheless chemically peculiar.

Abundance patterns are an important clue to the origin of the abundance peculiarities. We contrast patterns due to nuclear and chemical differentiation processes. The roAp and related stars show vertical as well as horizontal abundance variations, and abnormal line profiles. Photospheric abundances in these stars are surely abnormal (nonsolar), but as long as the models are uncertain the derived abundances will be very crude.

For more than two decades, observations of CP stars in the X-ray and radio regimes have been made with increasing sensitivity and pointing accuracy. We discuss the current evidence linking magnetic and nonmagnetic CP stars to sources of galactic X-rays and radio radiation. There seems no doubt that high energy phenomena are associated with, if not produced by, some CP stars. This circumstance admits the possibility that the release of high energy particles (p's, n's and $\alpha$'s) during such events may initiate nuclear reactions on the surfaces of the CP stars. We briefly reconsider the viability of such processes for producing exotic species like Pm by proton bombardment using recent data for solar and stellar flares.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Important results of magnetic CP star research in the last decade from photometric and spectroscopic observations are discussed. Hipparcos parallaxes confirm CP stars are Main Sequence stars. Photometric monitoring of the rapidly rotating stars provides evidences for the rotational braking on the Main Sequence. High signal-to-noise spectra with high resolution and time-resolution give strong support for chemical separation processes operating in stellar atmospheres (abundance stratification). There are also observational evidences for the departure of the temperature structure of cool CP star atmospheres from that for normal stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Observations of chemically peculiar (CP) stars have been conducted for decades and have revealed a variety of spectrum anomalies, most prominent among them are line enhancements of heavy elements. The earlier observations were limited to the optical region and the use of less sensitive detectors, yet are responsible for much of our current characterization of the CP star phenomenon. More recent observations embrace a wider expanse of the electromagnetic spectrum and employ more sensitive detectors that continue to unveil new levels of spectrum peculiarity. The traditional criteria used to distinguish normal from peculiar stars have become blurred, thus in some sense replacing the concept of peculiarity with one of continuity. This presentation will address the observations of the traditional nonmagnetic CP star groups over the past decade, paying particular attention to new avenues of research that have a bearing upon the interpretation of the atmospheres of CP stars and the origins of this phenomenon.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report on the detection of secondary spectra in five spectroscopic binary systems: HD 434, HD 861, HD 108642, HD 178449, and HD 216608. High signal-to-noise high resolution spectroscopic observations were carried out at the Bulgarian NAO Rozhen as part of an extended project concerned mainly with Am stars in binary systems. Our knowledge about early type binaries has serious gaps. This is true especially when it is only based on older photographic techniques. We concluded that photographic data involving longer orbital periods (where Doppler shifts due to the orbital motion are comparable or even less than the rotational broadening of the spectral lines) and early type stars (that have only a few and usually broad lines) should be revisited or at least used with caution. We demonstrate that for the five systems how CCD observations made with 2-m class telescopes can discover the binary nature or secondary spectra of many currently unresolved SB1 systems. Important astrophysical information such as the atmospheric parameters and the mass ratios are used to unravel previous misinterpretations of the data leading often to spurious orbits.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The use of the Four College Automatic Photometric Telescope at the Fairborn Observatory in Southern Arizona with its ability to observe stars every clear night and for continuous runs has played a major role in our very successful cooperative program of Strömgren differential uvby photometry of magnetic CP (mCP) stars. Out of about 100 stars observed only a few have variable light curves. But more may be found when additional observations are made. We review the published theoretical basis and observations as well as present new results for CU Vir, V1093 Ori, MW Vul, and HR 7224. We interpret some observations as possibly requiring a more robust theory of the variability of mCP star light curves.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The kinetic equilibrium of Nd II -Nd III in the atmospheres of A-type stars is investigated for the first time with a model atom containing 1651 levels of Nd II, 607 levels of Nd III, and the ground state of Nd, IV. NLTE leads to an overionization of Nd II resulting in weakening the Nd II lines at mild neodymium overabundances relative to the solar Nd abundance ([Nd/H] $\lt$ 2.5) and produces the opposite effect at the higher [Nd/H] values. NLTE abundance corrections grow with effective temperature and reach $\sim$0.6 dex at $\Teff$ = 9500 K. The Nd III lines are strengthened compared with LTE. NLTE abundance corrections range between -0.3 dex and -0.2 dex for $\Teff$ between 7500 K and 9500 K. Therefore NLTE effects may explain ionization discrepancies up to 0.8 – 0.9 dex, derived with the LTE approach.

NLTE effects are even larger for a stratified Nd abundance distribution compared with a homogeneous one resulting in positive NLTE abundance corrections up to 1.4 dex for the Nd II lines and negative ones as small as to -0.5 dex for the Nd III lines. The influence of uncertainty in the photoionization cross-sections on NLTE results is investigated. NLTE calculations were applied to Nd analyses in the atmospheres of roAp stars $\gamma$ Equ and HD 24712.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html