We present the standard models for small and intermediate main sequence stars and we discuss some of the problems arising with semiconvection and overshooting. The surface abundance of Li serves as a test for other physical mechanisms, including microscopic and turbulent diffusion, rotation and mass loss.

We discuss the status of current models for the early evolutionary stages of stars in the initial mass range 10-40 M⊙. Effects of the pre-main sequence evolution, mass loss, internal mixing, and changes in atomic and nuclear data are outlined and confronted with several basic observational facts, which are unexplained by standard models. We conclude that especially internal mixing processes deserve much more attention in future investigations, and we show why convective mixing may be less efficient than generally assumed, but more mixing should be present in the radiative zones of at least a fraction of all massive stars.

We present the results of new evolutionary calculations of the pre—main-sequence phase of stars in the mass range 1 to 5 Mʙ. The novelty of our approach is that, unlike in the standard theory of PMS evolution, the initial conditions of the gravitational contraction phase are not arbitrarily set, but derive from protostellar theory.

In this paper we review the observations which lead to the believe that intermediate mass pre-main sequence Herbig Ae/Be stars are surrounded by accretion disks. A discussion is given of their properties and their contributions to surface and inside phenomena of the central stars.

CESAM is a consistent set of programs and routines designed for the calculations of stellar evolution. Untill now it allows the computation of the evolution from PMS or ZAMS to helium flash for stellar masses of some solar mass. It is constructed in such a way that all the physics works as external routines. The numerical techniques are based on the B-spline formalism. This formalism used both for the integration of the differential equations and for 1D and 2D interpolation schemes of various tables of physical data.

Work has begun on the construction of a grid of theoretical isochrones based on important recent refinements in stellar physics. These refinements have already been applied to the construction of a standard solar model, which, for the first time, free of any adhoc assumptions, reproduces the observed p-mode oscillation spectrum within the errors of the physics (Guenther et al. 1992).

The effects of Coulomb corrections on the evolution of globular clusters stars are discussed. Coulomb corrections alter the equation of state by about 1% in most of the stellar interior, and for stars of fixed initial parameters, this results in an 8% increase in the ZAMS luminosity and an 8% decrease in the age at the main sequence turnoff. Ages for globular clusters measured by comparing to the turnoff luminosity of theoretical isochrones are lowered by ≈ 4% when Coulomb effects are included.

With the aid of detailed stellar model computations, we analyse the consequences of applying a mild overshooting to 1.0 M⊙ to 1.2 M⊙ stars with solar metallicity. It is shown that solar models with a small convective core can be constructed, resulting in a decrease in the 8B neutrino flux that can amount to 30% relative to the value obtained in models without convective core.

We computed the main sequence structure of masses from 2 to 0.7 M⊙ for two choices of helium (Y) and metal (Z) mass fractions. Models are given for solar composition (Y=0.285, Z=0.018) and for a composition appropriate to a younger population I (Y=0.32, Z=0.03). We took particular care both in the choice of the most recent opacities presently available and in the treatment of the input physics and numerics (see also D’Antona et al, 1992).

Opacities from Rogers and Iglesias (1992) were adopted where available, complemented with those by Kurucz (1991) at T≤10000 K and, elsewhere, by the Los Alamos opacities (Huebner et al. 1977). The equation of state by Mihalas et al. (1988) was used at ρ ≤ 0.01, and elsewhere the relation P = P(ρ,T) was taken from Magni and Mazzitelli (1979), while the adiabatic gradient was obtained from a Saha type e.o.s..

For many years we are witnessing a lively debate on the existence and extent of convective overshooting, mainly in the cores of main-sequence stars. This is an important issue, since even a small amount of overshooting increases considerably the mass of the finally hydrogen exhausted core and lenghthens the main-sequence lifetime correspondingly. The available evolutionary calculations assume either moderate overshooting, d/Hp = 0.25, (d = overshooting distance, Hp = pressure scale height; Maeder & Meynet 1988) or strong overshooting, d/Hp ≈ 0.50 (Bertelli et al. 1986). Presently theory is unable to quantify the exact amount of overshooting, and one has to resort to empirical determinations.

Recently, Stothers (1991) collected all available information from the literature on stellar parameters and evolutionary calculations and concluded that, within the errors, d/Hp = 0 is an acceptable result, with a conservative upper limit of d/Hp < 0.2. However, such an approach is hampered by observational errors (like distance or temperature uncertainties, rotation) that are difficult to quantify and that may mask any definitive result. Detailed investigations of detached binaries may help in this matter (Andersen et al. 1990) but the number of suitable binary systems is probably not very large.

We present a selection of results from a large set of numerical simulations of the spin-down of a solar-type star containing a large scale magnetic field in its radiative interior. Our computations are dynamical, in that they take into account both the generation of the toroidal component by the wind-induced shear endits back-reaction on the azimuthal flow. Our results demonstrate the existence of classes of internal magnetic fields that can accomodate rapid spin-down near the ZAMS, and yield weak internal differential rotation by the solar age.

The apsidal motion test in eclipsing binaries is one of the few observational procedures available to study the stellar interior. A careful analysis of empirical data and theoretical models has been performed since more than ten years ago by our group, and in the present communication we show the most recent results obtained from the confrontation between theory and observations.

We have carried out an extensive comparison between new stellar models (Claret & Giménez 1992) making use of the latest opacity library by Rogers & Iglesias (1992) and the best available data from double-lined eclipsing binaries (Andersen, 1991). The level at which we can theoretically reproduce the observations of classical absolute dimensions, namely, R and Teff for a given mass, is shown in Figure 1, while Figure 2 shows the fulfillment of the requirements of a common age for the two components of the same system. A very similar good agreement is obtained when we take into account a moderate amount of convective core overshooting. Small differences can be easily explained in terms of metal abundance variations with respect to the adopted average value Z = 0.02. A more detailed check is shown in Figure 3 for some selected systems.

A numerical method is presented, which is able to determine quantitatively the age t and the initial abundances of helium and metals (Y0, Z0) of a given binary system. Input data are simply the values of mass m, temperature Teff and surface gravity g, of both stars. The procedure works for detached main-sequence binaries, and it does not require any preliminary hypothesis about surface abundances or age. It is assumed that the two stars evolve parallely with no interaction.

The ζ Herculis system is one of the closest well-kown binaries. A tentative modelling shows that though the brighter star is easily described with a mass compatible with the observed one and an age of approximately 3.4 billion years, difficulties arise for the secondary, which plea for more precise observations.

Rotationallly induced mixing can explain several observational peculiarities of massive stars. Homogeneous evolution of differentially rotating stars including spindown due to stellar winds is studied by means of homology transformations of ZAMS models.

The origin and evolution of blue straggler stars (BS) in clusters is still an unsolved problem for our understanding of stellar evolution. Several rivalling theories exist, of which that of Saio & Wheeler (1980) is the most interesting one since it assumes additional mixing of fuel in the interior of BS. This theory predicts larger luminosity-to-mass ratios for a given position in the HR diagram compared to the standard evolution theory. This theory has been taylored for the old galactic cluster NGC 7789 which contains many bona fide BS, but it can also be used for other clusters of similar ages. The other theories rely on the standard theory of stellar evolution and assume (1) binary mass exchange or even merging, or (2) delayed star formation. In either case follows a “normal” luminosity-to-mass ratio. We report here on a careful spectroscopic investigation in order to discrimate between these rivalling theories.

Observed periods of pulsating stars provide information about the properties of the stars. As examples I here consider double-mode pulsators, solar-like oscillations and δ Scuti stars. The ongoing expansion in the observational efforts in this area may be expected to lead to increasingly detailed tests of stellar evolution theory.

Rotation and a magnetic field break the spherical symmetry of a star viewed as a pulsating system, lifting the degeneracy of oscillation frequencies, and leading to (sometimes prominent) observational consequences. Theoretical studies of rotational and magnetic effects in pulsating stars are reviewed, starting with simple configurations with slow rotation and weak magnetic fields.

Excitation mechanisms of oscillations in stars are discussed, in particular on the problems of the Beta Cephei pulsations and of the solar oscillations. A long-standing mystery about the excitation mechanism of the Beta Cephei pulsations seems to have finally been solved, which is due to availability of the new “OPAL” opacities. The very mechanism is the classical к-mechanism, however, due to enhanced heavy element opacities at temperature around 2 × 105K. By using the new opacities, three independent groups have found pulsational instabilities for the fundamental radial mode and a few low-degree non-radial modes in Beta Cephei models. Stochastic excitation of solar oscillations by turbulent convection is then discussed. It is shown that the noise generation by quadrupole radiation concentrated at the top of the convection zone may well explain the frequency dependence of the observed solar acoustic power pumped into individual p-modes.

Nonlinear effects determining the amplitudes of unstable modes of oscillations in stellar modes, are reviewed. The two basic processes - saturation of the driving mechanism and resonant mode coupling - are discussed within the framework of the amplitude equation formalism. There are difficult problems in the theory of multiple mode interaction that must be solved to make a prediction of amplitude spectra possible. The observed spectra for δ Scu stars and other multiperiodic variables, exhibit no simple pattern that would suggest a solution of the theoretical problems.