We perform a model atmosphere analysis of ξ Boo A, and make estimates for the secondary component. The abundances of 25 elements for ξ Boo A are obtained with the parameters: Teff=(5300 ± 100) K, log g= 4.1 ± 0.2, ξt = 0.5km/s. The atmosphere of ξ Boo A is deficient in Y and Zr, but overabundant in Ba and rare earth elements. We confirm the high Li abundance in ξBoo A (logA(Li) = 2.1), while Li is strongly underabundant in the secondary relative to the Sun. From comparison with evolutionary calculations we find that ξ Boo A has mass 0.95 M⊙, radius 1.4 R⊙ we also determine a rotational velocity v sin i = 8 km/s. We conclude that, in spite of its relatively high rotation rate and high chromospheric activity, it is already evolved from the main-sequence.

We have calculated synthetic spectra for HR 1099 for the wavelength region λ6425-6445 Å, which includes the Cal λ6439 Å and FeI λ6430 Å lines, often used for the surface imaging of rapidly rotating late-type stars. The best agreement between the theoretical and observed spectra can be achieved with solar abundances in the both components of the system, except for Ca in the primary. Theoretical calculations give the possibility of estimating the influence of microturbulence, damping and blending on Ca I and Fe I line profiles.

I will summarize some of the principal scientific results presented at the Conference on “Mechanisms of Chromospheric and Coronal Heating” held on 5-8 June 1990 in Heidelberg, Germany. The Conference included invited and contributed papers on observations that point to specific heating mechanisms and theoretical papers on the heating mechanisms themselves. There were many opportunities for useful interaction between proponents of these two approaches to understanding stellar chromospheres and coronae. I will concentrate on what is being learned from the empirical side, and then summarize the heating mechanisms discussed and for which types of stars they may be applicable.

We have monitored Ca II H with a spectral resolution of 8 × 104, in asimple of late-type MS stars over a four-year period. The high resolution enables us to add information on velocity fields to the usual flux monitoring. We detect changes in wavelength of different parts of the Ca II H feature, which can be interpreted as velocity fields in the lower chromosphere, with downflow and upflow of order 0.5 km s−1, depending on the star. Flux variations in Ca II H emission can be ascribed, via velocity tagging, to long-term change in plage cover rather than short-term modulation by (incompletely sampled) rotation cycles.

Here we present preliminary results on the study of the chromosphere of Altair from Lα high dispersion profiles. We report also the detection of Lα chromospheric emission from α Cep (A7V) and β TrA (F0 V) with the IUE. We show that chromosphere may exist up to B – V = 0.22. The possible heating mechanism, magnetic or nonmagnetic, for the chromosphere of these stars is also discussed.

Using high resolution spectroscopy of the T Tau star RY Tau in the regions of Hα and Na D lines, we discovered line profile variability, which is interpreted by a hypothesis of stellar ’prominences’: cool gas clouds are moving inside the stellar wind, ascending and descending on a time scale of a few days. These clouds are supposed to be controlled by the magnetic field of the star. The range of radial velocities of the clouds (−100 to +100 km/s) is twice as large as the rotational velocity (v sin i) of the star. The form of variability of the Hα profile suggests the existence of two components: the (variable) stellar wind and the circumstellar disc.

We have measured the He I D3 line, λ5876 Å, in 32 F-type young open cluster stars and in 4 field stars. Combining our results with previous observations of D3 we strengthen the following conclusions: (a) Sub-surface convective zones appear, on the main sequence, around F0 (B – V ~ 0.3). (b) The stars between F0 and F5 (0.3 ≲ B – V ≲ 0.42) are chromospherically “active” irrespective of age, which implies a heating mechanism not affected by evolutionary effects. In this range the activity in D3 and X-rays are uncorrelated. (c) For stars later than F5 the correlation among all the activity indices, chromospheric and coronal, shows a common energy source. Convective zones here should be deep enough to generate magnetic fields capable of supplying this energy, and also for braking the rotation.

It has been widely demonstrated that chromospheric emission is dependent on rotation rate (Hartmann and Noyes, 1987, Catalano, 1990, for recent reviews). Since the rotation decays with time, the chromospheric activity of single main sequence stars is also dependent on age. Catalano and Marilli (1983) suggested that the chromospheric emission of solar mass stars decays exponentially with the square root of the time, as recently confirmed by Barry (1988). Magnetic activity depends also on the convection parameters, i.e. on the mass. Since the evolution of surface rotation is faster for lower mass stars (Catalano et al., 1988), the internal rotation distribution, and therefore the dynamo action, should change with time in a different way for different masses.

In order to get insight into the evolution of dynamo action we have studied the CaII chromospheric emission as a function of mass and age. Here we present some preliminary results of observations of late type stars in young clusters and old moving groups.

ER Vul (HD 200391) is a double-lined spectroscopic binary, at a distance of 45 pc, catalogued as a short-period RS CVn binary, and has been found to present eclipses. The components have spectral types G0V/G5V, masses 1.07/0.98 M⊙, radii 1.23/1.23 R⊙ and Mv 4.8/4.6m (Strassmeier et al., 1988, and references therein). The system presents emission reversals of the H and K absorption lines (Bond, 1970; Eggen, 1978), but no information on the character of these emissions has been previously reported.

Changes of emission cause variations of the Hα absorption line of HD 199178. The correlation with the simultaneous light curve may depend on the relative strength of the axisymmetric and non-axisymmetric components of the evolving spot configurations.

The Hα line of hydrogen is a very powerful tool for the study of the solar chromosphere (plages, flares, prominences). Although it is difficult to derive in a simple way quantitative information, like chromospheric temperature and electron density, because of the very extensive depth of its formation and the very complex mechanism of excitation, Hα has been successfully used to study the chromospheres of active stars (Herbig, 1985). Moreover this diagnostic has proved to be very useful for the study of surface inhomogeneities and transient phenomena like flares in systems of the RS CVn type and other active stars (Bopp, 1981; Bopp et al., 1988; Fraquelli, 1982).

With the aim of investigating the behaviour of chromospheric activity, we have undertaken an extensive monitoring of Hα emission in a selected group of RS CVn binaries (Strassmeier et al., 1988). We report here preliminary results of the observations obtained in 1988–1989.

Hα and HeI D3 lines were observed in five active cool stars over their relatively short rotation periods. Three of the stars (AB Dor, Rstl37B, HD82558) are in the pre main sequence phase (as deduced from their lithium abundances), while two are post main sequence FK Comae type stars (HD32918 and BD-223467) (Vilhu et al., 1991). We present the results of these observations and point out the physical similarity of the Hα and He D3 transitions (Fig.1).

We have obtained 21 ~one-hour observations of the prototypical spotted star, BY Dra, with the VLA. These data are sufficiently well spaced in time to allow for complete rotational and orbital phase coverage; the resultant radio light curves are compared to their optical counterpart. We also plot the emission versus time without phase-folding, in order to search for longer periods of activity. Splitting the data into smaller (three minute) time bins allows for the determination of the normalized amplitude distribution of the flux; we compare this with known stellar optical and X-ray flare distributions.

Algol-type binaries are basically known to undergo hydrodynamic processes related to mass exchange between components. Recent observations on radio, X-ray emission and flare-like events have raised the question of possible magnetic activity in the secondary component of these systems (Hall, 1989).

From a microwave emission survey we have shown that the radio emission from Algol systems cannot be accounted for by thermal emission from an hot corona (T ≥ 107K) and that their radio luminosities compare very well with those of the magnetically active RS CVn systems (Umana et al., 1990).

An analysis of the X-ray emission of a large sample of single MS stars shows that X-ray luminosity is strongly correlated with the photospheric radius (Fleming et al., 1989) which implies the dependence of activity on bolometric luminosity Lbol, effective temperature Teff, and thus stellar mass M (Micela et al., 1985; Bookbinder et al., 1986). The role of the emission area on the coronal activity of RS CVn systems has been noted independently by Majer et al. (1986), and Demircan (1986).

By using new data on some well known RS CVn systems, we produce new evidence that above idea is indeed valid. A plot of X-ray luminosity Lx versus (B – V) for the sample of well known RS CVn systems in Fig. 1 shows that cooler, long period, more evolved systems with larger surface area have stronger X-ray emission.

A theoretical interpretation is offered for the power-law dependence of the flare occurrence rate in red flare dwarfs. Flares on red dwarfs are regarded as being physically akin to solar flares, with the energy source being the magnetic field. A power law flare energy relation then emerges naturally. Model estimates of the corresponding spectral index β fit the observed values in most flare stars. Within the bounds of the interpretation offered the energy dependence of a flare event is determined by the character of the turbulence in the convection zone. The observed parameters of the dependence can be used to diagnose the character of the turbulence in the convection zones of flare stars.-

Dynamo theory can in principle be tested by comparing its predictions with a variety of types of solar and stellar observations. The progress apparent from papers presented at this Colloquium is summarized, and areas where further observations would be of particular value are stressed.