The STEPHI network (STEllar PHotometry International) is a multinational cooperation stablished in order to get as perfect data as possible on several δ Scuti stars, within a reasonable period of time. STEPHI is currently integrated by nearly 15 people, belonging to the first four institutions cited above. However, astronomers of other institutions have eventually contributed to the network. The idea was born in 1986 and the network has been working to full operation since August 1989. A campaign, observing one or two δ Scuties is normally undertaken every 15 months. They are usually order of three weeks long, avoiding full-moon periods. Up to now, four campaigns have been performed, the last ending February 1992.

Very good outcomes on five δScuties [63 Herculis in June 1987 (Belmonte et al, 1991), GX Pegasi in September 1989 (Michel et al, 1992a), v650 Tauri in November 1990 (Belmonte and Michel, 1991; Michel et al, 1992b) and, finally, BN Cancri and BU Cancri in February 1992] have been obtained. Indeed, some very useful aditional information on comparison stars (see e.g. Belmonte et al, 1990) and sky conditions (transparency, brightness, scintillation, etc..) has been obtained as well. Aditionally, some key information about mostly all multiperiodic (four or more periods) δ Scuti stars studied up to March 1992, with good quality data, is presented in this report.

Stellar seismology of δ Scuti stars offers the best prospect for determining the interior properties of normal non-magnetic A-F stars on or near the main sequence. The discovery of rich eigenfrequency spectra in the light variations of several δ Scuti stars (e.g., θ2 Tau: Breger et al. 1989; GX Peg: Michel et al. 1990) should make this possible for low-degree modes (l ≤ 4). However, due to cancellation effects, modes of higher l cannot be studied through photometry or radial velocity measurements.

The detection of rapid line-profile variations among rapidly rotating δ Scuti stars (e.g., Walker et al. 1987; Kennelly et al. 1992) has extended our sensitivity to much higher modes. These variations are consistent with moderate- to high-degree modes, where |m| ~ l. (In the simplest case, they could be pure sectorial modes with |m| = l and pulsation amplitudes strongly confined to the stellar equator.) To use these data effectively, we need an objective way to identify the modes and their time evolution.

The special opportunities offered by δ-Scuti stars are the following:

1. - They regularly oscillate in several modes, and with high precision observations one has measured of the order 5 modes (Michel and Baglin 1991

2. - The excitation is a very delicate and therefore a very sensitive measure of the internal structure.

3. - They are found in open clusters which constitute a very rich scenario for tests of the theory of stellar structure and evolution. This makes it feasible to observe several stars simultaneously and gain more results than for single stars.

4. - The brightness is moderately high and still they live long enough to be present in a variety of clusters and in the field as well.

In the last few years Asteroseismology has contributed to the knowledge of stellar interior, in some extend. This contribution has been done with -observations, providing the characteristics of the acuostic modes of stellar oscillations; and with theory relating them to inner parameters, locating stars in the JCD diagram (Christesen-Dalsgaard J., 1989). At the beginning only classical pulsating stars had high enough amplitudes able to be detected. But techniques have been improved and now the goal is the detection of oscillations in all kind of stars, specially solar-like ones. Up to now, few cases of possible detections have been reported, the best evidences are: Belmonte et al (1990) in an FV star, Brown et al (1991) on the subgigant Procyon, and Pottasch et al (1992), in α Cen.

We describe technical aspects and performance characteristics of an Advanced Fiber Optic Spectrograph (AFOE), planned for the Mt. Hopkins 1.5 m telescope for asteroseismology of sun-like stars.

Probing the interior of the Sun, a slow rotator G2 V main sequence star, and connecting the atmospheric structure to inside phenomena is one of the objectives of the SOHO mission. An overview of the helioseismology and of the coronal instruments is given. It furnishes a guide line for missions that are dedicated to the probing of stellar interiors in using microvariability and activity as tracers, such as the PRISMA study.

The EVRIS experiment is an exploratory mission devoted to stellar seismology. It will observe approximately ten bright stars, for 20 days each, during the cruise of the Russian “MARS 94” mission. The photometer will be able to detect amplitudes of modes as small as a few 10−6 magnitude.

The detection of a variable period is very important as it gives information on the evolutionary state or on the binary nature of a pulsating star. If a pulsator is moving in a binary system, its light curve is frequency-modulated by the orbital period (light-time effect). Mode switching or chaos also may be the cause of the changes in the amplitude and period of the light variations.

The determination of period variability is difficult. Usually the conventional Fourier-spectrum cannot give any information about a possible period variation. The 0-C diagram is useful to detect a variable period, but it does not give the amplitude and phase values.

Some details of the data analysis used to determine high precision Doppler shifts from data obtained using an absorption cell are briefly discussed.

My brief from the principal organizers of this colloquium is to draw your attention to some of the prejudices upon which the observations that have been discussed here have had or should have had some impact, and to remark on how that might revise our views about physics: it should not be a balanced summary of the meeting, but a prejudiced review.

We have taken part to an exceptionally rich colloquium, characterized by a large amount of information in all fields of physics, and a remarkable collection of observational data. Our understanding of what is going on inside the stars has changed in a radical manner during the last years, on one side because the observations are bringing new kinds of information, and on the other side because theory in its development is taking into account a number of processes, some of them having been completely ignored only ten years ago.

It is impossible to draw here a complete list of problems. There are many cases where some inconsistency could be found in the theory itself or some contradiction between observational data and theory. I recommended that such a list should be drawn carefully. The participants to the round table discussion will give their contribution, helping to open the way to new fields of research and to new discoveries.

We present fax UV observations of Pleione made with the IUE satellite at two epochs: in 1979, when the star exhibited a strong shell spectrum and, in 1991, when it showed a Be-type spectrum. Between these two epochs, the 2200 Å bump changed dramatically, thus, rendering it inadequate for interstellar extinction/distance determination.

We report the analysis of 154 hours of nearly continuous high-speed photometric data on the pulsating DB white dwarf (DBV) GD 358 obtained during the Whole Earth Telescope (WET) run of May 1990. The Fourier transform (FT) of the light curve is dominated by power in the range from 1200 – 1700μHz with more than 180 significant peaks in the total transform. We also see significant power at the sums and differences of the dominant frequencies, indicating the importance of nonlinear behavior. We can use this data to obtain an accurate total stellar mass, and surface He layer mass. The implied surface He layer mass, if correct, provides a significant and surprising challenge to stellar evolution theory, as well as the theory of chemical mixing.

The evolution of the chemical abundances at the surface of both components of Algol-type binaries is examined. First, we have examined some mixing processes which can affect element distribution in components of Algol-type stars. Second, the C abundance determinations from theoretical models are compared with values known from observational analysis. We conclude that there is no significant difference in chemical composition in components between case AB and early case B mass transfer. We have found a real difference between systems which evolve from initial mass ratios of 10/4 and 10/9.

We present a new model to explain stellar mass distributions in different stellar environments. In our model, the protostar phase is terminated, when the protostellar core embedded in a molecular clump experiences a collision with another star or protostellar clump, which ejects the protostellar core from its parent clump. Such dynamical interactions are necessarily important, if stars preferentially form in dense clusters. We show that, in a simple model, the initial mass function approaches a simple, asymptotic form, which strongly resembles observed mass functions. The model has important consequences for star formation in different environments. We also discuss the implications of the model for our understanding of pre-main-sequence stellar evolution.

From both astronomical facts and theory it is inferred that the primary energy of main sequence stars most probably comes from gravitational work around a neutron star. The new mechanisms involved can in principle account for the observed facts, including those unconsistent with conventional models.

Flares are very frequent phenomena on the sun and on other stars and are actively investigated with precise observational techniques. These studies are particularly stimulated by recent X-ray and UV observations on spacecraft (Skylab, SMM). From all these observations the flares on the sun and stars seem to be of the same physical nature but of different energetic power. A number of investigations devoted to comparison between the solar and stellar flares led to the conclusion that they have the similar mechanisms for the energy release and transport and for their manifestations in different ranges of electromagnetic radiation.

The hydrogen emission is easy to observe in these events, and thus it is convenient for diagnostic analysis of physical conditions in the flares. However, the impulsive events were considered to be observed in X-rays so only. Zharkova & Kobylinsky (1992, 1993) have recently shown that the hydrogen continuous radiation of the solar flares is sensitive to the electron beam parameters. We also develop an approach to detection of the flares of non-thermal or thermal origin by observing the hydrogen Balmer and Pashen continuum emissions with high temporal resolution.

Based on a series of high dispersion Coude spectra taken with 188-cm telescope of Okayama Astrophysical Observatory over an extensive period of time (nearly 20 years), spectroscopic behaviours were measured and discussed. Synchronous variations of V/R (intensity ratio of violet to red emission peaksjvariability with the radial velocity curves (RV) for Balmer and Hel 3888 shell absorption lines were confirmed. Using the peak separations of the Balmer emission lines, envelope extension was also estimated. Main difficulty was found in explaining the envelope extension of the primary star of the system which overflows the Roche-lobe limit when we consider the binary nature of the system.

The problem of angular momentum loss in binaries and its influence on stellar evolution is considered. The results of 2D hydrodynamics computer simulation of mass transfer is presented.

The logic of naming stars could be based more on physical principles as happened in the naming of resonances in Particle Physics. A possible system might be based on extensive Monte Carlo studies of protostars followed by standard Stellar Evolutionary Model calculations.