Ideally one would like to understand the origin of coronal emission in the perspective of stellar evolution, including the role of convection, magnetic field, and angular momentum. Everyone agrees that all this has something to do with coronal emission, but to prove what mechanisms are operating in detail is a different matter.

The study of stellar X-ray coronae started a dozen years ago with the launch of the Einstein Observatory (Vaiana et al. 1981). From the beginning it emerged a richness of data, not anticipated in the original mission plans, involving the majority of stars and ages. Since then, the major developments of stellar coronal physics based on data from Einstein and EXOSAT have been reviewed (cf. for example Rosner, Golub & Vaiana 1985; Pallavicini 1989 and reference therein). ROSAT will certainly enrich the present scenario: the improvement in coverage will eliminate some of the present biases, and the improvement in spectral resolution will contribute to a better classification of X-ray emission. Presumably, only the substantial increment in sensitivity attainable with the AXAF and XMM missions will allow other major surprises.

T Tauri stars (TTS) are low-mass (M ≲ 1M⊙) pre-main sequence (PMS) stars (for a general review, see Bertout 1989). They have long been known to be variable from near-TIV to near-IR wavelengths, on timescales ranging from a few minutes to a few decades. They are observed to flare in many wavenlength rages, from X-rays to the radio, and all the existing evidence is consistent with a very strong magnetic activity, in many ways analogous to solar activity (for a review, see, e.g., Montmerle et al. 1991).

At the time of writing (i.e., September 1991), the ROSAT all-sky survey has been completed and almost the entire sky has been scanned with an imaging X-ray telescope down to a limiting flux of approximately 2 10−13erg/s/cm2 in the pass band 0.1 — 2.0 keV; in regions of deeper exposures near the poles of the ecliptic considerably fainter flux limits have been achieved. While the processing and analysis of this huge body of data is still in progress and hence final results on the number of detected sources and their distribution in flux are not yet available, the total number of detected X-ray sources will be around 60 000. Preliminary results from optical identifications of selected fields show that about one quarter of the X-ray sources discovered at high galactic latitudes come from by comparison nearby stellar sources (Fleming 1991), while at lower galactic latitudes up to one half of the detected X-ray sources are of stellar origin; in areas occupied by star forming regions (for example, Orion) or open clusters (for example, Hyades or Pleiades) a large number of the detected X-ray sources can be identified with young stars, yielding up to 80 percent of the total source count as galactic stars. For the whole of the ROSAT all-sky survey we may therefore expect about one third of the total sources to be of stellar origin. The vast majority of these stellar X-ray sources is of coronal origin (i.e., late-type low mass stars). Only a relatively small number of stellar X-ray sources will be associated with early-type massive stars where the X-ray emission is thought to arise from instabilities in their radiatively driven winds or metal-poor degenerate stars where the X-ray emission comes from portions of the atmosphere considerably hotter than the optically visible photosphere. From the preliminary analyses performed so far it is already clear now that supersoft sources such as white dwarfs do not constitute a major fraction of the X-ray source population found in the ROSAT all-sky survey and the number of newly X-ray discovered white dwarfs will certainly be considerably less than one thousand. The X-ray emitting late-type stars are commonly referred to as ”active” stars, and the ROSAT all-sky survey catalog will comprise the most extensive list of such objects.

Although coronae for stars other than the Sun have previously been detected only in the X-ray and radio portions of the spectrum, the HST and future spacecraft sensitive to ultraviolet (UV) and extreme ultraviolet (ETIV) light will have the spectral resolution to study the dynamics and spectroscopic diagnostics of hot coronal plasmas. In the UV region accessible to HST, forbidden lines of FeXII at 1242 and 1349Å, of FeXXI at 1354Å, and other species seen in solar flares, are predicted to be present in the spectra of active stars. Upcoming observations with the Goddard High Resolution Spectrograph (GHRS) by S. Maran will search for these lines in the dM2e star AU Mic and other stars.

Soft X-ray images of the solar corona obtained during the last 20 years have systematically shown an intimate relationship between intense emitting structures and magnetic fields (Vaiana and Rosner 1978). The magnetic field confines a 106 K plasma, which is an intense source of soft X-ray photons. Therefore, it is natural to expect the bright X-ray structures to follow the field’s geometry. But this relationship does not seem to be just geometrical. It is generally believed that the energy necessary to heat the plasma comes from the dissipation of magnetic stresses, which are continually being re-generated by subphotospheric convective motions. However, there is still great uncertainty about the precise physical mechanisms involved in the production and release of the magnetic energy.

Modelling of coronal active regions in terms of loop structures began around 1974 (see Jordan 1975) and was stimulated by images of the corona in X-rays and the uv, obtained from rocket flights. Vaiana was a pioneer in this field (see Vaiana et al. 1973). The Skylab missions provided a fuller range of imaging and spectroscopic observations, and much of the available information is still based on these data (see Orrall 1981). Since 1975 a very large number of papers have been published on the various aspects of loop structure, heating and stability (see review by Mewe 1991). Here I can mention only a few points concerning the relation between observable parameters and the energy balance and heating requirements of active region loops.

Around the turn of the century Professor Kristian Birkeland at the University of Kristiania (Oslo) carried out laboratory studies of the emission of charged particles from the Sun and their interaction with comets and magnetized planets. He found that cathode rays emitted by a magnetized sphere are bent toward the equator along trajectories similar to the structures observed in the solar corona during the 1901 eclipse. He also concluded that comet tails are formed by charged particles from the Sun interacting with dust and gas around the comet.

Birkeland carried out extensive observing programs of the aurora, geomagnetic storms, and the zodiacal light. During a 9 months campaign from mid 1902 he observed recurrent geomagnetic storms from stations in Northern Norway, at Novaja Semlja, Iceland, and Spitsbergen, and correlated the occurence and periodicity of magnetic storms with solar activity. He found that the period of recurrent geomagnetic storms was generally larger than the period of “near equator” solar phenomena. He also found that “Very often large maxima of storminess are not accompanied by any sun-spots at all”. (Birkeland 1913, p. 524).

Ground-based and satellite observations have shown that all late-type dwarfs possess hot outer envelopes, and that the chromospheric and coronal emissions observed from these envelopes vary significantly for a given, fixed spectral type. In addition, there is growing evidence for nonhomogeneous and locally strong magnetic fields in the atmospheres of these stars. It is obvious that any heating theory must account for these two observational constraints as well as for the mean level of heating.

There are at least two general classes of models that deal with the required heating. The first class assumes that outer stellar atmospheres are heated by hydrodynamic (mainly acoustic) or magnctohydrodynamic (MHD) waves, and that these waves are generated by turbulent motions in the stellar convection zones. The second class considers dissipation of currents generated by photospheric motions as the primary source of energy. Neither observation nor theory has been able to definitively determine which one of these two general classes of models dominates in the atmospheric heating. The main aim of this paper is to briefly present recent developments in the MHD wave heating theory. The key problems that will be addressed are: where and how efficiently are MHD waves generated, and how do these waves propagate and dissipate energy?

Some well established observational facts about most wind-type astrophysical outflows are the following: (i) they are strongly nonspherically symmetric and at least two-dimensional; fast solar wind streams from polar coronal holes and jets from stars and galaxies are two representative and characteristic examples, (ii) some kind of nonthermal heating is required, at least during the initial acceleration stage of the outflow; therefore, the flow is far from beeing adiabatic and the assumption of polytropicity with an arbitrarily specified index γ, although useful from the mathematical point of view to solve the governing equations and provide some physical insight into the problem, is nevertheless artificial. (iii) the ubiquitous magnetic field seems to play a decisive direct, or at least, indirect role in heating stellar coronae and driving stellar winds. Nevertheless – perhaps for the sake of simplicity – most studies on astrophysical outflows and winds so far have neglected to incorporate the above three basic features of nonspherical expansion, nonpolytropic equation of state and magnetohydrodynamic description of the problem. We have recently embarked in an effort to model wind-type outflows by incorporating those basic physical constraints (Low and Tsinganos, 1986; Tsinganos and Low, 1989; Tsinganos and Trussoni, 1990, 1991; Tsinganos and Sauty, 1992a,b).

A consistent picture of large-scale structure appears to be emerging from different types of observations including the spatial distribution of galaxies, clusters of galaxies, narrow pencil-beam surveys, and quasars. I describe these observations below. A network of large-scale superclusters, up to ~ 150 Mpc in scale, is suggested. The supercluster network surrounds low-density regions of similar scales, suggesting a “cellular” structure of the universe. (Ho = 100 km /s/ Mpc is used).

Redshift surveys of galaxies have been over the past decade the major source of information for studies of the large-scale structure of the Universe. Following the completion of the original CfA Redshift Survey, several groups have joined the endeavor, probing different regions of the sky to different depths in a remarkable long-term effort to study the nature of the galaxy distribution and its statistical properties at different scales. Here I summarize the current status of the ongoing surveys drawn from optical galaxy catalogs. The review is not intended to be complete but rather to demonstrate the vitality of the area and to point out that exciting new data should be forthcoming in the next few years.

We have recently completed a survey of the peculiar velocities of 1355 southern spiral galaxies. The Tully – Fisher relation was used to estimate the distances to the spirals. The photometry was done in the I-band using CCDs on the lm telescope and the Anglo-Australian Telescope at Siding Spring Observatory. The rotational velocities were measured from 1042 H-alpha rotation curves obtained with the 2.3m telescope at SSO and 609 H 1 profiles obtained with the Parkes radio telescope.

Deviations from the Hubble flow directly probe of the underlying total mass distribution, assuming the gravitational instability picture. We discuss the origin of motion of the Local Group with respect to the Cosmic Background Radiation and review the peculiar velocity field deduced from distances to hundreds of elliptical and spiral galaxies, including new results for the Shapley Supercluster. Bulk-flow solutions which are free of Malmquistbias are presented, indicating coherence length larger than that expected from the optical and IRAS dipoles or from Cold Dark Matter models.

Lyman a companions to high-redshift quasars appear associated with radio activity in the quasar. Of the five known companion systems seen at typical Lα fluxes of a few x 10-16 ergs cm-2 sec-1, all are associated with quasars which are radio sources, and lie within a few arcseconds of the quasar. We summarize results of searches for the companions, and report on subsequent investigations into the spectral energy distribution and radio studies of these objects. Multi-color imaging from 3400 Å to 2.2/μ shows the companion to PKS 1614+051 possesses a blue SED similar to an irregular at that redshift. VLA observations show double-lobed structure around Q1033+137 with depolarization of the counterjet lobe.

We have carried out a number of imaging surveys of fields around quasars to study their global environments (e.g. Yee and Green 1987, Ellingson, Yee and Green 1991). The richness of the galaxy cluster environment of each quasar was determined using the galaxy-quasar spatial covariance amplitude, a quantity which is normalized for the expected luminosity and spatial distribution of galaxies (Longair and Seldner 1978, Yee and Green 1987). We find that ~40% of the brightest radio-loud quasars inhabit rich clusters of galaxies (Abell class 1 or higher) at z≳0.5 whereas only fainter AGN inhabit clusters at more recent epochs (Figure 1). This can be understood if quasars in rich clusters evolve much faster than those in poor environments.

Bruzual & Chariot (1991, BC91 hereafter) have supplemented the purely photometric isochrone synthesis models of Chariot & Bruzual (1991) with an updated library of stellar spectra and evolutionary tracks in order to compute the spectral evolution of stellar populations. BC91 used the evolutionary tracks of Maeder & Meynet (1989), with the main-sequence lifetimes for stars in the mass range 1.3 M⊙ ≤ m ≤ 2.5 M⊙, revised as prescribed by Maeder & Meynet (1991). BC91 included an accurate description of the late evolution of low and intermediate-mass stars as hot nuclei of planetary nebulae and through the white dwarf cooling sequence. While stars in these stages contribute negligibly to the optical/near-IR light, the self-consistent model of BC91 for an initial burst of star formation can account for the UV excess observed in quiescent early-type galaxies. Models that do not include post-AGB stars require additional star formation to account for this UV excess.

The test of the world geometry with the number count of galaxies is one of the most classical subjects. Yet it is only quite recent that observations started to yield useful information on the world geometry (Tyson 1988). We have been studying this cosmology test over the last few years (Fukugita et al. 1990; see also Yoshii and Takahara 1988), recently with more emphasis given to galaxy evolution and selection effects of the observation. In this talk we summarise the main result from our study (Fukugita et al. 1991; Yoshii and Fukugita 1991b).

The shape and amplitude of the galaxy – galaxy correlation functions, ξgg(r), are among the most widely used measures of the large-scale structure in the universe (Totsuji & Kihara 1969). The estimates, however, might be seriously affected by the limited size of the sample volume, or equivalently, the limited number of available galaxies. In fact, while the observable universe extends c/H0 ~ 3000h-1Mpc, most observational works to map the distribution of galaxies so far have been mainly applied to samples within ~ 100h-1Mpc from us. Thus a CfA redshift survey slice, for example, of 8h < α < 17h, 26.5° < δ < 32.5°, and cz ≾ 15000km/sec (de Lapparent et al. 1986, 1988) represents merely ~ 2 x 10-5 of the total volume of the observable universe. This clearly illustrates the importance of examining possible systematic biases and variations in the estimates of two-point correlation functions from instrinsically limited data. We studied such sample-to-sample variations by analysing subsamples extracted from large N-body simulation data.

Textures will produce positively skewed non-Gaussian primordial fluctuations. This implies that galaxies and quasars formed earlier in this model than in a Gaussian model with the same power spectrum. The model also predicts the existence of more high velocity dispersion clusters and richer superclusters. We describe the texture model and our numerical simulations of the origin of structure in the model.

Astrometry is the oldest of the astronomical endeavors. Indeed, we still have the observing journals of Galileo, the journals of Flamsteed, and those of most of the other old transit-circle astronomers. We have most of the early photographic plates taken for the astrographic catalog project, and almost all the parallax plates. However, as technology progressed, we have become somewhat less precise in recording what we did. It is for this reason that this joint meeting was called: to impress on the modern astronomer that, in general, he only extracts a fraction of the information from his observations, and that these observational data may, in the future, provide an invaluable source needed for the progress of the science.