In the last decade stars showing variability of unclear origin have been detected among early F-type stars mainly as secondary results in the context of observational programmes devoted to the study of δ-Scuti and CP stars.

For this reason only a small number of these objects has been thoroughly studied: their positions in HR diagram is shown in fig.l.

Figure 1: HR diagram of the observed stars.

The stars in NGC 2516 have been singled out in a survey work by Antonello and Mantegazza (1986), HD 164615 has been studied by Abt et al.(1983), the variability in 9 Aur has been discovered by Krischinas et al.(1990,1991) and HD 23375 in Pleiades has been pointed out by Breger (1972). The solid line in Fig. 1 is the ZAMS and the dashed lines are the borders of the i-Scuti stars instability strip.

For active late-type dwarfs, the total energy losses including both chromospheric and coronal radiation as well as the energy losses caused by stellar flares reach 3·10−3 L001. To maintain such high energetic losses requires a high efficiency of the dynamo action in the convection zone. Mass loss by active late-type stars can probably approach the rate of 10−11 M⊙/yr if we take into account the large-scale structure of their coronae and transient events. The phase of activity can continue for up to 1% of the lifetime of these stars on the main sequence due to braking by such a stellar wind.

High-resolution (0.145A/diode, high signal-to-noise (> 100) Reticon spectra of 57 RS CVn systems, which were observed with the coudé spectrographs of the McDonald 2.1m telescope and Yunnan lm telescope, were used to analyse a correlation between Li abundance and chromospere activity. Li abundances of 57 RS CVn stars, which include 76 detected components have been determined through the determination of equivalent widths of Li doublet (6707.761A + 6707.912A) and Kuruz’s model atmosphere. The model metal abundance and line analysis of 10 RS CVn stars were determined from the present spectral lines data. The main results are the following: 1, Lithium abunances of 76 RS CVn components for 57 RS CVn systems are obviously reduced with the the effective temperatures gradually decreasing. 2, Li abundance has staistically decreasing trend in RS CVn stars with the rotational velocities vsini increasing. 2, The position of 168 RS CVn stars in the (U-B, B-V) Figure show that they can be devided into two groups. The most of them seem to be not poor-metal stars. Li abundances of these stars seem to have no obious correlations with their stellar chromosphery activity levels, but 7 RS CVn systems, in which cool component is more active show that the more active component has a lower Li abundance.

The Rosat Observatory was launched on June 1st, 1990, and after a brief Performance Verification phase, performed an almost complete sky survey. One of the two main telescopes on the Observatory is the UK Wide Field Camera (WFC). The WFC is the first extreme ultraviolet (EUV) instrument to survey the whole sky, and during the survey detected several hundred active late-type stars.

The WFC (Wells et al., 1990) was built by a consortium of five UK institutes, four Universities and the Rutherford Appleton Laboratory. During the all-sky survey, two filters were used. These allow broad-band photometry to be performed. The SIA filter (lexan/carbon/boron) and S2A filter (lexan/beryllium) were alternated on a one day basis. The SIA filter covers the energy range 0.085-0.180 KeV, centred on 0.124 KeV, while the softer S2A filter covers the range 0.070-0.110 KeV (centred on 0.090 KeV) (10% efficiency) [see Kent et al. (1990) for a full description of the WFC filters].

Many scientific programs, most of them linked to stellar physics (such as asteroseismology, stellar rotational modulation, surface structures, Doppler imaging, Zeeman-Doppler imaging, variable stellar winds) require a continuous spectroscopic coverage during several days.

MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international project for setting up a network of high resolution spectrometers coupled to telescopes of the 2m class, well distributed around the world, and partly dedicated to continuous spectroscopy.

The strategy to reach this objective was defined during two workshops organized at Paris-Meudon Observatory in 1988 and 1990, and consists of three steps: 1) organize multi-site spectroscopie campaigns using resident instruments on various telescopes around the world and transportable fiber-fed spectrographs where adequate spectroscopie equipment is not available; 2) design and develop a cross-dispersed echelle spectrograph, well suited for the scientific programs that require multi-site observations; 3) propose this MUSICOS spectrograph for duplication at several collaborating sites.

The Herbig Ae stars are PMS objects of intermediate mass. Their location in the H-R diagram indicates that they are in the radiative phase of their quasi-static contraction toward the main sequence, i.e. that they do not possess outer convective zones, according to the standard stellar evolution theory (Iben, 1965; Gilliland, 1986). In spite of the expected absence of subphotospheric convective envelopes, these stars show remarkable signs of activity: emission in the Mg II h and k lines, presence of the CIV resonance lines at 1550 A and He I 5875.7 A line, Ca II IR triplet in emission, etc... Considering that stellar activity, witnessed by the same type of indicators in other parts of the H-R diagram, is generally attributed to dynamo magnetic fields and/or acoustic waves generated in the convection zone, these active phenomena are quite paradoxical in the Herbig Ae stars.

The main question concerns the origin of their activity: is this activity linked to phenomena occurring within the stars, like e.g. dynamo-generated magnetic fields, or to an external agent, like e.g. a boundary layer between an accretion disk and the stellar surface? We already have some indirect clues that the activity of the Herbig Ae stars might be of magnetic origin (Praderie et al., 1986; Catala et al. 1986) observed a rotational modulation of lines formed in the wind of AB Aur, prototype of the Herbig Ae stars. By analogy with the solar wind, they proposed that the modulation might be due to the corotation of azimuthal structures in the wind, controlled by a surface magnetic field.

Young stars, as a rule, are too faint for measurements of magnetic field either by photographic method with the use of Zeeman analizer, or photoelectrically from hydrogen lines. That is why it is necessary to look for indirect ways of magnetic field detection, for example, by measurement of polarization. Ae/Be Herbig stars without a magnetic field are surrounded by a gaseous envelope in the form of a globe or a spheroid, flattened along the rotational axes (as dependent on stellar rotation velocity), and also by a gaseous-dust accretion disc in the plane of equator. There are powerful flows in gaseous envelopes of stars, connected with mass loss and accretion. If a star is a magnetic oblique rotator (as a magnetic star of the Main Sequence), then the gaseous envelope may acquire the shape of alon-gated ellipsoid with the major axes coincident with that of dipole (Dolginov et al., 1979). From the poles there arises a jet flow controlled by a magnetic field, as in He-r and He-w stars, having already reached the Main Sequence (Barker et al., 1982). Calculations show (Dolginov et al., 1979), that maximum polarization in the extended envelope p ≈ 4% arises when the ratio of ellipsoid axes is ≈ 2.5b. The electric vector of the dominating oscillation of the light wave is perpendicular to the plane through the axis of symmetry of the ellipsoid and the line of sight. Naturally, the magnetosphere rotates together with the star, involving the gaseous envelope, resulting in the variation of the degree and direction of polarization. Additional polarization is created by the polar jets, where the direction of the dominating oscillations of the electric vector is perpendicular to the axis of the polar stream, and value of maximal polarization may reach 5% along the beam.

Our understanding of Solar structure has increased dramatically in the last couple of decades thanks mainly to the opening of new windows of observation providing high quality data to theoreticians with access to powerful computing facilities. Two of the new windows were UV and X-ray images of the Sun, allowing a detailed view of the upper solar atmosphere, and the development of very high resolution spectrometers allowing us to exploit the solar oscillations to probe the internal structure of the Sun. It is the goal of PRISMA to extend these techniques to other stars, which using the Sun as a calibration point will allow us to explore stellar structure and evolution in ways not possible now.

In this poster I will present a possible selection of instruments able to achieve this goal, and explain some of the rationale in their design. A more general overview is presented by T. Appouchaux also in these proceedings. It must be stressed that these are not the definitive instruments to be flown on PRISMA, but rather result from a study to show the feasibility of such a mission. Should PRISMA be chosen as the next ESA medium sized mission, an ‘Announcement of Opportunity’ wiH be issued by ESA and the responses of all people interested in constructing the instrument will be considered.

Recently it has beeen suggested that the latitude distribution of the main surface features of solar activity is intimately related to the angular velocity profile inside the Sun through the working of a MHD dynamo in the boundary layer between the convective and the radiative zones (Belvedere et al. 1991).

Although the present observational capabilities are not very encouraging, here we want to point out, in the framework of the analogy to the Sun (solar-stellar connection), that space observations of surface distribution and latitudinal migration of active regions on stellar surfaces, which could be carried out in this decade with more sophisticated techniques, may conversely allow us to infer the rotation profile, and consequently the angular momentum distribution, in stellar interiors. This methodology may in principle be considered alternate or complementary to the classical one based on observation of acoustic oscillations.

In stars which pulsate in more than one mode, be it acoustic or gravity modes, radial or non-radial, we have a chance to study the interior structure by use of seismic techniques. Generally the greater the number of modes observable the greater the chances will be to obtain valuable direct information about the stellar interior not accessible by any other technique.

The observational problem of acquiring high quality data divides the stars where seismic techniques can be applied into two groups: A group of stars with intensity amplitudes in the range of millimagnitudes and the stars with very small amplitudes around 10 μmagnitudes or less. The first group contains the white dwarfs, the rapidly oscillating Ap stars, the δ-Scuti stars and the β-Cephei stars. The modes are excited directly or indirectly by some form of к-mechanism due to an opacity source which vary from class to class. The second group is made up of stars with outer convective envelopes which generate acoustic noise that may excite resonant modes, often a very large number.

This paper is a discussion of the possibilities for observing these stars with ground based techniques, and also a statement about what can not be done from the ground.

The capability of ground-based photometry to detect and monitor low-level stellar variations is discussed in the particular case of amplitudes in the range of few magnitudes to few percent. The technical and atmospherical constraints are briefly recalled. The variability level of the so-called constant stars is discussed as function of the spectral type, the luminosity class and the effective temperature. The photometric aspects of the Hipparcos mission are presented here since this satellite allows a systematic detection of microvariables and the obtention of light curves across the whole HR-diagram with an unprecedented efficiency.

We will discuss the opportunities opened by space experiments to make measurements that cannot - or only marginally - be made from ground.

The long uninterrupted observation of the Sun as a star with the IPHIR experiment can be used to set limits to the significance of the determination of p-mode parameters as a function of time and the length of the time series of measurements.

Detection of individual modes of oscillation near 5 minutes period is proposed, based on the comb-like signature which is similar to that found in the sun. The mean peak amplitudes (ap) of the strongest individual modes are 75 ≤ ap ≤ 120 cm/s, and the mean separation between alternate modes is 55.3 ± 3.6 μHz. Apparent modal lifetimes are on the order of 2 days, but we suggest this may be caused by interference of adjacent modes.

Recently three independent groups(Cox et al. 1992; Kiriakidis et al. 1992; Moskalik and Dziembowski, 1992), using opacity tables published by Iglesias and Rogers (1991), demonstrated that β Cep star models are pulsationally unstable. The instability is driven by the classical к- mechanism acting in the layer with temperatures near 2 × 105K where there is a bump in metal opacity. The groups reported results of calculations made for rather narrow ranges of stellar parameters and oscillation modes. We conducted an extensive search for unstable modes in complete evolutionary models of B-type stars of luminosity classes III - V. Our aim was to determine the domain of instability and examine its role not only in β Cep stars but also in variable stars located in the nearby areas of the H-R diagram.

An unexpected new aspect of our calculations is the use of the improved opacity data. In a very recent work Iglesias, Rogers and Wilson (1992) showed that effects of spin-orbit interactions significantly enhance opacity in the critical region for driving the pulsations in β Cep stars. These effects and improved information about the solar metal mixture have been included in the updated opacity tables kindly provided to us via electronic mail by Dr. Rogers. The consequences of this change in opacity for stability of B-type star models are indeed quite important. Contrary to previously announced results, that only the fundamental mode is unstable, we now find the first two radial overtones to be unstable, as well. Thus, the discrepancy between the theoretical prediction and the mode identification suggested by observers has been removed. Furthermore, no longer is a high metal abundance (Z > 0.03) required to explain the occurrence of pulsation in most of the objects. In fact, the theoretical instability domain in the H-R diagram, based on the models calculated with Z = 0.02, agrees better with the observational β Cep domain than that based on the models Z = 0.03.

Period analysis of Be stars 28 Cyg and EW Lac has shown that their photometric variations are multiperiodic. Dominant periods have counterpart in low-order NRP modes identified in the independent line profile studies.

The search for pulsational variables that occupy the hotter realms of the instability strip in the Hertzsprung-Russel diagram (HRD) have been traditionally restricted to the regions where models indicated that pulsation is tractable. Recent surveys of stars have revealed, however, that stars that were thought not to lie in the instability strip do indeed pulsate. This raises questions about the extent of the instability strip.

While this has implications for all hot pulsating stars, we use here primarily our survey results of Ap stars to compare with the models. We report results here from a high-speed photometric survey of 120 Ap stars that was conducted between 1985 and 1991 to search for rapid variability. The absence of pulsations in the hotter Ap stars (roughly B8-A3) is noted and deemed significant on the basis of the number of stars observed in this temperature range, as well as the overall quality of the data. This, however, does not preclude their existence, especially since HD 218495 was recently discovered to be a rapidly oscillating Ap (roAp) star (Martinez, Kurtz and Kaufmann 1991), and has a spectral type of about A3.

Color-magnitude diagrams of the survey stars are presented, with the known roAp stars included for reference. The diagrams are presented in the Strömgren and Geneva systems. The color-magnitude diagrams demonstrate the completeness of the survey in covering Ap stars at a wide range of temperatures. We find no obvious means of using color indices to differentiate roAp stars from non-pulsating Ap stars.

The observations were carried out in August, 1983 using the 75 cm telescope at Sierra Nevada Observatory in Spain. This telescope is equipped with a six-channel uvbyβ photometer for simultaneous measurements in uvby and the narrow and wide Hβ channels, respectively. The observations were collected during four nights in the four uvby filters and one more night was devoted to measuring β Cas in the n and w bands of the Hβ Crawford system. In all cases, a calibrated neutral filter was used for β Cas. The comparison star was Cl=HR 9085 (V=5.m55, F0III).

The analysis of frequencies was carried out using the Discrete Fourier Transform method, as described in López de Coca et al. (1984), to the observational uvby points collected. The periodograms showed a principal peak at v=9.91 cd−l. After prewhitening for this frequency, the resulting periodograms did not show any trace of another peak, suggesting the monoperiodic nature of this star, in agreement with Antonello et al. (1986). Under the assumption of monoperiodicity of β Cas, the classical O-C method can easily be applied. Nine times of maxima were obtained for the four nights by using the method described in Rodriguez et al. (1990), where each light maximum has been derived as an average over the four uvby bands. When the O-C method was applied, the least squares fit converged to a linear ephemeris with the following elements: To=2445568.d5097 (±0.d0007) and Po=0.d10091 (±0.d00002). This linear ephemeris satisfactorily reproduces our data of β Cas.

At Merate Observatory the study of δ Sct stars began in the sixties and spectroscopie and photometric campaigns were continuously undertaken in order to clarify the controversial points. As an obvious extension of the research, the observation of δ Sct stars was proposed for telescope-time allocation at European Southern Observatory, in order to take advantage of the ESO facilities and of the considerably better sky of La Silla. We put in our observing programme δ Sct stars showing cycle-to-cycle variations and, possibly, an amplitude larger than 0.05 mag in order to have a better signal-to-noise ratio; they are listed in the table. It is important to notice that we always tried to collect a number of measurements as large as possible (from 1988 we collected more than 1000 measurements per star), because it is a well-established fact that periodicities with

an amplitude of a few thousandths of magnitude can be present in δ Sct stars. Moreover, when several modes are excited, an adequate frequency resolution becomes necessary and, therefore, a sufficient time baseline is requested (Poretti and Mantegazza 1992). A poor data sampling also gives an apparent good fitting for several solutions, all leaving a small residual rms, and there is no possibility to choose among them: different authors can pick up different solutions, generating the conflicting interpretations often found in δ Sct star literature.

Constructing a theoretical model of 1.88 M⊙ pulsation properties of 63 Her have been analyzed. For the model, In question, pulsation periods in the fundamental mode and first two overtones have been derived by solving the linearized pulsation equations. The model has been found to be vibrationally stable.