We have investigated a large sample of faint Post-T-Tauri stars located far from dark cloud complexes, for far-infrared emission and infrared excess, using the IRAS data-base. ~12% of the stars studied here were detected by IRAS at 12 μm. Their 12 μm luminosities L12 are in the range of 4×1030 − 6×1033 erg/sec and correlate well with the stars, bolometric luminosities (Lbol).

IRAS has detected emission from the photospheres of the stars studied here and for a few of them it may have observed far-IR excess.

Extinction laws observed in spectra of Be stars are shown to be evidently different from the mean extinction curve. They do not contain in many cases the prominent extinction feature - the 2200A bump. The results are derived both from multicolour (ranging from the far-IR until the ultraviolet ANS) photometry and from TD-1 UV spectra. It is strongly suggested that the observed extinction phenomena are originated in circumstellar, disk-shaped shells as the shape of resultant extinction curve suggests both the presence of big, core-mantle grains and continuous infrared emission of circumstellar origin.

The observations of stellar flares in optics, radio and X-ray show the general similarity of the flares on UV Ceti-type stars and on the Sun. At the same time there exist a lot of data showing that the analogy between these processes is not full. From this point of view the analysis of the theoretical models of stellar flares is given.

We examine the response of the quiescent chromosphere to the large (E)UV and optical continuum and spectral line radiation field arising from a flare. We show that during a UV Ceti type flare, which displays a large U-band enhancement, a major part of the Balmer line flux may arise from the “quiescent chromosphere”, rather than the heated flare plasma itself. This leads us to distinguish two main phases in the Balmer lines, as first proposed by Houdebine et al. (1989): an early, mainly impulsive, phase, driven by radiative pumping of the quiescent chromosphere, which is mainly correlated with continuum variations, and a later one, related to the thermal flare phase, which arises from the cooling of the flare plasma itself. The effect of the radiative pumping is much larger for stellar (dMe) than for solar flares, due to substantial differences in the flare, relative to the quiescent, level and the quiescent chromospheric density and temperature.

The solution of steady-state equations for hydrogen is carried out. The diffuse radiation field was taken into account in these equations the value βs≡NRB - the escape probability of photons from the medium. This value is calculated for a layer of motionless plasma with finite optical depth. Our results are compared with analogical ones using Sobolev's method.

The Balmer decrements observed from flare stars during the flares and out-of-flare are analyzed on the basis of our new consideration of hydrogen atom kinetics. The steep Balmer decrements correspond to the emission source with electron density ne ≤ 1013 cm−3 optical depth at the Lα line centre τ(Lα) ≤ 107 for T=104, The low-sloping Balmer decrement, being typical for flare maximum of red dwarfs as well as for several flare stars with measurable X-ray emission out-of-flare, are described by ne = 3 · 1013–1014 cm−3 and τ(Lα)= 107 − 108 for T=(1−1, 5) · 104 K.

Four flares were observed on the late-type binary YY Gem in March 1988 during a total monitoring time of 408 min. The flares were unusual in that there is a periodicity in their occurrence, being separated by 48 ± 3 min. Considering the flares to be formed as a stochastic process, we find that the probability of these events occurring by chance is 0.5%. Modelling indicates that for quite reasonable input parameters (e.g. a spot field strength of 1000 G and a filament with mass per unit length of 106g cm-1), the flare periodicity can be explained in terms of filament oscillations. The only requirement is that there should be a filament at these heights where the magnetic field drops inversely proportional to the height.

The paper presents the basic ideas of pinch-model for UV Cet star flares. The observational consequences and predictions are discussed.

The principal possibility of γ-ray manifestation of stellar flares in the framework of pinch-model is shown. Validity of assumption that, γ-bursts are connected with magnetically active dwarfs (MAD) is discussed.

Ambartsumian's celebrated hypothesis that stellar flares and other phenomena of stellar instability are due to a novel source of energy and a novel means of transporting this energy to the outer layers of the star has drawn the attention of electrodynamicists to a number of fundamental problems. One of these problems, namely the energy transport problem, is the subject of this communication. Herein, by assuming that the matter of the star is an isotropic collisionless plasma, from Maxwell's field equations and Newton's equation of motion with nonlinear Lorentz driving force, we have derived a vector differential equation for electromagnetic wave propagation. This equation contains the Debye radius and the plasma frequency as parameters, and reduces to the well-known wave equation when its nonlinear terms are neglected. We have indicated that the nonlinear equation has cohesive (solitary) wave solutions for both the longitudinal and transverse components of the electromagnetic field. Such cohesive waves are appropriate for transporting energy from the prestellar core of the star to its outer layers since they hold their shape, are free from dispersive distortion, and can carry energy in discrete amounts.

Line profiles calculations were carried out for spherically-symmetrical stellar wind models with discrete structure. It is shown the possibility to explain two-component emission profiles typical for T Tauri-type stars in the framework of such stochastical approach.

The origin of dwarf or low mass stars is one of the most interesting and challenging problems of modern astrophysics. In recent years advances in observational technology particularly at infrared and millimeter wavelengths, have produced an avalanche of revealing new data, unexpected discoveries and new mysteries about the process of star formation. From this new knowledge a complete empirical picture of stellar origins is being synthesized and a more profound and penetrating understanding of the physical process of star formation in our galaxy is beginning to emerge. It is now apparent, for example, that energetic bipolar outflows are a fundamental aspect of the formation of low mass stars and understanding how a star can form by the act of ejecting mass may be the key to unlocking the secrets of stellar genesis.

X-rays turned out rather unexpectedly to be of fundamental importance in studying, and especially in discovering, pre-main sequence stars. The bulk of the data we now have comes from observations using the Einstein satellite, operating in the imaging mode between ~0.4 to ~4 keV (for reviews, see, e.g., Feigelson 1984, Feigelson, Giampapa, and Vrba 1990); because many of the detected sources turned out to suffer a relatively large absorption (Av up to several magnitudes, see below), EXOSAT, sensitive to softer X rays, unfortunately proved to be of little use.

The present review addresses selected questions on UV Cet-type red-dwarf stars primarily concerning the physics involved in the various aspects and phases of the flare phenomenon, rather than the average activity behaviour of the flare stars. In fact, while flare activity level and general trend are reasonably well established, a fully consistent physical picture of both solar and stellar flare events is still missing. Some recent results are presented with the aim of showing which observations are needed and the relevant role that coordinated multiband studies can play.

A number of years ago, the nobel-prize winning Chilean poet Pablo Neruda paid a visit to the Byurakan and wrote these comments in his Memoirs (1978:Penguin Books, New York, pg. 243.): “I shall never forget my visit to the astronomical observatory of Byurakan, where I saw the writing of stars for the first time. The trembling light of the stars was picked up; very fine mechanisms were taking down the palpitation of the stars in space, like an electrocardiogram of the sky. In those graphics I observed that each star has its own distinct way of writing, tremulous and fascinating, but unintelligible to the eyes of an earthbound poet” Unintelligible to the eyes of a poet, but as we have seen from this symposium, to the eyes of the astronomer, the phenomenon of flare stars is becoming more and more comprehensible. Before I came here to Byurakan I must confess that I (like Neruda) knew very little about the flare star phenomenon. As a result of this symposium I have learned much about these objects and now have a keen appreciation for their importance for studies of early stellar evolution. In this regard, I find it a particular priviledge and extremely appropriate to have a meeting on the subject of flare stars here at the Byurakan Observatory, where so much seminal work on this topic has been done. I have been impressed by the dedicated efforts of the astronomers at the Byurakan Observatory in flare star research. Their efforts represent an important contribution to galactic astronomy.