Solar p-mode multiplet oscillation frequencies and expansion coefficients for degeneracy splitting have been measured in the summers of 1986 and 1988–1990. Temporal variations not unlike the differences between 1988 and 1986 reported by Libbrecht & Woodard (1990) continue into 1990. These differences are possibly associated with the solar cycle. We report here on our first findings about the variation of the internal angular velocity and asphericity deduced from these frequency differences.

Large amplitude infrared 2.23 μm solar intensity oscillations were detected in photometer observations obtained at Oslo Solar Observatory in 1987 and 1988. Five wavelength regions ranging from 0.67 μm to 2.23 μm and 7 circular entrance apertures ranging from 0.5 to 4.3 arc min were observed simultaneously at all wavelengths with the same detector. The 2.23 μm region showed remarkably higher amplitudes than the other 4 wavelength-regions. The observed power was concentrated to the 2.5 - 3.5 mHz region suggesting that we observe the well known 5 min oscillations (Leifsen and Maltby, 1990).

Solar Evolutionary Models are briefly reviewed and while the models are robust, there are uncertainties in the input data which justify rather larger errors. The 1992 experimental results from GALLEX, SAGE II and Kamiokande are shown to be consistent with calculated fluxes of solar neutrinos whereas the Chlorine results continue to be significantly low though this experiment has a problem with the high variability with time of its results in contradiction to Kamiokande. It is concluded that the evidence for a solar neutrino problem is not compelling and New Physics are not demanded. Further experiments are essential to search for neutrino masses and to study the Sun.

Recent advances in high S/N spectroscopy are reviewed with particular emphasis on new data for the abundances of chemical elements of importance for the modelling of stellar interiors.

It seems well established that young, nearby B-type stars have abundances of CNO elements that are about a factor of two lower than in the Sun except for a small fraction of nitrogen-rich stars. The existence of such stars among main sequence stars remains to be explained.

Among normal A-type stars (excluding Am and Ap stars) large deviations from solar abundance ratios occur with interesting anticorrelations of the abundances of C and Si. This suggests that diffusion processes and/or gas-dust separation of the chemical elements in the protostellar cloud play an important rôle.

The significance of a detailed knowledge of the atmosphere of a star for a proper understanding of its interior is illustrated and discussed. It is concluded that significant progress in the understanding of stellar atmospheres is needed in order to match methods, discoveries and needs in the study of stellar internal structure and evolution.

In this paper, I review recent progress in the domain of indirect imaging of stellar atmospheres. Since all stars other than the Sun are too far away for us to disclose any feature on their surface directly, one has to use an indirect tool to image their atmosphere. This method (Doppler Imaging) takes advantage of a star’s rotation to identify and characterise inhomogeneities on its surface. This is done by studying both how inhomogeneities (carried in and out of the observer’s view by rotation) modulate photometric and spectroscopic data with time, and how surface features distort the shape of line profiles at a given rotational phase. In the last decade, Doppler Imaging has first been applied to abundance mapping of magnetic Ap stars, then extended to temperature and magnetic mapping of cool active stars. Allowing a solar-like way of studying the surface of stars, this technique provides a wealth of information about stellar atmospheres and, in particular, about the physical processes which dictate their dynamics and energy balance.

We compare the log g parameters of early A and late B type stars determined from model atmospheres with those issued from models of stellar evolution. There is a systematic deviation between both determinations for log g < 3.7 .

For 9 stars of A5–G2 spectral type interval IV and V luminosity classes effective temperatures, bolometric corrections, radii, angular diameters and luminosities were obtained by means of the infrared flux method in the modification by Blackwell et al. (1980).

The energy distributions (EDs) of 6 peculiar (3 Aps and 3 λ Bootis) stars are presented from 1200Ȧ up to about 12000Ȧ. For those stars having infrared photometry, the integrated flux from 1200& to 22000Å has been calculated and the Infrared Flux Method (IRFM, Blackwell and Shallis, 1977) applied to derive effective temperatures and angular diameters. For all stars, the effective temperature Teff, the surface gravity logg and metallicity [M/H] were also derived by matching the EDs to a grid of model atmospheres. A chisquare techniques is used to sort out the best fit to the observed EDs.

The IRAS satellite has observed some early-type stars in the infrared part of the spectrum. Most of this infrared radiation is due to free-free scattering in the dense stellar wind of these stars. We modelled this phenemenon by solving the equations of hydrodynamics as well as the radiative transfer equation (for a H and He mixture). This yields continuum fluxes at visible and infrared wavelengths which have been compared with the observations.

For several years it has become quite common to derive stellar parameters like effective temperature, Teff, and surface acceleration, g, by means of properly calibrated photometric indices, and to use these values for the derivation of important properties of stellar aggregates (viz. ages, star formation history, distances, etc.). Photometric observations, however, fail to give informations about one important property of a star: its rotational rate!

The main effect of rotation is to increase the size of the star, mainly in the equatorial region, leading to lower surface temperatures and accelerations. Since in non–spherical stars Teff and g depend on the latitude, the observed values, which are, of course, averages over the visible hemisphere, depend on the angle of inclination, i. Collins & Sonneborn (1977) utilized the rigidly rotating stellar models of Sackmann and Anand (1970) to compute emergent fluxes and photometric indices for various stellar masses, inclination angles i and rotational parameters being the break–up rotational rate. These indices, viz. C0 and β, represent then averages over the visible part of the rotally distorted stellar models. Closer inspections indicated also that emergent fluxes and line profiles of rotationally distorted stars can be matched by the predictations of standard model atmospheres to a very high accuray, even for w close to unity (Wenske 1992, Diplom Thesis).

We present results based on the analysis of lithium abundances in a sample of ~ 50 pre-main sequence stars covering a wide range of masses (from 2 to 0.3 M⊙) and luminosities (corresponding to ages of 1-100 Myr). Stars with masses estimated to be ≳ 1 M⊙show lithium abundances close to cosmic with little scatter (± 0.3 dex). Stars with masses less than Solar present a wide range of lithium abundances, with a clear trend to lower abundances for lower luminosities (greater age). The observed Li abundances constrain theoretical predictions of lithium depletion in rotating pre-main sequence stars.

For a set of normal A and early F type stars, we discuss the possible contributors which can affect the Li feature and their effect on the determination of the Li abundance.

We are conducting a search for lithium in very low mass objects with the aim of discriminating between stellar and substellar objects. Lithium is expected to be preserved in brown dwarfs with M/M⊙ ≤0.06, while it is known to be efficiently destroyed in low mass stars. In this paper we present high resolution observations in the region of the λ 6708 Li I resonance line of 5 very low mass dwarfs. In none of them lithium is detected, implying a Li destruction in their atmospheres of about four orders of magnitude. Our results suggest that these objects are probably very low mass stars rather than brown dwarfs.

Standard stellar evolution predicts that F stars should retain their initial surface lithium (Li) abundance because their convection zones are too shallow to destroy it at their base. Yet, observations reveal a severe Li depletion (the “Boesgaard Gap”), perhaps by as much as about two orders of magnitude, in a narrow Teff range. Several physical mechanisms, not usually included in stellar evolution calculations, have been proposed to account for this Li deficiency. These include diffusion, mass loss, meridional circulation, and rotationally-induced mixing driven by angular momentum loss. Identifying which of these (if any) might really be at work is not only of vital interest to stellar evolution, but may also have serious implications elsewhere (e.g. cosmology, Deliyannis et al. 1991). We bring attention to beryllium (Be) observations in F stars, which are crucial for discriminating between scenarios. Particularly important is the star 110 Her, which is depleted in Be by about a factor of 5 -10, but still has a detectable Li abundance (depleted by a factor of 100 - 200). Depleting surface Be without having depleted nearly all of the surface Li requires specific circumstances; we discuss how this depletion property severely constrains or eliminates most of the proposed mechanisms. One mechanism, rotationally-induced mixing, predicts relative depletions for these elements that agree well with what is observed.

We are now seeing a new generation of stellar models based on the recent opacity tables by Rogers and Iglesias (1992). Various changes in other important physical ingredients like nuclear reaction rates, convection parameters, mass loss rates, helium/metal ratios, relative abundances of heavy elements etc. are also included. Besides the Sun the most obvious objects for confrontation of the new grids with observations are binary components, pulsating stars, and well-studied open and globular clusters. Among these candidates the detached, double-lined eclipsing binaries play a unique role as our only source of absolute dimensions which 1) can be considered valid for single stars and 2) are accurate enough for serious tests of the input physics of modern stellar models.

We present new beryllium abundances for three unevolved stars with [Fe/H] in the range −0.4 to −1.6. The Be II doublet is detected in the three stars. A spectral synthesis analysis has been conducted to derive the abundances. We briefly discuss their Be and Li abundances, together with a wide sample of stars from the literature, in the context of mixing processes below the convection zone.

The results of the He, N and C abundance determinations for MS early B-stars, summarized in the review [1], lead to the inference that there are evolutionary variations in atmospheric contents of these elements. The conclusion was made that CNO-cycle products emerge on a surface of B-stars during their MS phase yet and such appearance can be considered as a probable manifestation of some mixing process between outer layers and stellar interiors. In particular the N enrichment with the age was found, which reachs about 1.0 dex for most massive B-stars (see [1,2]). However Gies and Lambert [3] (further GL) investigated recently the C, N and O abundances in a sample of early B-stars and did not confirm such great N enrichment. Since the problem of early mixing is very important for the understanding of first evolutionary stages of massive stars, the GL data need further and thorough analysis.

We have obtained the oxygen abundance in a sample of 44 F-type dwarfs in the Ursa Major group and the Hyades open cluster, using an NLTE analysis of the O I infrared triplet at λλ 7771-7775 Å. These clusters show the “lithium gap” in the F spectral range, and it has been suggested that if this phenomenon were due to microscopic diffusion, their Li-depleted stars could also show oxygen and/or nitrogen depletion. Our results here show a substantial measure of uniformity in the oxygen abundances. The data might indicate a very small dip (< 0.1 dex) in the O abundance for stars located in the Li gap. Microscopic diffusion seems to be the only known mechanism able to produce an oxygen dip. If this mechanism were responsible for the Li gap, it would have to account for a depletion of up to two orders of magnitude in Li and, at the same time, less than 0.1 dex in oxygen. We note that turbulent mixing could modify a microscopic diffusion pattern in order to yield the observed abundances of the two elements.

Progress in observational techniques (i.e. high S/N spectroscopy) and modelling (e.g. RE models including convection, nonLTE codes, accurate atomic data) make significant to carry to extremes the numerical fit of a portion of the stellar spectrum and try to interpret the residual differences between calculations and observations in terms of stellar characteristics new respect to the employed model.

We have started to apply this synthesis method to a set of high resolution, high S/N spectra of late-type dwarfs in the region of the Na I D lines, obtained at ESO with the 1.4m CAT on May 1989 (Franchini et al. 1992). The dwarf stars are modelled in radiative equilibrium (RE), and the Na I D line synthesis accounts for nonLTE and blend effects.