Observations with the Einstein X-ray telescope show that quasars, as a class, are luminous X-ray emitters. By coupling our X-ray observations with quasar optical number counts, we can show that the quasars contribute significantly to the diffuse X-ray background. In fact, the X-ray data strongly suggest that somewhat above 20m the slope of the optical counts must flatten.

Astrometry - the determination of positions, motions, and coordinate systems and the entirety of the products of these observations- was the first specialty in astronomy. The value of astrometry for all of the fields of astronomy does not have to be emphasized here; it is clear that the foundation of astronomy and astrophysics is provided by astrometry. What is perhaps not so clear is the fact that, although considerable progress has been made in the seventies, the numerical estimates for many of the basic quantities are much less certain than the average user realizes. Fortunately, astrometry stands at the brink of a technological revolution, involving both ground-based and space techniques. Once completed, this will lead to one or more orders of magnitude improvement in both the precision and quantity of most astrometric observations, with enormous implications for other fields of astronomy.

A description of the instrument is given. It is expected to obtain positions, annual proper motions and parallaxes for 100 000 stars in a 2.5 years mission. The accuracy (s.e.) will be for stars of mB < 11 and about at mB=14. An extension of the mission to 3.5 years would improve the annual proper motion to for stars brighter than mB = 9.

This paper describes the Space Telescope and the instruments it will carry which will be used for astrometry. Particular attention is paid to the two imaging cameras and to the Fine Guidance Sensors. A brief outline of the kinds of programs which are to be carried out is also given.

I think the best way to start my talk is to recall what kind of spectroscopic, information is available at present. Spectroscopic information can be of various kinds; starting from the simplest it may be:

a- a spectral type (unidimensional)

b- a spectral type plus a luminosity class (bi-dimensional type)

c- a description-more or less detailed

d- a quantitative analysis, which can be more or less sophisticated

Assuming that we have the absolute parallaxes and homogeneous set of proper motions for a large number of stars as described by the previous speakers - how should we use them in studies of galactic structure and dynamics?

Concerning the parallaxes I will be very brief - the subject has already been discussed by Dr. Pagel. Clearly, the main importance of the absolute parallaxes obtained is in the calibration of photometric or spectrophotometric absolute magnitude scales. For a sufficient number of stars observed in the Hyades cluster at a distance of 50 pc and in the Scorpius-Centaurus association the systematic error, which lies at the bottom of all subsequent calibration of distance indicators, may be reduced to the same order as the accuracy of photometric measurements.

I have understood the title of this talk given to me by Dr. Westerhout to imply a review of our present ability to provide new radial velocity measurements for the many stars to be observed by the Space Astrometry projects, as well as an indication of the technical developments still required. And indeed, the quantity and quality of the existing radial velocity determinations are such that only a small fraction of the data needed can be considered available.

The challenges presented by the two projects, the Astrometry Satellite and the Space Telescope, are somewhat different, corresponding to the complementary nature of the two programmes: The dedicated Astrometry Satellite will observe a very large number of relatively bright stars, while the Space Telescope is expected to give astrometric results for much fainter, but also far fewer objects (estimated currently at perhaps 20 per year). The two programmes will therefore be considered separately.

The chemical compositions of external galaxies are usually found from their HII regions or composite absorption line spectra of their nuclei. In the Magellanic Clouds, however, individual stars are observable, but objects heretofore studied necessarily have been luminous la supergiants which border on the brink of instability. The image photon counting system on the Anglo-Australian Telescope makes it possible to observe fainter, more stable Ib supergiants. Energy distributions and Balmer line profiles for these stars can be fitted with theoretical predictions by Kurucz. A joint effort by J.E. Ross and B.J. O’Mara of the University of Queensland, Bruce Peterson of the Anglo-Australian Observatory, and a group at the University of California, Los Angeles to analyze three lb supergiants suggests that metals of the iron group are depleted by a factor of 2-3.5 with respect to the normal chemical composition of our own galaxy.

This review concerns recent work on the determination of overall metallicities [Fe/H] in a number of globular clusters and the systematics of mixing effects displayed (usually) by weak CH and strong CN. Special attention is given to the globular cluster ω Centauri, where both metal abundance variations and mixing effects occur and are closely intertwined. Recent observations carried out at the Anglo-Australian Telescope by E.A. Mallia and D.C. Watts have revealed large variations in the strength of metallic lines across the red giant branch of this cluster.

This review summarizes the techniques and limitations involved in determining small rotational velocities in late-type stars. Recent results from photoelectric line profiles of field main sequence G stars will also be presented.

Historically three techniques have been used to measure stellar rotational velocities: (1.) calibration of a “clean” line width in terms of computed profile models, (2.) use of the Fourier transform frequency domain (dissection of rotation and turbulent velocity fields from the thermal profile), and (3.) monitoring of the rotational modulation of a chromospherically active or spotted star. The results discussed below concern technique (2.). Technique (3.) has not been exploited much yet but holds considerable promise for the future, in particular for the measurement of ultra-slow rotation.

In binary stars of short period, axial rotation of the components tends to be synchronized with the orbital revolution. Rotation of B. and A stars is therefore slowed down, while for the later-type stars, it is accelerated. This latter fact probably contributes to the phenomenon of the RS CVn stars. Notable deviations from synchronism among short-period systems are probably related to mass transfer between the components.

Binary stars play a double role in astrophysics. On the one hand, they reveal properties and structures that would otherwise be almost unobservable, and therefore help considerably to understand the character and evolution of single stars. On the other hand, they create objects and phenomena that otherwise would not exist at all, thereby actually creating more problems than they help to solve. I think that the first aspect makes the binary stars extremely useful; the other aspect makes them extremely exciting and interesting. This dual role also appears in connection with rotation. The “revealing” property is best documented by the fact that the phenomenon of stellar rotation was discovered in binary stars, through the “rotation effect” distorting the radial velocity curve of an eclipsed star. A more recent example is the probable identification of a star considerably flattened and distorted by differential rotation, in agreement with the theoretical models advanced by Bodenheimer and Ostriker (1970). The star is the secondary component of BM Orionis, which is the faintest of the four stars forming the famous Orion Trapezium. BM Orionis was once suspected of harboring a black hole, since during the apparently total eclipse of the primary B3 star, is spectrum remained visible and no other spectrum could be detected (Doremus, 1970). A higher dispersion of our Lick spectra permitted us (Popper and Plavec, 1976) to detect weak lines of the secondary star, which is most likely a late A giant still contracting to the main sequence. The flat bottom of the light curve at the primary eclipse is best explained by a model first suggested by Hall (1971), involving a disk-shaped, differentially rotating star. It would be difficult to recognize the shape of such a star if it were not member of an eclipsing system.

Kraft (1970) obtained the rotational velocities for large numbers of stars located in the field and in clusters of different ages. He noted that (a) among the field stars those stars with strong Call K emission had larger rotational velocities than those without; (b) stars in the Hyades and Pleiades (which are much younger than the field) had both larger rotational velocities and stronger Call K emission than field stars; (c) there was a pronounced break at spectral type early F in v sini as a function of spectral type and (d) the distribution of angular momentum per unit, mass J(M⊚) was proportional to M0.57 for main sequence stars with mass M > 1.5 Mʘ. This distribution predicted a v sini of ˜75 km/sec for stars of lower mass (e.g. G type) but such high velocities were not seen in the Pleiades nor in the sun. This implied a more rapid deceleration of v sini for lower mass stars and led to estimates of the e-folding time of ˜4×l08 years for stars of 1.2 M⊚ to reduce their v sini from that of the Pleiades to that of the Hyades and ˜4×l09 years to go from the Hyades to the sun’s v sini. We note also that the age of the Pleiades is approximately equal to the pre-main sequence lifetime of a 1.0 M0 star so that the zero-age main sequence cannot have J(M) α M0.57 for ˜1 M0 stars. Skumanich (1972) showed that both the Call k emission and the rotational velocity decayed as the (age)-½ for main-sequence stars.

The theme of this Joint Meeting of Commissions 10, 27, 40, 42, 44, and 48 is “Close Binaries and Stellar Activity”. At the time this meeting was initially conceived, we took stellar activity to mean activity such as we observe on the Sun: spots, flares, chromospheric emission, variable radio and X-ray emission, coronas, winds, and so forth. That is why the series of seven reviews presented at this meeting will deal for the most part with convective stars, in particular with the RS CVn-type and BY Dra-type stars, which we know are active in so many of these ways.

It had been planned that Steven N. Shore would review briefly the situation from a theoretical viewpoint (briefly, I say, because theoretical treatment of the problem is still in the infant stages). Reference can be made, however to the contribution he presented at I.A.U. Symposium No. 88 in Toronto.

One way to open this meeting, I hope a useful way, is simply to list some unsolved problems. Those on my list may not be equal in importance or difficulty. Different problems would no doubt appear on someone else’s list. It may even turn out that, after we have heard these reviews, a number of my unsolved problems have already been solved or soon will be, but that would be a pleasant surprise rather than a disappointment.

Physically quite dissimilar, the BY Dra and RS CVn stars have the common characteristic of extraordinarily high levels of stellar surface activity.

The RS CVn group, as defined by Hall, consists of detached binaries both components of types F, G, or K. The orbital periods are from a few days to a few weeks, and the mass ratios are generally near unity. Spectroscopically, the stars are remarkable in that they show very strong Ca II H and K emission outside eclipse. Masses and radii are known for many of these systems; physical parameters indicate these systems contain a moderately evolved subgiant, with mass slightly greater than 1 M⊚. The ages (which may be verified by data on visual companions) are a few 10 years.

In contrast, the BY Dra variables are late-type dwarfs, with spectral types ranging from dK5e to dM4e. The e designation indicates the presence of Balmer emission, which is generally visible in moderate strength at Ha and only very weakly present in the blue. The BY Dra stars do exhibit strong emission at H and K, however, and also show UV Ceti-type flares. Approximately 75% of the BY Dra variables are known as double-line spectroscopic binaries.

The reason for presenting this review at this Joint Meeting, in spite of the fact that there was, a few days ago, a special Joint Discussion on recent UV observations is partially that somehow the general subject of close binaries was overlooked at the Joint Discussion, although unique close binaries, such as cataclysmic binaries and X-ray binaries, were included. There exist a large number of vigorous research projects being carried out with UV telescopes in the field of close binaries. I shall concentrate on IUE results, although other satellite and balloon-borne telescopes have continued to produce interesting UV observations of close binaries.

There are a number of active IUE programs on this theme. In the time allocated to this review I shall only be able to present highlights of selected programs in the hope of giving a glimpse of what is going on. The programs on close binaries from the first year of IUE operations include those by P. Conti, M. Hack, J.B. Hutchings, R.H. Koch, Y. Kondo and G.E. McCluskey, M. Plavec, and J. Sahade. In the second year, additional astronomers are also observing close binaries with the IUE.

Since the discovery in 1974 by Heise et al. (1975) with the ANS satellite of X-ray flares from YZ CMi and UV Cet, only a few attempts to observe X-rays from flare stars have succeeded. On the other hand, the discovery of X-ray emission from Capella by Catura, Acton and Johnson (1975) in a rocket flight has been followed by so many detections of RS CVn binaries by the low energy detectors (0.15 - 3 keV) of the HEAO A2 experiment that, while Catura et al. estimated that many variable soft X-ray sources probably exist, Walter, Charles and Bowyer (1978) could identify the RS CVn systems as a class of quiescent sources. They have higher temperatures than at first predicted, so they are ideal for detection in the energy range ¼ keV to a few keV, and the high space density provides many close candidates. Further information on these sources is now available from the GSFC Solid State Spectrometer Experiment on the Einstein Observatory, which with energy resolution of 140 eV can resolve the major complexes of line emission from Si, S, Fe and less abundant elements that are an important part of the emission of 10 million degree plasmas. The imaging experiments on the Einstein Observatory have detected X-ray emission from subsets of all types of stars, and results on the luminosities, variability and temperatures are just beginning to come out.

Excluding single stars, star systems with very strong X-ray sources, thermally radiating winds and circumstellar envelopes, marginal detections, and binary systems that are far from “normal”, there are about 21 close binary systems that show clear signs of stellar activity in the form of variable radio emission. Sixteen of these are RS CVn binaries. Typical events are smoothly varying with time scales of from a few hours to a few days. In the RS CVn binaries UX Ari and V711 Tau variable circular polarization is sometimes observed, with occasional appearence of components with only one frequency and one circular polarization, part of which shows “oscillations” with “periods” of about 4 minutes. Different stars and different events typically have inferred electron energies of about 5 MeV and inferred magnetic fields of 1 - 30 Gauss. The radiation mechanisms are usually assumed to be synchrotron or gyro-synchrotron; however, radiation from plasma processes cannot be ruled out in some cases, and it will be very important to establish or rule out this possibility. Most radio binary events show clear signs of self-absorption, so the variations appear most strongly at the higher frequencies. Typical rise times of events are about 30% of decay times. Maximum radio luminosities range from 1013 to a few times 1017 ergs s-1 Hz-1. Most radio flares are mainly at cm-wavelenghts and have observable and inferred energies 104 - 106 times those for the largest solar events, a scaling which is similar to that for X-ray emission measures of these stars when compared to solar coronal loop emission measures. Most importantly, VLBI measurements of Algol indicate that the sizes of the radio emitting regions for very strong events are of the order of 2 - 3 stellar radii. Single stars appear to be active at radio wavelengths much less commonly than close binary systems. In many cases this may be due to the way dynamo action and convection near the surface are affected by increased rotation forced by the synchronization induced by the binary system.

The motivation for obtaining optical polarization data of close binary systems and/or stars exhibiting stellar activity (i.e., flares, winds, chromospheres, spots, etc.) is that polarization is not only dependent on the physical condition of the stellar environment but is actually rather sensitive to geometric factors and can act as an excellent diagnostic of the latter.

High resolution, multichannel spectropolarimetry of certain emission-line stars was demonstrated to be a powerful new technique in studying the physics and kinematics of stellar envelopes. Two sources of polarization were discussed viz., (l) fluorescent (resonant scattered) emission lines similar to those found in the solar chromosphere and corona, which can be highly polarized and sensitive to magnetic fields, and (2) Doppler broadened emission lines formed in a rotating/expanding stellar wind. The Doppler broadening effectively provides spatial resolution, so that as one scans a line polarimetrically, scattering from different regions of the stellar wind is observed according to its kinematical motion.

RS CVn-type systems exhibit many similarities to active regions on the Sun. In the last few years considerable progress has been made in quantifying the differences between the atmospheric properties of solar plages and quiet regions. These differences, as summarized in Linsky’s (1979) review, can be simply explained by additional nonradiative heating and include the following: (1) temperature enhancements in the photosphere, (2) heating of the temperature minimum and its displacement downward in mass column density, (3) enhanced radiative losses in the chromosphere and transition region (typically by a factor of 10), (4) steeper chromospheric temperature gradients, and (5) enhanced pressures and densities at the top of the chromosphere, throughout the transition region, and at the base of the corona (again typically by a factor of 10).

The transfer of mass from one component of a binary system to its companion can involve ejection of the mass by radiation pressure, by Roche lobe overflow, by centrifugal action through the coupling of the upper atmosphere to the stellar rotation through magnetic fields, and by the dynamics of the atmospheric fluid under the action of an energy source at the photosphere or lower chromosphere. I will primarily address the latter mechanism, which commonly goes under the title of stellar winds. The major determinant of a stellar wind is the energy source. In the case of a binary system, the dynamic effects of the system on the flow pattern, particularly for close binaries, is a second major determinant, once an energy source is available to drive the wind.

In addition to ejection, the transfer of mass requires capture of the ejected matter by the companion. The first issue is free particle vs. continuum flow. I will only consider continuum flow, as I doubt the applicability of the free particle concept to any binary system with significant mass transfer, because of the scale height and because of the likely presence of magnetic fields in the interstellar plasma. For more distant binaries, rather simple considerations show that the capture of matter for continuum flow can be enhanced over that for free particle flow.