Photometric and spectroscopic variability of late-type stars frequently has been interpreted as evidence of magnetic activity. The standard picture of stellar activity – inherited from solar observations – includes cool, dark “spots” in the photosphere and hot, dense regions in the chromosphere and coronae. The immediate cause of each of these phenomena is a closed topology of the local magnetic field. Because stars appear as mere points of light, these localized phenomena have not been directly resolvable on stars other than the Sun. Most observed effects are produced by an asymmetric distribution of starspots. If the distribution is symmetric, it would escape detection by most current techniques of light-curve and line-profile modeling. Even more troubling, the stellar properties measured with these techniques describe only a difference between contrasting hemispheres, not an absolute measure.

In the past, much effort has been devoted to determine stellar magnetic fields by means of the analysis of two spectral lines with different magnetic sensitivities (see, e.g., Robinson, 1980; Saar, 1988; Basri and Marcy, 1988). However, these methods are largely influenced by the uncertainties caused by, for instance, the presence of blends, uncertainties of atomic parameters, etc. Gray (1984) and, more recently, Basri et al. (1992) used a multiline analysis to measure stellar magnetic fields, reducing the importance of those uncertainties, thus increasing, in a statistical sense, the confidence of the results.

Here we present a new method based on the analysis of the equivalent widths of many Fe I spectral lines. The central idea is to obtain information on the magnetic field from the magnetic strengthening of the equivalent width of spectral lines. This technique has been applied to a weak-emission-line T Tauri star (TAP35) and a main sequence star (ξ Boo A), using data published by Basri et al., (1992).

Rapidly oscillating Ap (roAp) stars are cool magnetic CP2 stars which vary with periods of a few minutes and amplitudes less than 0.01 mag and 1 km/s in light and velocity. Analysis of their p-mode eigenfrequency patterns and splittings gives information about evolutionary state, rotation rate, magnetic field geometry and internal field strength (see Kurtz 1990; Matthews 1991). We present here an example of how roAp pulsations can be used to obtain an estimate of the temperature structure of an Ap atmosphere.

The pulsation amplitudes of roAp stars decline more rapidly with increasing wavelength than other known pulsators. Matthews et al. (1990) explained this by the wavelength dependence of limb darkening and its weighting effect on the integrated amplitude of an (l, m) = (1,0) mode.

We derived preliminary abundances from the ultraviolet spectrum of Vega by comparing high-resolution Copernicus spectra with spectra computed with the ATLAS9 code and with updated line lists. The Teff=9400 K, log g=3.90 model, which fits the visual flux and the Balmer profiles is able to fit the ultraviolet flux either with [M/H]=−l and microturbulent velocity ξ= 2 km s−1 or with [M/H]=−0.5 and ξ=0 km s−1. The line spectrum is fitted by ξ=2 ± 0.5 km s−1 and by an iron abundance log (N(Fe)/Ntot)=−5.2 ±0.1 dex. All the observed elements indicate underabundances ranging from about −0.35 dex (C ) up to about −1 dex (Ni, Zn). The origin of the underabundances is an open question.

We discuss in this poster the problem of the formation of Hα lines in the stochastic winds of T Tauri stars.

There are two basic approaches to the equation of state for stellar envelopes and interiors. The traditional method chooses the so-called “chemical picture”, in which the notion of atoms is maintained despite the plasma environment. A mixture of atoms, molecules, ions, electrons and nuclei is considered, and the occurring ionization and dissociation reactions (thus the name chemical picture) are treated according to the entropy-maximum (or free-energy-minimum) principle. The alternative method is based on the so-called “physical picture”, where only fundamental particles (electrons, nuclei) explicitly enter. Through the means of activity expansions, the problems of plasma physics and statistical mechanics are treated simultaneously and on the same footing. For helio- and asteroseismology, an accurate and precise equation of state is essential. Progress towards a better equation of state can be made in several ways: purely theoretical efforts, checks with experiments, including astrophysical data, and comparisons between different theoretical formalisms. Comparisons are useful to assess the domain of temperature and density where the theoretical complications matter, and to determine the diagnostic potential of astrophysical observables for equation of state issues.

A review is given of opacity calculations made during the past decade.

This review focusses on the most recent work which has been achieved concerning turbulence in stellar interiors. Among all possible causes for such turbulence, the most powerful is certainly the convective instability in unstably stratified regions, but little was known until now beyond the fact that thermal convection is capable of establishing an almost adiabatic stratification. The latest numerical simulations (Stein & Nordlund 1989; Cataneo et al. 1991) show that turbulent convection is highly intermittent, with strong downdrafts carrying most of the energy flux.

These plumes penetrate in the stable region below, which they render nearly adiabatic over some distance (Zahn 1991); the penetration depth should be measured soon through acoustic sounding of the Sun (Berthomieu et al. 1992). Such plumes are likely to exist also in a convective core, and to penetrate into the radiative region above (Roxburgh 1989).

The effect of gravitational settling and radiation driven diffusion on the evolution of stars near the main sequence is reviewed. New simplified formulae for calculating diffusion are proposed that improve on previous such formulae. The reliability of available diffusion coefficients is discussed and areas where further work is needed are identified. Newly available opacity calculations are used to estimate the effects of radiative acceleration on Fe.

The size of the modifications to the evolution are shown to be modest: a reduction of order 10% on the evolutionary age of globular clusters. There are indications from the Li abundance in the high Teff halo stars that some turbulence is present below the convection zone. Models calibrated using solar properties reproduce the Teff at which the Li gap is observed in F stars as well as its depth without any arbitrary parameter. Similarly the presence of AmFm stars is explained over the Teff range where they are observed.

Electron– and proton–impact line widths and shifts for important Ca II, Sc III and Ti IV lines, have been calculated using the semiclassical–perturbation formalism. The obtained results were used to investigate the behaviour of Stark broadening parameters within the K I isoelectronic sequence.

Opacities are of vital importance in astrophysics necessary to model stellar atmospheres and interiors, and below about 6000 K diatomic molecules make an increasing contribution to that opacity. Moreover, at around 3000 K polyatomic molecules, such as H2O for an oxygen rich solar composition, start playing an important rôle, and additionally at high pressures collision induced H2 – H2 and H2 – He absorption is important for late-type main sequence stars.

In order to investigate the importance of molecular opacities, in Sharp (1992) we computed and compared opacities for a solar and a three times enhanced CNO abundance, and some further comparisons were made in Sharp et al. (1992). Some of these results are briefly summarised here.

We propose a new approach to calculate the populations of the different ionic states and ionization stages that may exist in a plasma in thermodynamic equilibrium. A self-consistent scheme is solved for each ionization stage, which couples the local density of free electrons and the atomic structure of this stage.

Various models are used in order to develop an accurate equation of state (EOS) for stellar matter (see e.g. Saumon and Chabrier 1991). The complexity of physical effects, for instance degeneracy, relativity, Coulomb- and non-Coulombic interactions requires detailed knowledge from the many-particle physics.

For many applications it is desirable to have theories for these effects over a wide range of densities and temperatures. One of the main sources of uncertainties in the EOS models is the exchange-correlation contribution at any degeneracy and relativity. Previous investigations determine thermodynamic quantities of a partially (e.g. Graboske et al. 1969, Kraeft et al. 1986, Däppen et al. 1988) or fully ionized (e.g. Hubbard and DeWitt 1985, Ichimaru et al. 1987) plasma, resp. Interpolation procedures (Magni and Mazzitelli 1979, Fontaine et al. 1977) combine both cases in the EOS but show non-continuous behaviour.

We study the diffusion process occurring just below the superficial convection zone of Am stars, improving the methods used sofar. We are now able to compute, in a more realistic way, the evolution of the concentrations during the stay of the stars on the main sequence for a large number of elements. This allows to better constrain the different properties (mass loss, depth of the superficial convection zone, transition between convection and radiative zone) entering the modelling of Am stars in the framework of a diffusion-dominant description.

The mechanism and properties of mass loss are poorly known for Ap stars. Present upper limits on the mass loss rate are of 10−10 M⊙yr−1, a value which does not permit any element separation. Abundance maps could be a very powerful tool to constrain the mass loss rate and the wind geometry of Ap stars, as surface abundances are sensitive to rates as small as 10−15 M⊙yr−1. We here propose a diffusion-mass loss model for 53 Cam and compute abundance distributions in the photosphere of 53 Cam. The mass loss geometry is determined from the Ca II K line profile and its time variation. We obtain that the diffusion-mass loss model explains many spectral features of 53 Cam, both in the UV and visible domains.

The generation of internal waves in the radiatively stable stellar region by the turbulent motion at the boundary of the overlaying convective zone is similar to the same case in the deep ocean or in the earth atmosphere (Townsend, 1965), and can be described in a simple way as following: When an turbulent fluid element arrives at the boundary of the convective region with a non-zero momentum, it beats and it deforms the interface between both regions. This disturbance of the equilibrium state excites a train of internal waves propagating below the convective zone in the horizontal and vertical directions for the frequencies lower than the characteristic one for the stable stratification (Brunt-Väisälä frequency).

That mixing take place in the radiative zone of many stars, is an event that cannot be forgotten when we try to explain observational results as the lithium abundance in the atmosphere of different stars, its dependence on spectral type, age or rotation velocity... During the last years many processes have been proposed as being responsible of this mixing: overshooting, turbulence induced by rotational instabilities, internal waves, etc... We will consider, following the results obtained by Press (1981), the role of this last mechanism in the transport of lithium to the burning level, not as generators of turbulence (in Press, 1981, and García-López and Spruit, 1991, it is shown that turbulence induced by internal waves decays very quickly inside the radiative zone), but as generators of a diffusive process due to non linear dissipative effects.

Observations of lithium in F type main sequence stars and subgiants lead to specific constraints on the rotation-induced mixing. The fact that lithium is smoothly depleted in subgiants down to factors as large as 1000 at the end of the sequence can be explained if lithium suffered extra depletion inside the star while it was on the main sequence. We propose a simple consistent model, based on the discussion of rotation-induced mixing developed by Chaboyer and Zahn (1992), which could account for these observations, as well as the observations of lithium on the main sequence.

Angular mementum distribution is one of the most important factors for stellar structutre and evolution. Among other mechanisms, angular momentum is transfered by non-axisymmetric oscillations (nonradial oscillations). In this mechanism the angular momentum is carried mainly by the Reynolds stress, which is proportional to the product between radial and azimuthal components of oscillation velocity; i.e., (Φ direction is the direction of rotation velocity). In the linear oscillation analysis, the phase difference between and is with A finite value of δ, which arises from excitation or damping of the oscillation, makes the time average of finite. Positive angular momentum is transfered from the driving zone to the damping zone by a prograde mode (Osaki 1986).

The integral constraint on convective overshooting (Roxburgh 1976, 1978, 1989) is re-derived for for statistically stationary two-dimensional convection between stress-free horizontal boundaries maintained at constant temperatures, the vertical boundaries being periodic. We report on numerical simulations of such two-dimensional compressible convection in a fluid where the central regions are convectively unstable and the surrounding layers are stable. The results support the conclusion by Roxburgh (1989) that the contribution from the radiative flux is well approximated by the flux carried by the mean temperature gradient, but demonstrate that viscous dissipation is important, reducing the extent of overshooting.