It is possible to estimate the galactic mass density in the solar neighbourhood either directly by summing up the mass densities of individual subsystems of stars and interstellar matter or indirectly from dynamical considerations.

Observational data on the number density of visible stars lead to mutually consistent results on the stellar component of the mass density. The mean of different estimates is ⍴stars=0.052±0.010 Mʘpc−3. By adding the probable contributions of intrinsically faint undetected objects and of interstellar matter the value ⍴=0.09±0.02 Mʘpc−3 has been obtained for the total mass density.

The discoveries of both steady and bursting X-ray sources in globular clusters have made them as interesting for high energy astrophysics and X-ray astronomy as they were for unlocking galactic structure in Shapley’ s day. Globular clusters are massive (˜ 105 M⊙) spherical clusters of primarily low mass (⋞0.8 M⊙) evolved stars and may be the oldest systems in the Galaxy. Thus when several globular clusters were tentatively identified as containing X-ray sources (Giacconi et al. 1974), it was apparent that either dynamical or evolutionary processes must be occurring among the ancient stars in globular clusters to produce the relatively short-lived X-ray sources. The existence of these X-ray sources has prompted considerable discussion as to their origin, since they exist with ˜100 times the probability per unit mass in globular clusters as in the Galaxy as a whole (Katz 1975; Clark 1975). They may be yet another example of X-ray binaries (e. g., Clark 1975; Fabian et al. 1975) or be due to accretion onto a single collapsed object (black hole) at the center of the cluster (Bahcall and Ostrlker 1975; Silk and Arons 1975). At present both of these classes of models are possible and generally consistent with the data, though each has its particular difficulties. We shall review the observational data and then the models in an effort to point out the present balance of evidence for the binary vs. black hole models as well as the most promising directions for future work. Given the possibly high incidence of X-ray bursters in globular clusters, our discussion will refer to both observations and recent models for bursters. While this review will primarily address the X-ray sources in globular clusters, it is obvious that their eventual understanding will contribute much to studies of globular clusters in general and will prompt future studies in particular areas.

Based on the observations of M giant stars in the north galactic polar objective-prism survey of Upgren (1960) and the data summarized by Blanco (1965) the overall space density of all M-type giants as a function of distance from the galactic plane at the position of the sun can be approximated by,

where z is in kpc and ρ(z) is the number of stars per 106 pc3. This relationship is derived from the observed fall-off in space densities up to a distance of about 2 kpc.

The question arises as to the validity of extrapolation equation (1) to larger z distances so as to predict the number of faint M giants expected per unit area near the galactic poles. Adopting for the M giants a mean visual absolute magnitude of −1.0 (Blanco 1965), one finds that equation (1) predicts that less than one giant fainter than V~12 should be expected in a region of 200 square degrees. This expectation formed the hypothesis of a thesis study (Sanduleak 1965) in which it was assumed that the very faint M stars detected in a deep, infrared objective-prism survey at the NGP were main-sequence stars, since this could not be ascertained spectroscopically on the very low-dispersion plates used.

A model given by Henriksen and Chia (1972) for short duty cycle X-ray flare sources has been extended by Henriksen (1976), and applied to the recently discovered X-ray bursters. We use their notation here unless otherwise indicated. The model envisages a rapidly rotating magnetic white dwarf or neutron star located in a dense stream or cloud of gas such as might be found in close binary systems or in dense interstellar clouds. The basic charging mechanism for the flare is the accretion of matter into the hydromagnetic wave zone of the rotator at the ‘Bondi’ or ‘tail-shock’ rate (corresponding to a thermal cloud or a stream,-respectively), AB.

We consider spherically symmetric accretion flow onto a strongly magnetized neutron star. We show that, under certain conditions, the flow is intermittent and that the resultant accretion luminosity (X-rays) from the stellar surface is akin to that observed in the bursting X-ray sources. We investigate the properties of such burst flows under a variety of conditions, in the hopes of providing a basic theoretical framework on which realistic models of the observed bursting sources can be built. This work was supported in part by NSF Grant PHY75-08790.

The fate of nuclear energy carried by matter accreted onto a neutron star was considered e.g. by Rosenbluth et al. (1973). They examined pycnonuclear reactions on an originally cold star, and found that the whole star is thereby heated up to 106−107 °K. The thermonuclear reactions that can be ignited then, have been studied by Hansen and Van Horn (1975), who computed stationary burning shells, finding, however, that in most cases the shells are thermally unstable.

We consider here the possibility that X-ray bursts are due to instabilities of this type. The observational constraints fix the parameters uniquely.

This paper will truly be an introduction — a presentation and a discussion of the basic problems, to set the stage for detailed research results that will be reported by others.

The problem of star distribution perpendicular to the galactic plane has one central theme, which can be played in two modes. The theme is the drop-off of star-density with z, and the two modes are the general density profile and the stratification of populations. (This last phrase is a convenient one, but I should warn against taking it too literally. By “stratification” I mean a gradual change, rather than a sharp separation; and I want to emphasize even more strongly that the word “population” should be used in a general and abstract sense, rather than as a sharp separation into two, or five, or any other discrete number of components.)

Recent progress in identifying the optical counterparts of X-ray sources has been slow, mostly because candidates are faint and X-ray data show no periodicities by which the identification can be confirmed. This report therefore deals with the investigation of some candidates to seek confirmation of their identity with the X-ray emitters, as well as details which are new or important concerning the few known binary sources. I shall also make some general remarks on the properties of the sources as they now appear, but leave a critical assessment of the masses to the next speaker.

The observational base of our investigation consists of a two-dimensional MK classification of about 11.000 stars in the 42 Kapteyn Areas situated along the gal.lat-s from −17° up to +72° and 200 Ap, Am stars discovered in the same KA. The dispersion of the applied objective-prism spectra is 160 Â per mm. The data are of high accuracy, close to the Michigan level, and uniformity, which make them reliable. The limit is close to the 12-th ph.mg.

The general conclusion of the undoubted importance is stated: the galactic concentration of dwarfs is closer than it had been assumed until now; on the other hand - the giants are not so closely concentrated to the galactic plane as it has been accepted.

At the Tbilisi European Regional Meeting 1975, I reported on results of uvby, Hß photometry of F-type stars in the north and south galactic polar caps (Blaauw and Garmany, 1976), based on stars in the McCormick proper motion fields between latitudes 60° and the poles. A relation was shown to exist between the quantity Am. as determined in this photometric system, and the distance, z, from the galactic plane; Δm1. being a measure of the metal abundance in these stars. The spectral range we deal with is defined by b-y = 0.25 to 0.40, corresponding with F0 to G2. It was found that, from the solar neighbourhood near z = 0, to z = 700 pc, the mean relative metal abundance M/H decreases by a factor of about one third.

The presence of compact x-ray sources in binary systems provides an opportunity to determine observationally the masses of neutron stars and possibly of black holes. These masses are important for equations of state at nuclear densities, for theories of gravitation, and for late stages of stellar evolution. A mass range for each source is determined by finding the sets of orbital elements that are consistent with the available x-ray and optical data. As such analyses are subject to various uncertainties, the resulting mass ranges are rather wide. Nevertheless, significant mass estimates are obtained, accompanied by interesting results on other system parameters.

The halo program of the Basel observatory, initiated by Becker in 1965, is based on a three colour photometry in test fields along the circle through the galactic centre and the galactic poles. The more favourable direction of the blanketing vector relative to the main sequence in the two colour diagram for RGU makes it possible to separate at least statistically the disk population and the halo populatior within the interval of absolute magnitudes + 3 ≦ MG ≦ + 8. It is therefore possible to derive density functions for both populations and for different intervals in absolute magnitude for each test direction within the test plane defined above. This allows one to draw isodensity curves in the test plane and, assuming rotational symmetry of the halo, also isodensity surfaces. The last assumption is tested at least locally by test fields with different inclinations towards the test plane (Fenkart, R.P. and Wagner, R., 1975).

The most important recent observational discoveries in the field of X-ray binaries are probably those of the slow pulsars and of the winds of normal early-type main-sequence stars. These facts yield key information on the evolutionary history of the X-ray binaries and on the rotational slow-down mechanism for a neutron star in a stellar wind, as will be pointed out in section 3. In the theoretical field, the X-ray binaries have triggered much fundamental work, notably on the detailed processes of mass transfer and on tidal evolution, which will be considered in sections 2, 4 and 5.

A systematic photometric investigation on stars of spectral type G4 to K4 is performed in both polar caps in symmetric coaxial cones. According to a calibration of the Geneva system, Mv, Teff and [M/H] ratio have been now derived for about 4OO stars. For the computation of density law, a special care has been taken to avoid selection biaises related to variations of mean age or metal abundance. A change of luminosity function for evolved stars in function of z has been detected. Using the geometrical properties of the cones and direct counts in the solar vicinity, we derive the following density law for stars with Mv ∊ [0.0, 1.3] and (B-V) ∊ [0.7, 1.3]

The subject of accretion flows in binary X-ray systems has been reviewed recently by Lightman et al. (1977), and the subject of accretion disks by Shu (1976). Here I shall emphasize developments that have occurred since those reviews were written, and concentrate on issues concerning the gas flows rather than the radiation mechanisms.

The coordinated campaign to observe x-ray binaries sponsored by the IAU Commission 42 (Close Binary Stars) and Commission 44 (Astronomical Observations from Outside the Terrestrial Atmosphere) was initiated in January 1974 with Y. Kondo as its coordinator.

The main objectives of the campaign have been: (a) dissemination of information on satellite x-ray experiments and ground-based observing programs; (b) transmission of suggestions and recommendations from campaign participants; (c) designation of campaign dates for observation of specific objects. These functions have, in part, been performed through Campaign Circulars and Special Bulletins; the most recent, Circular No. 23, was issued a few weeks ago.

In order to study interstellar extinction near the NGP a polarization observation programme has been undertaken at the Metsähovi Observatory of the University of Helsinki. 32 stars (b>80°) have been measured with an accuracy of a few hundredths of a percent. To this list observations of 42 stars made by Appenzeller (1968) were added. The stars measured have distances up to 400 pc (Figure 1).

As a contribution toward settling the question of the distances of Murray-Sanduleak (M-S) stars (Murray and Sanduleak, 1972), two of these stars were added to the U. S. Naval Observatory (USNO) program in 1973. After the initial plates had been taken, it appeared that parallax for an additional star could be measured on one series, while parallaxes for two additional stars were possible from the second. As a result provisional parallaxes are now available, Table I.

In terms of USNO parallaxes, these are considered provisional because they are based on half the number of plates on which the final parallaxes are ordinarily determined; only small changes are expected when additional material is added beyond the present.