A method to determine the r.m.s. horizontal velocities in stellar convective zones (assuming that non-locality effects are not too strong) is given and applied to the solar case. The results may serve as useful input parameters for both convection theory and dynamo theory.

Numerical models of a convection zone embedded between two stable layers have been calculated; one is shortly presented and in particular a few items concerning the lower overshoot region are discussed.

We have obtained detailed 2-dimensional models of the surface layers of solar-type stars from extensive numerical simulations solving the time-dependent, non-linear equations of hydrodynamics for a stratified compressible fluid. The calculations take into account a realistic equation of state (including ionization of H and He as well as formation of H2-molecules) and use an elaborate scheme to describe multi-dimensional, non-local, frequency-dependent radiative transfer. The hydrodynamical models include the photosphere as well as part of the convective subphotospheric layers, with an open lower boundary located about one pressure scale height (~ 320 km) below TRoss = 1, allowing a free flow of gas out of and into the model. A fixed specific entropy, s*, is (asymptotically) assigned to the gas entering the simulation volume from below, which uniquely determines the effective temperature of the hydrodynamical model. For details about the physical assumptions, numerical method and characteristics of the resulting convective flow at the solar surface see Steffen (1991); Steffen and Freytag (1991).

Helioseismology and asteroseismology put high demands on the accuracy and consistent numerical realization of the equation of state (for a review see Däppen, these proceedings). This is explicitly illustrated by the helioseismic determination of the helium abundance of the solar convection zone in a recent investigation by Kosovichev et al. (1992). In that work it was observed that details of the treatment of the heavy elements matter more than is intuitively expected. Naively, one would expect an uncertainty of less than 10−4 in the key thermodynamic quantity, the adiabatic gradient Γ1. This is because in material of solar composition the heavy-element abundance is less than about 1.5 × 10−3 by number, and under solar conditions the dominant nontrivial contributions to the seismically relevant thermodynamic quantities predicted by modern equations of state agree to a few per cent, even for the much more abundant hydrogen and helium. However, Kosovichev et al. (1992) found that uncertainties in the treatment of the heavy elements translate into discrepancies in Γ1 of the order of 10−3, which is enough to disturb the helioseismic helium-abundance determination significantly. We briefly present the reason below. A forthcoming paper will show more detailed results, though some further information can already be found in papers by Kosovichev et al. (1992) and Christensen–Dalsgaard & Däppen (1992).

An improved theory of fluid hydrogen at high density, based on a detailed treatment of inter-particle correlations and a self-consistent treatment of pressure ionization, has become available recently (Chabrier 1990, Saumon and Chabrier 1991, 1992). We present a preliminary comparison between this new EOS (hereatfer SC) and equations of state frequently used in astrophysical contexts, namely: Fontaine, Graboske and Van Horn 1977 (FGVH), Däppen et al. 1988 (MHD) and Magni and Mazzitelli 1979 (MM).

The SC theory predicts a first-order phase transition in the region of pressure-ionization (the so-called Plasma Phase Transition, or PPT), between an essentially neutral mixture of atoms and molecules (xe– < 10−2), and a partially ionized plasma (xe– ≈ 50 %), with a critical point located at Pc = 0.614 Mbar, Tc = 15300K and pc = 0.35 g/cm3.

The present status of our theoretical and experimental knowledge of the rates of hydrogen-burning reactions in hydrostatic stellar conditions is very briefly reviewed. Special emphasis is put on reactions of the proton-proton chain, CNO cycles, NeNa cycle and MgAl chain whose rates are still affected by uncertainties that can have an impact on predictions regarding astrophysical issues of interest. This concerns in particular the solar neutrino problem, or the nucleosynthesis accompanying the CNO, NeNa and MgAl chains of reactions, as it is revealed in particular by the first dredge-up episode. A brief discussion of the possibility of reducing the present rate uncertainties is presented.

Hipparcos, the astrometric satellite of the European Space Agency, launched in August 1989, will provide absolute trigonometric parallaxes for about 100 000 stars brighter than the limiting V-magnitude 12.5. In spite of the difficulties caused by a faulty orbit, the mission is successfully going on. A lifetime of more than 4 years is expected which will allow to reach the original objectives of the mission: 2 milli-arcsec for stars brighter than V-magnitude 9 and 4 to 5 milli-arcsec for fainter stars. The observing programme is based upon a single, uniquely-defined Input Catalogue. Field and open cluster stars of almost all effective temperatures, chemical compositions and gravities are being observed. Most types of binaries and variable stars are well represented, but Hipparcos is discovering new ones. Significant parallaxes (relatives errors smaller than 20%) will be obtained for stars closer than 100 pc (about 36 000 stars). Reliable stellar structure models should fit all the possible observational constraints: the main contribution of Hipparcos parallax data will be the huge improvement in the individual luminosities. The determination of stellar masses from the study of visual binaries will also be improved. The preliminary results of the first-year data reduction are an encouraging indication of the expected quality of the final results. On the basis of a mission duration of 4 years, the final results are foreseen for 1995.

This is a review of our understanding of the effects of rotation on the stellar interior through an examination of observed surface stellar abundances. First, the evolution of angular momentum from the pre-main sequence through the giant phase is summarized. Attempts are then made to decipher the signature of abundance anomalies due to meridional circulation and rotational braking with reference to available theoretical models. Finally, special cases such as tidally-locked binaries and chromospherically-active giants are examined to determine if their rotational peculiarites have induced abundance anomalies.

The current status of empirical data on stellar masses and radii of sufficient accuracy to give constraints on stellar models is reviewed. Results from the best-studied eclipsing binaries can already trace the main-sequence evolution of 1-10 M⊙ stars in considerable detail and will be even more useful when supplemented by chemical abundance data.

Taking the deceptively simple question of the observed width of the main sequence as an example, it is shown how careful attention to the details of the data is required to reach robust conclusions about such features of modern stellar evolution models as opacity tables or convective overshooting. Only detailed modelling of specific systems with known masses, radii, and metal abundance constrain the theory strongly enough that a truly critical test is achieved. The same is true when using tidal interactions in binaries (apsidal motion, rotational synchronization, and orbital circularization) as another probe into stellar interiors.

The most important ingredient of a stellar model is its initial mass. Hence, real stars with known masses are important test objects for stellar models. Initial chemical composition is the other important starting parameter and should therefore also be known (Clausen, 1992). Finally, the most direct indicator of the evolution of a star is its radius. Together, these parameters determine the effective temperature and luminosity of the model, to be tested against a real star.

We report the detection of 10 new double–line eclipsing binary systems, with first measurements of masses and radii. These results are the preliminary outcome of an observational campaign for detecting new double–line eclipsing systems, carried out at the Observatoire de Haute Provence (OHP) in France, with the 1.52-m telescope and its CCD high–resolution spectrograph Aurelie.

In this paper we apply the Wilson-Price procedure to the solution of the binary system AO Cam using simultaneously all the available information and a statistical criterion to judge about the quality of the solutions found.

Photometric and spectroscopic observations of V3903 Sgr are analyzed, and absolute dimensions (masses and radii) are determined to a precision better then 3%.

The orbital period of a close binary system (P ~ a few days), formed by two solar type components, evolves under the influence of angular momentum loss (AML) by electromagnetic braking and of spin↔orbit angular momentum transfer (AMT) by tidal coupling. Because of AMT, which tends to reestablish the spin-orbit synchronization destroyed by AML, the loss of AM takes finally place at the expenses of the orbital reservoir and produces shrinking of the orbit and spinning-up of the components. The variation with time of wK and w, respectively the orbital and the rotational angular velocity, is expressed by a system of (stiff) ordinary differential equations and the results of the (numerical) integration strongly depend on the AML and AMT laws. A series of recent papers (Maceroni and Van ’t Veer 1991, 1992, Van’t Veer and Macerord 1992, hereafter MVI, MV2 and VM) presents the resulting orbital period evolution function (PEF) with a variety of choices. A typical result, in terms of orbital period, and for a binary formed by two identical G5 V components, is shown in fig. IA. The solution shown here corresponds to the braking law a) of fig. IB, to the AMT according to Zahn (1977) and to stars rotating as rigid bodies; different assumptions have been tried in MVland VM.

Let us recall briefly the main points of the theory as developped by Chandrasekhar and Münch (1950): if the axis of rotation of a star is inclined with an angle i to the line of sight, the apparent velocity is given by y = v sin i where v is the true rotational velocity. Assuming that the axes of rotation are randomly distributed in space, the frequency function f(v) of the true velocities may be related to the corresponding φ(y) of the apparent ones by the integral equations:

The central moments of f(v) can be derived directly from the observations by means of (1) without the knowledge of the function itself; for instance, the mean and the mean square deviation are given respectively by:

The accuracy of spectral, photometric and astrometric catalogues of stellar absolute magnitudes of F-subgiants and dwarfs has been estimated from a comparison of the data for stars in common.

New low temperature opacities have been computed and applied together with the new OPAL opacities to recalibrate the α Centauri system. Our solution is in favour of a Z value of about twice Z⊙.

A new uvby and Hβ monitoring program of low mass eclipsing binaries is currently being carried out in the framework of a 5 year observational program which also involves radial velocity determinations. The scope of this work is to provide very accurate absolute astrophysical parameters: mass, radius and effective temperatures, for main sequence late-type stars. One of the main goals is to improve the mass-luminosity relation in the low and intermediate mass range. A second objective is to perform accurate tests of the most recent grids of evolutionary models. This program is complementary to that currently being in progresss by the Copenhagen group.

In this contribution we present the photometric preliminary results obtained for three of the systems included in our long term survey: BH Vir, ZZ UMa and CR Cas for which, primary eclipses have been observed. Particular attention is paid to the determination of reddening, distances and radiative properties. A more detailed study will be carried out when the light curves and radial velocity measurements are completed

We have initiated a programme of spectroscopic observations of RS CVn short-period group, with orbital phase resolution. The systems of this group are all eclipsing binaries with both components at the Main Sequence, and most of them have similar spectral type components. The high rotational velocities and their short orbital periods (less than 1 day) hinder the spectroscopic study of these stars. We presents the first results yielded by Hα line observations of the systems XY UMa and WY Cnc. Both systems were observed during 1991 with reasonably wide orbital phase coverage. The analysis of the spectra is made by comparison with a binary model, constructed from the observed spectra of normal stars of the same spectral type as the RS CVn system components. The model accounts for the partial contribution of each component at any orbital phase within eclipses.

The Pre-Main-Sequence (pre-MS) is defined as the phase following the protostellar stage and ending with the ignition of hydrogen in the stellar core. Models in hydrostatic and thermal equilibrium such as typical “Hayashi track” models are basically adequate to describe the internal structure of pre-MS stars, although for a fraction of pre-MS objects there can be phases of accretion also at late stages, which may influence the surface abundances of light elements.

The interior evolution of pre-MS structures is focused in the two main stages’ of Deuterium and Lithium burning for the low mass (M ≤ 1.5M⊙) stars. The location in the HR diagram of theoretical tracks is still subject to large uncertainty, even more apparent today that new opacities and treatment of turbulent convection are available, and the results can be internally compared. Uncertainties amount to a factor ~ two for mass and age determination of individual objects, and affect the absolute location for both the D-burning and the Li-burning regions. Qualitative constraints, such as that there can not be Li-depletion at L ≥ L⊙, remain valid. Abundances of7Li at the surface of pre-MS stars, if measured at a stage which can definitely be considered previous to the possible occurrence of nuclear burning, and, consequently, previous to other depletion mechanism which may occur later on, provide constraints on galactic evolution of this element of cosmological interest.