Various models of the X-ray background are discussed. It is postulated that the only explanation consistent with all the existing data is discrete sources. Present observational material suggests that known classes of active galactic nuclei also dominate the source counts below the lowest detectable flux levels.

In this talk I shall discuss two topics related to modeling of cosmic structure formation. In the first part I shall briefly review the problem of cosmologica! parameters, which are crucial for any of model calculations. An emphasis is made on whether we need a non-zero cosmologica! constant. In the latter part cosmic microwave background (CMB) anisotropics are discussed as a probe of the models for cosmic structure formation.

Constraints on models of cosmic structure formation that can be drawn from current limits on large angle microwave background anisotropies are now competitive with those from recent small and intermediate angle experiments and are relatively insensitive to the reheating history of the Universe. Here I give limits on Gaussian scale invariant adiabatic fluctuations and describe the role that the large angle results play in constraining models with enhanced large distance galaxy clustering power inferred from correlation function measurements is described.

The search for anisotropies in the Cosmic Microwave Background (CMB) is fundamental to observational cosmology: it requires observations on a range of angular scales and at a range of frequencies to distinguish CMB structure from foreground galactic structure. We have made significant progress in setting new limits to CMB anisotropies on angular scales of 3°-12° using scaled observing systems at 10 and 15 GHz. This regime of angular scales is particularly matched to the predictions of Cold Dark Matter (CDM) and isocurvature scenarios of galaxy formation in the early Universe.

We present new limits on the small scale 3 K emission anisotropy. At scales of 1°-3.3° all seven harmonics have amplitudes below 10-5 in ΔT/T at our wavelength of 7.6 cm. At the 6.1x103 to 17.5x103 (rad-1) scale, this limit is about 2.8x10-4 (at 2.7 cm wavelength) and the observations may be fully explained by contributions from discrete sources. The low baryonic content of the Universe and present limit on the ΔT/T at the horizon scale suggest that (ΔM/M)gravitating ≪ (ΔM/M)baryonic. speculate that the 3 K anisotropy measurements prove a) the existence of the dark matter, b) the nonbaryonic nature of this dark matter and c) that the space distribution of this matter is much more homogeneous than that of the visible one.

The use of aperture synthesis allows one to search for fluctuations in the CBR on angular scales below 1 arcminute. I report here tentative results of an experiment carried out with E. B. Fomalont, R. Windhorst and J. Lowenthal using the Very Large Array of the National Radio Astronomy Observatory in New Mexico. We used the instrument in its tightest configuration and at a wavelength of 3.6 cm; the corresponding angular resolution was ~10”. We were able to set limits on fluctuations in the CBR on a range of angular scales, 10”-90”, and these are among the most sensitive upper limits on CBR fluctuations yet published.

The scientific objective of the COBE Differential Microwave Radiometer (DMR) investigation is to use the cosmic microwave background (CMB) radiation as a probe of cosmology. Stringent limits can be set on the amplitude of fluctuations of the potential energy at the surface of last scattering of the CMB photons. Large angular scale (>1 .) fluctuations in the gravitational potential energy are directly related to temperature anisotropies in the CMB by the Sachs-Wolfe Effect (Sachs & Wolfe 1967), which describes the gravitational redshift of photons as they are last scattered at a gravitational potential energy per unit mass, ϕ, giving δϕ/c2 = 3δT/T. The DMR instrument was described by Smoot et al. (1990), recent scientific results by Smoot et al. (1991a,b), and calibration by Bennett et al. (1991). The DMR consists of two independent radiometers at each of three frequencies: 31.5, 53, and 90 GHz. Each radiometer measures the difference in temperature between two 7. FWHM beams separated by 60.. The scanning pattern of the instrument causes all 60. separation pixel-pairs to be observed thousands of times.

Values of D/H measured in the methane on the giant planets and Titan indicate the presence of two distinct reservoirs of deuterium in the outer solar system. The dominant reservoir is in hydrogen gas, the second, multi-component reservoir is found in the hydrogen that is bound in condensed compounds. Both reservoirs appear to have originated in the interstellar medium. In contrast, the values of D/H in water vapor on Mars and Venus (especially) exhibit a large enrichment from the “condensed matter” starting value. Interpretation of this enrichment may illuminate the history of water on these two planets.

New and important data have been obtained during the 1985-1986 return of comet Halley, including in situ observations of the nucleus and the coma. Since the interpretation of the observations is not straightforward, the results are presented in a rather conservative manner. Some clues to the solar system formation are suggested, e.g. the shape of the nucleus, its low density, the estimated mass of the Oort cloud, the elemental abundances in comet Halley. Constraints related to isotopie abundances (deuterium enrichment, possible anomalies in carbon isotopes) and to cometary dust (complex organic compounds, submicron sized dust particles) are extensively discussed.

Since the beginning of the XVIIth century, rings of dispersed matter play an important role in theories accounting for the origin of the solar system and proto-planetary nebulae. Planetary rings afford a good opportunity of studying some of the accretion mechanisms which operate during the early evolution of a proto-planetary nebula. Three new planetary rings have been recently discovered. Space and ground-based observations have completely renewed our image of ring systems. Simultaneously, a wealth of theoretical and numerical models have flourished. Collisions between ring’s particles and gravitational perturbations of nearby satellites should explain most of the ring’s structures. However, important questions are still unanswered. We do not understand why the rings are so dissimilar. We do not know the ring’s origin and their stability over billions of years. Most of the ring’s complex structures, the existence of arcs, and color and optical depth variations are not explained. Confinement mechanisms, which are so important in cosmogony, seem to be at work in planetary rings today.

A common origin of the sun and the planets from the collapse of interstellar gas is now widely accepted. Regardless of how stars form, which is considered as the previous step of the whole story, the starting point is a flattened rotating cloud containing a mixture of dusts and gas (the so called Kant-Laplace Nebula). On the other hand the observations of young solar-mass stars show with increasing evidence that the gas is dispersed away on a time scale less than 107 years and this provides us with a clear time constraint for model building since the formation of the giant gaseous planets have to take place on a shorter time scale.

Multispectral photometric and polarimetrie observations of the Beta Pictoris circumstellar disk, made with a coronagraphic CCD camera at the Las Campanas Observatory between 1984 and 1989 (Smith et al 1992a), indicate that the nearly edge-on, highly flattened disk extends to a distance of more than 60 arcsec (1000au) from the star, with a nearly constant projected thickness of 2.8 arcsec (45 au) on the plane of the sky. A peculiar morphological property of the disk is its apparent asymmetry, with the northeast extension appearing somewhat longer and brighter than its southwest counterpart. Because of its implications for disk dynamics and morphology, this anomalous structure is perhaps the most puzzling phenomenon yet associated with the Beta Pictoris disk and deserves further study.

Results from the IRAS satellite showed that many pre-main sequence stars exhibited unexpectedly large fluxes in the infrared spectral region. Several studies have shown that the simplest and most satisfying explanation of this excess emission is that it arises in optically-thick, dusty, circumstellar disks (Rucinski 1985; Adams, Lada, and Shu 1987, 1988; Kenyon and Hartmann 1987; Bertout, Basri, and Bouvier 1988; Basri and Bertout 1989). The masses of these disks are estimated to range between 10-3M⊙ to 1M⊙ (Beckwith et al. 1990; Adams et al. 1990), large enough that disk accretion may have a significant effect on the evolution of the central star. Indeed, Mercer-Smith, Cameron, and Epstein (1984) suggested that stars are essentially completely accreted from disks, rather than formed from quasi-spherical accretion (Stabler 1983, 1988).

Launched in August 1989 into a geostationary transfer orbit, the Hipparcos astrometry satellite failed to reach its intended geostationary orbit through the failure of its apogee boost motor. Present indications are that the possible operational lifetime should nevertheless extend beyond the end of 1992, with extremely high-quality scientific data being returned for some 65 per cent of the time. For an assumed 3-year operational lifetime, the original mission goals (positions, parallaxes, and annual proper motions with an accuracy of about 2 milli-arcsec) should be achievable (the pre-launch expectations of the planned 2.5-year mission would be achievable with a 3.5-year mission in the revised orbit). This paper describes the in-orbit performance of the satellite most relevant to the scientific goals.

The Hipparcos observing programme is composed of 118 000 selected stars, one QSO (3C273), 48 minor planets, and 2 satellites of major planets. These objects were selected on the ground of scientific proposals submitted by the world-wide astronomical community, while taking into account die specific operating mode of the satellite and using at best its observing possibilities. The content of the stellar part of this observing programme and the way in which the star selection was performed is described in details in a paper accepted for publication in Astronomy & Astrophysics.

The methods developed to predict instrumental magnitudes, to monitor the ageing of the optics, to calibrate photometrically the payload and finally to build up a high accuracy photometric reference system are tested against space observations. The quality of ephemerides for long period variables are also discussed.

Even before the launch difficulties, it was planned to monitor the trend of the accuracy acquisition resulting from the adopted observing process and eventually to modulate the initial values of the observing strategy parameters whenever it may help correcting distorsions in the final uniforme accuracy. Of course this monitoring of accuracy acquisition became more important when, as a result of the revised orbit, large departures from the nominal observations take place. The observing strategy and the associated modulation process behave according to expectation and may smooth out on the long range inhomogeneities. However important effects due to the complete lack of long sequences of observations in the early period in some regions will not be compensated unless the real mission extends substantially beyond the end of 92.

The data selected for publication and the stellar content of the 118,000 stars constituting the observing programme of the Hipparcos satellite are briefly described.

Forty eight minor planets and two planetary satellites have been retained in the Hipparcos programme. The observations of minor planets will be used to try to determine the dynamical equinox and the rotation of the Hipparcos reference system (S. Söderhjelm and Lindegren, 1982); they can be also used to determine some physical properties of minor planets. As for the observations of planetary satellites, they will allow to obtain accurate ephemendes of these objects and to improve those of the primary planets. The inclusion of minor planets and satellites in the observing programme raises very specific problems: owing to their motion, positions of these objects are given in the Input Catalogue using ephemerides; preparatory astrometric work needed for improving these ephemerides (ground-based observations and improvement of orbital elements) has been described in Bec-Borsenberger (1989, 1991a). To be sure that the ephemerides are sufficient to get the 1 arcsecond predictability needed for Hipparcos, a reduced programme of ground-based observations will last all along the Hipparcos campaign (with the automatic meridian circles of Bordeaux and La Palma observatories). Presently, an annual update is adequate in order to ensure this precision. The improved orbital elements and predicted ephemerides for years 1991 and 1992 are published (Bec-Borsenberger, 1990, 1991b): the residuals obtained from the last set of improved elements with the meridian observations made for the Hipparcos programme give rms in right ascension and declination between 0.19 and 0.38 arcsecond for all Hipparcos minor planets. Since the launch of Hipparcos, the two satellites have been observed and about a third of the minor planets of the programme have been found; the previsions allow to hope to observe this year about ten more minor planets.