In this series of papers we report on an effort to deduce the morphology of the chromosphere and corona of AR Lacertae, an eclipsing RS CVn system (Hall 1976), via observations at UV, radio, and X-ray wavelengths during a complete orbital cycle. In another paper (Gibson, Walter and Basri 1982, Paper I), we present the observational data and general constraints on the morphology of the coronae. The full study will be published elsewhere (Walter, Gibson, and Basri 1982).

Ultraviolet and X-ray surveys of the W Ursae Majoris type stars are reviewed. These systems exhibit extended coronas and transition regions that are confined close to the optically determined surfaces. Correlations of X-ray activity with period or rotational velocity indicate a turn-over or saturation of emission at the short periods or high velocities found in the W UMa-type systems. For a number of systems, ultraviolet emission appears to be anti-correlated with the strength of X-ray emission. These observations are discussed in terms of solar structures, activity, and evolution.

Among the RS CVn stars showing solar-type activity, with spectral types ranging from F to K and total masses up to 4 M⊙, there are two peculiar groups with period less than one day:a)agroup with components well inside their Roche lobes (Short-Period-Group, hereinafter SPG) and b) a group with their components in a thin or marginal degree of contact, with lightcurves of W UMa-W type (hereinafter WWG).

Angular momentum loss (AML) may be introduced as part of the explanation of the peculiar overabundance of W UMa-type contact binaries (W UMS) (for a recent review, see Vilhu, 1981). Among the various possible mechanisms to achieve this are (i) overflow through the outer (L2) Lagrangian point in a deep contact phase (Kuiper, 1941; Nariai, 1979), (ii) magnetic braking, where magnetic lines of force “stiffen” and thus enhance the efficiency of angular momentum loss associated with a stellar wind (Huang, 1966; Mestel, 1968). Such subjects have been investigated in a number of more recent detailed studies.

We discuss observational results for indicators of elevated activity of contact (W UMa-type) binaries. References are made to results for fast rotating single and detached binary (MS) stars: they are discussed in the accompanying paper by Vilhu and Rucinski.

We have studied transition-region (NV + SilV + CIV) and chromospheric (MgII) emission observed with IUE (Vilhu and Rucinski, 1982; Rucinski and Vilhu, 1982; Ayres et al., 1981) together with coronal (soft X-ray) emission observed with the Einstein satellite (Cruddace and Dupree, 1982; Walter, 1982) for contact (W UMa) and detached main sequence binaries. The components are main sequence stars (or near ms) and rapid rotators due to spin-orbit coupling. They can thus be expected to give information of dynamo-processes in rapid rotators, although the binary effects clearly produce extra complications in this discussion.

It is well known that the original Lucy CCE model (Lucy, 1968a, 1968b) was unable to represent W-subclass light curves of W Ursae Majoris stars. Lucy's zero order barotropic model, with gray atmosphere limb darkening produces a deeper transit eclipse and zero color variation with phase. These predictions disagree with observation. Changing the limb darkening coefficient alone, within acceptable limits, cannot deepen occultation eclipse enough to make it deeper than the transit eclipse, as observations require.

Almost twenty years ago a large flare event was observed on the prototype contact binary W UMa by Kuhi (1964). Similar events have been reported on 44 i Boo (Eggen 1948) and U Peg (Huruhata 1952) . In this paper we present photoelectric observations at three wavelengths of a flare on VW Cep. This is the first event of this kind to be reported for this star. VW Cep is a triple system. The main contributor to the visual flux is the eclipsing binary, consisting of a Kl primary and a G6 secondary (Kopal 1978), classified to be in contact. The orbital period is 6h 41m. Seven per cent of the total flux in the visual filter is due to the third component, a late K type dwarf at a distance of 12 AU from the eclipsing system (Hershey 1975).

The WUMa contact binary VW Cep has a period of 0.2783 days, spectral type G5+K1 and mass 1.1+0.4. Mʘ. Our three-colour photoelectric measurements made in Sept. and Oct. 1964 confirmed the existence of the Kwee effect in this system. We believe that if the Kwee effect is to be explained in terms of moving gas streams the influence of magnetic fields must be considered. It is assumed that there is a deep convection zone on the primary accompanied by a dipole magnetic field. This field is distorted by the moving gas streams to form a ring-like configuration with an enhanced field strength close to the inner Lagrangian point, L1 (see figure l), where large groups of spots form. We suppose the magnetic field in the spots to be 5000G and the general field to be ~10–20 G.

Comparisons of the activities of UV Cet-type flare stars and of T Tau-type stars with solar activity permits the conclusion that non-sta-tionary processes in the UV Cet-type stars and in the Sun are of an indent ical physical nature but that they differ qualitatively from active events in the T Tau-type stars. The identity of the activity in flare stars and the Sun makes it possible to study successfully stellar activity with the help of known models of various solar events and, on the other hand, to have a more general approach to the physics and evolution of solar activity on the basis of established features of numerous flare stars of different ages and masses. The hypothesis on hydromagnetic activity of the T Tau-type stars is sketched; within this framework, one supposes that the main feature of such stars is an occurrence at every point of the stellar surface, of conditions necessary for the existence of dark spots, i.e. a lowering of the photospheric brightness due to strong local magnetic fields.

It is demonstrated that the light variations of DI Cephei can be explained as due to the combined effect of variable completely grey circumstellar extinction and variable continuous and line emission.

At the Van Vleck Observatory of Wesleyan University, Middletown, Connecticut, we have initiated a program of monitoring T Tauri stars with the 24-inch reflector (Perkin Telescope). During the first observing season, we have obtained UBVRI and Hα (30Å and 150Å) data for five stars: T Tau, RY Tau, SU Aur, RW Aur, and CO Ori. The data will be presented elsewhere (Herbst, Holtzman and Phelps, 1982). Here we re-port some of our conclusions.

While the T Tauri stars are the best known of the late-type pre-main-sequence (PMS) stars, there are also some late-type PMS stars with only weak line emission in their visible spectra. Several years ago we noted that the weak-emission PMS stars have B-V colors too blue for their V-I colors and suggested that their surfaces might have regions of differing temperature. During October 1981 we used the USNO 40-inch and Kitt Peak National Observatory No. 4 16-inch telescopes to monitor, over a 7 night interval with UBVRI photometry, four of these weak-emission PMS stars: HD 283447, V410 Tau, and X-ray stars 1 and 2 of Feigelson and Kriss (1981). The PMS nature of these stars is established from (1) their membership in the Taurus dark cloud T-association and (2) their location within the T Tauri band region of the H-R diagram.

We have recently reported the discovery of a cool (650–800 K) low-luminosity companion to the pre-main-sequence star, T Tauri (Dyck et al. 1982). We proposed that the optical star and its infrared companion form a physical pair with a N-S separation of 100 a.u. However, there remained in our 2-5 μm speckle interferometry an ambiguity of 180° in the position angle of the secondary. In addition, Cohen et al. (1982) noted an 800 milliarcsec (mas) offset between the visual and 6 cm radio positions at T Tau. Both of these positional discrepancies have now been clarified by accurate visual and radio astrometry of T Tau, and by further near-IR speckle interferometry.

IUE spectra of six pre-main-sequence (PMS3 stars are analysed and the resultant emission measure distributions compared with that of T Tau for which a chromospheric model has been calculated. The general shape and absolute level of the mean emission measure distributions are remarkably similar, indicating the relevance of the T Tau chromospheric model to other PMS stars. Evidence for the influence of large scale motions and/or stellar winds on the transition region and coronal emission measures is found. The relative importance of different energy balance terms is discussed.

RW Aur is one of the most active T Tau-type stars with essentially variable emission line spectrum. In order to study the nature of this variability,patrol spectroscopic observations of the star were carried outfrom November 1979 to March 1982 at the 2.6-meter telescope of the Crimean Astrophysical Observatory. The spectral region λλ38OO-67OO A has been observed with a dispersion of about 40 A/mm. All nights were supplied with simultaneous spectrophotometry of the star.

The correlation between the rapid rise of intrinsic polarization, increasing U-B and enhanced equivalent width of Balmer, H and K emission lines is interpreted as a consequence of the activation of colour centres in circumstellar silicate dust by X/UV photons from flares. The temperature dependence of bleaching is discussed and the number of centres producing polarization changes is estimated.

The interaction of energetic particles with frozen gas layers plays a relevant role in a variety of astrophysical scenarii and also in T Tau nebulae because of the large fluxes of particles ejected by these stars during their continuous flaring activity and the observed presence of ice grains at least around the very young star KL Tau (Cohen, 1975).

Narrow-band spectrophotometry observations (Cohen, 1975) showed that the extremely young star HL Tau has an absorption feature at 3.1 μm well matched by extinction from pure ice solid grains (radius a≈0.3μm, mice ≈10-4 gr cm-2). This feature, not present in others (more evolved) T Tau stars, suggests that ice may be only found around the very youngest T Tau stars and progressively disappears as starsbecome increasingly visible through their circumstellarshells (Cohen, 1975).

MHD effects in stars are seen in their most spectacular form in the processes which are typical of flares. At first sight, it appears that the phenomena of dark spots (whose long lifetimes give an impression of quasi-equilibrium) are inevitably less interesting. However, this is not necessarily true. Laboratory experiments in recent years have shown that there are many more ways to drive a plasma out of equilibrium than to preserve equilibrium. In that sense, then, it is perhaps “easier to understand” why flares should occur in a stellar atmosphere (where convective jostling of field lines creates potential for driving a large number of instabilities) than why a long-lived feature such as a dark spot should persist. Various instabilities which may contribute to flares are discussed by Priest and Spicer (this volume). Here, we summarize work on the equilibrium structure of cool spots in the sun and stars. Since spots involve complex interactions between convective flows and magnetic fields, we need to refer to observations for help in identifying the dominant processes which should enter into the modelling. This summary therefore begins by discussing certain relevant properties of spots in the solar atmosphere.