The Hipparcos astrometric parameters were obtained through the merging of abscissae data obtained by the FAST and NDAC Data Reduction Consortia. Using a different reduction scheme, other astrometric solutions may be obtained, although not statistically different: a weighted solution using FAST and NDAC astrometric solutions is shown, which gives both astrometric data and associated covariance matrix. This is applied to the acceleration and stochastic solutions. The intersection between merged and weighted solutions is then studied from the point of view of the possible signature of astrometric binaries.

New astrometric binaries were found during the Hipparcos data reduction process and their orbits were determined or improved. Other astrometric binaries may however be detected by searching for a quasi-sinusoidal signaturein right ascension and declination. For this purpose the observations of the candidate stars have been combined at a few different epochs, using the Hipparcos astrometric intermediate data.

A simultaneous fit of the astrometric and orbital parameters has then been done. However in most of the cases other ground-based data is needed in order to confirm the suspected binarity and to give a reliable orbit.

The ESA Hipparcos astrometry satellite did not contain an imaging detector (such as a CCD) but used a modulating grid of alternating transparent and opaque bands to modulate the light from a given object. The Fourier coefficients of the modulated detector signal were the basis for the astrometric and photometric processing leading to the Hipparcos Catalogue. These coefficients are included (for a subset of the stars) in the CD-ROM version of the Hipparcos Catalogue as the ‘Hipparcos Transit Data’. In this work, the Transit Data are used in a completely different and novel application: the modulation components from a single scan are considered to be equivalent to the interference fringes from an interferometer oriented along the scan direction and having baselines of 0, ≃ 10 and ≃ 20 cm and a wavelength of 550 nm. Using all the scans collected over the mission lifetime, it is possible to use the complex visibilities (containing bothamplitude and phase information) calculated from the Transit Data to createaperture synthesis images of various type of objects such as visual or orbiting binary systems. More information, including sample images, can be found in: Quist C.F., Lindegren L., Söderhjelm S., 1997, Using HipparcosTransit Data for Aperture Synthesis Imaging, Hipparcos Venice ‘97 Symposium, ESA SP-402 (in press).

Prof. M. Grenon asked the team of the General Catalogue of Variable Stars to attribute GCVS names to new Hipparcos variables in order to present, in the Hipparcos Catalogue, the information on these stars already with final designations. We received lists of 5665 variables. Of them, 3157 qualified to the standard GCVS criteria and will appear in the 74th Namelist of variable stars.

The main problems in compiling the Namelist were the following:

1. 1. Cases of spurious variability for double and multiple stars and for misidentified stars (wrong colour indices used in reductions). Some wrong identifications come from the SIMBAD data base.

2. 2. Lacking identifications with catalogues of spectroscopic binaries. Some stars attributed to slowly pulsating B variables are in fact ellipsoidal variables. HIP 98954 = V1472 Aql was classified as a semiregular star, but it is a rather unusual, probably eclipsing, star (Samus, 1997), a known spectroscopic-binary M giant with a 200d period (Lucke and Mayor, 1982).

The Second Cape Photographic Catalog (CPC2) contains 276,131 stars covering the entire Southern Hemisphere in a 4-fold overlap pattern. Its mean epoch is 1968, which makes it a key catalog for proper motions. A new reduction of the 5687 plates using on average 40 Hipparcos stars per plate has resulted in a vastly improved catalog with a positional accuracy of about 40 mas (median value) per coordinate, which comes very close to the measuring precision. In particular, for the first time systematic errors depending on magnitude and color can be solved unambiguously and have been removed from the catalog. In combination with the Tycho Catalogue (mean epoch 1991.25) and the upcoming U.S. Naval Observatory CCD Astrograph Catalog (UCAC) project proper motions better than 2 mas/yr can be obtained. This will lead to a vastly improved reference star catalog in the Southern Hemisphere for the final Astrographic Catalogue (AC) reductions, which will then provide propermotions for millions of stars when combined with new epoch data. These data then will allow an uncompromised reduction of the southern Schmidt surveys on the International Celestial Reference System (ICRS).

We present a first attempt to compute an astrometric solution of POSS-IIplates using TYCHO stars as the reference catalogue. Also, we have made use of a special catalogue of stars observed with the Carlsberg Meridian Circlein La Palma between 1994 and 1995, brought into the HIPPARCOS reference system. Such a catalogue consists of a double strip along the equator, fairly dense (approximately 10 stars per square degree), faint (average magnitude 12.5), with an average error at epoch of ~ 150 mas per coordinate. Since the limiting magnitude of TYCHO stars is about 11.5, almost exactly one magnitude less than the CAMC special catalogues’ average, the latter has served as a very useful external comparison, although a more interesting magnitude range would have been 14-15. In this respect, we note that the Schmidt plates under study cover a large area (6.4 x 6.4 degree) and have fairly long exposures (~ 60 min); therefore, a possible magnitude dependence of the model needs to be carefully investigated. The results shown in the table above, which were obtained using a 2nd order polynomial in conjunction with a mask method using 5 plates only are very promising. The average external error over the entire plate area is about 0.2 arsecper coordinate, after the CAMC error have been subtracted.

We investigated the improvement of photographic astrometric observations of outer planets by use of Hipparcos Catalogue stars. The position of Jupiter on the plates was determined from measurements of the Galilean satellites, combined with ephemerides accurate to better than 10 mas. In a 6 contants model of photographic plates reduction, when images are near the center of the field, only offsets have to be computed, by considering the scale and orientation known. We applied this idea to the galilean satellites by using137 photographic plates realised between 1967 and 1974 by D. Pascu, with the Mc Cormick 26-inch refractor, Virginia, and its twin in the USNO, Washington DC. A study of the scale factors reveals an expected anisotropy of the scale factors. One possible hypothesis is that this effect is a consequence of the thermal expansions of the telescopic components. The use of Hipparcos data in the astrometric reduction allows to obtain an external error of around 40 mas on the determination of the observed absolute position of Jupiter. It is 5 times better than the classic computations using photographic or CCD reductions. Furthemore, with such results, badly-known effects can be easly estimated like the anisotropy of scale. At the end, important impacts on the fit of planetary ephemerides to the visual observations are expected.

The 3435 photographic plates, which were obtained by using the 40cm astrograph at Zô-Sè station of Shanghai Observatory during 1901-1993, have been accumulated for observing open clusters, extragalatic nebulas, radio stars and variable stars etc. These plates cover 764 unoverlapping regions (2°.3 * 1°.7) on the sky. Most of the plates distribute along the ecliptic, about 10 000 Hipparcos stars, 27 000 PPM stars and 500 FK5 stars are included in these regions. Among these plates, there are 2718 plates having two observing epochs, and they cover 359 different regions; there are 142 and 92 regions where the intervals of two epochs are about 50 and 70 years, and the numbers of plates in these regions are about1494 and 1023 respectively. About 372 plates were photographed before 1949,which play an important action for improvement of proper motions of Hipparcos stars. Four methods used to improve the proper motions for Hipparcos stars were designed. If the 2718 plates mentioned above having two epochs were used for improvement of the proper motions of Hipparcos stars, the stellar number of 299, 1378, 377 and 1664 corresponding to the four methods will be obtained with final precision 0.7 mas/yr. If supplementary observations are made in 1997, the stellar number of 494, 2431, 1534 and 2899 will be obtained with the same precision.

The external systematic accuracy of the HIPPARCOS positions and proper motions in the equatorial zone as represented by the subset of its bright FK5 stars is estimated by comparison with modern ground-based catalogues of 1980-1990s as well as with the most precise compiled catalogues made in the 20th century. Significant zonal and regional systematic differences between Hipparcos and ground-based catalogues have been detected for the different epochs ranging from 1920 to 1990. The most typical behaviour of the analysed differences at the epoch 1991.25 is almost absence of small-scale deviations and presence of the large-scale features. For the epoch 1920, both large and small-scale discrepancies are more pronounced reaching up to 50 mas. This gives evidence of the inferiour consistency of the HIPPARCOS system with ground-based observations comparing with the best compiled catalogues.

The optical astrometry observations of latitude/universal time variations made with 48 instruments at 31 observatories are used to determine the Earth orientation parameters (EOP) since the beginning of the century. The Hipparcos Catalogue is used to bring more than four million individual observations, made in the interval 1899.7-1992.0, into the International Celestial Reference System. The Earth orientation parameters (polar motion, celestial pole offsets and, since 1956.0, also universal time UT1) are determined at 5-day intervals, with average uncertainties ranging from 8 mas (in the eighties) to about 40 mas (in the forties). Making use of very long series of ground-based observations, the solution also leads to the improvement of proper motions of about ten per cent of the observed Hipparcos stars, with precision of ±0.2 — 0.5 mas/yr. In addition, 474 auxiliary parameters, describing the rheological properties of the Earth and seasonal deviations of the observations at contributing observatories, are found. The new solution provides the EOP series suitable for further analyses, e.g., for studying long-periodic polar motion, length-of-day changes or precession/nutation.

Obervations of the positions of Europa (J2) and Titan (S6) by Hipparcos, and Ganymede (J3) and Callisto (J4) by Tycho are analysed to give checks on the latest JPL ephemerides of the planets Jupiter and Saturn.

The observed positions of the satellites are compared with DExxx, using the G5 theory of the Galilean satellites and D.B. Taylor’s theory ofTitan to calculate their offsets from the barycentres of the two systems.

Each Tycho observation can be resolved into RA and Dec because the slitsare inclined to the scan direction. Hipparcos, on the other hand, which hasa uni-directional scan, requires a minimum of two (preferably orthogonal) scans to resolve the offsets into RA and Dec. We have subdivided the Hipparcos observations in time and solved for corrections to DExxx at several epochs over the period of the mission.

The Hipparcos observations of J2, and Tycho of J3 and J4, put tight constraints on the orbit of Jupiter, and agree closely with the series of ground-based observations made by the Carlsberg Meridian Telescope. In particular, they help to resolve a problem in the declination of Jupiter associated with larger than expected errors in the tracking of Voyager II and radio positions from the VLA.

The theory of stellar interiors is generally considered as well settled, and many branches of astrophysics rely on its results. However, the true situation is not so bright. Despite great successes at its beginning, in the sixties, it has to deal with many physical processes, generally poorly understood, which have large influences on the results and the derived quantities, like for instance ages.

After a rapid description of these difficulties, I will recall the majorquestions of the confrontation between theory and observations, and how HIPPARCOS can and will help. As an illustration, a few preliminary results will be presented, and future prospects foreseen. But, this short review is byno means exhaustive; there are many more results dealing directly or indirectly with stellar structure theory which have already been presented at theVenice Symposium (Perryman et al, 1997a), and many more, I am not aware of, will certainly appear soon.

An extensive work on the luminosity calibration of the HR diagram has been performed using the LM method based on the Maximum Likelihood estimation (Luri et al. 1996). The method uses all available information: Hipparcos parallax and proper motion data as well as radial velocities and some measured spectroscopic or photometric parameters related to luminosity. On the other hand, the method takes into account the censorship of the sample and the errors of the data. Finally, it is able to identify and separate in a given sample, groups with different luminosity, kinematical or spatial characteristics providing not only the corresponding luminosity calibration relationship, but also improved individual absolute magnitude estimates.

In order to asses how reliable the luminosity calibrations based on the MK spectral classification are, the LM method has been applied to a sample of B to K stars with MK classification selected from the Hipparcos survey. All known binaries, variables and spectroscopically peculiar stars were removed from the sample. The final sample contains 22054 stars. Our results show that the relation between absolute magnitude and luminosity class has a large intrinsic dispersion. As a consequence, the assignation to a star of a mean absolute magnitude given by a MK classification calibration is a rough procedure. It is preferable to use the individual absolute magnitudes estimated using all the available information for each star, like the estimates provided by the LM method (Gómez et al. 1997).

Early release data were obtained under the 1982 announcement of opportunity for about two dozen stars belonging to seven classes whose basic properties or evolutionary status were uncertain at the time. A good many remain so. Many of the stars were too faint and far for parallaxes to be statistically significant, though many of the proper motions are. Two papers presenting the data have been accepted by archival journals (Trimble and Kundu 1997, 1998). Both bear the same title as this Abstract The first deals with the R Coronae Borealis variables, the second with the other six classes (FK Comae Stars, CVs, runaway T Tauri stars, and so forth).

As a preparation to the HIPPARCOS mission, a large observing programme on NLTT stars (propermotion > 0.18 ″/yr) was started in Genevaphotometry. The original programme consists of 10047 stars brighter than mR = 11.5, or mR = 12.5 if of colour class m. Among them, 7813 targets could be included in the HIPPARCOS programme, selected according to their observability and internal priorities in favour of large parallaxe stars (photometric distances < 100 pc) and high-velocity stars. The bulk of new nearby, halo, mild-metal poor and SMR stars in the HIP Catalogue originates from this proposal (N° 139). No less than 208 new nearby stars with π ≥ 40 mas were discovered south of δ +10°, the closest has π(HIP)= 182 mas. Radial velocities were obtained with CORAVEL at OHP and ESO. Most aspects of the early evolution of the Galaxy may be addressed with this sample. Here we discuss, as examples, the ages of the thick disk and of the galactic bulge.

A major result of the Hipparcos mission is the availability of accurate parallaxes for a much-increased sample of nearby subdwarfs. On the basis of those data, both Reid (1997-R97) and Gratton et al (1997-G97) have used main-sequence fitting techniques to obtain new estimates of globular cluster distances, deriving significantly larger moduli (by 0.15 to 0.3 mag) for extreme, metal-poor clusters such as M92 and M15. These re-calibrations are not solely the result of the revised parallax data - both D’Antona, Caloi & Mazzitelli (1997 - DCM) and Reid show that similar results can be derived from pre-Hipparcos subdwarf data if the calibrations are not tied exclusively to HD 103095. The longer distance scale leads to a brighter inferred turnoff, and, combined with the latest models (DCM), ages of only 11 to 13 Gyrs - substantially younger than the 16±2 Gyrs proposed by Bolte & Hogan (1992).

Recently, Pont et al (1997) have suggested that these revised distances reflect systematic bias in the calibration, notably underestimation of subdwarf abundances. Their distance modulus for M92 ( (m-M)0=14.68) is closer to the Bolte/Hogan value (14.65 mag) than those derived byG97 (14.82 mag) or Reid (14.93 mag). While we believe that Pont et al overestimate the bias, particularly, where high-resolution spectroscopic analyses are concerned (G97, Reid, preprint), there is no question that significant uncertainties remain in the cluster distance calibration. Colours,rather than luminosities, are particularly vulnerable, and a mismatch of 0.01 in (B-V) translates to δ(m — M)0 ~ 0.05 mag.

The parallax data gathered by Hipparcos for field subdwarfs allow a muchmore precise determination of globular cluster distances by direct sequencefitting than was previously possible. We determined the distance and age ofthe old, representative globular cluster M92 from a set of more than 500 subdwarf candidates with Hipparcos parallaxes. Precise [Fe/H] values were derived using the equivalent width of the CORAVEL cross-correlation function.

Our best estimate of the distance of M92 is (m — M)v = 14.67 ± 0.08 (including binaries with a statistical correction) or (m — M)v = 14.74 ± 0.08 (classic treatment, i.e. excluding binaries). The agreement of the cluster sequence with the position of extreme metal-poor field subdwarfs is remarkable [figure, left]. The distance found is slightly higher than previously thought. The corresponding ages, derived by comparing the luminosity of the turnoff and subgiant-branchstars with up-to-date evolution models [figure, right], are 14±1.2 Gyr or 13±1.2 Gyr respectively, implying a minimum age of 13 Gyr for the Universe.

Other authors have claimed that larger distances, and smaller ages, resulted from Hipparcos subdwarf parallaxes. Although there are some differences in the data sets used by each author, the main difference resides in the treatment of systematic and selection biases. We have examined at some length the biases affecting the determination of the mean luminosity of a set of subdwarfs selected in the usual way. By means of Monte Carlo simulations, we show that selection biases act in a direction opposite to the classic Lutz-Kelker bias affecting parallax data, and that they can be dominant. The biases introduced are of the order of 0.1 mag.

It is pointed out that, now that the whole Hipparcos catalogue is available, we shall be able to greatly reduce the systematic biases by refining the fitting procedure. Firstly, by imposing no [Fe/H] limits and fitting the subdwarf locus for all [Fe/H] values at the same time. Secondly, by imposing no parallax error limit and fitting the data in parallax space instead of magnitude space (comparing observed parallaxes directly with parallaxes predicted from the models).

Wavelengths of stellar spectral lines depend not only on the star’s motion. Until recently, accurate studies of shifts not caused by radial motion were feasible only for the Sun. Solar lineshifts are interpreted as gravitational redshift (636 m/s) and convective blueshifts (~ 400 m/s; caused by velocity-brightness correlations). In other stars, such effects may be greater (Dravins & Nordlund 1990). Accurate astrometric radial velocities are now available from Hipparcos (Dravins et al. 1997a; 1997b), permitting studies of such shifts also in some other stars.

For such stars in the open clusters of Hyades, Ursa Major and Coma Berenices, a spectroscopic program is in progress, analyzing wavelength shifts in groups of lines with different strengths, excitation potentials, etc., using the ELODIE high-precision radial-velocity instrument (Baranne et al. 1996) at Haute-Provence Observatory.

Space astrometry now permits accurate determinations of stellar radial motion, without using spectroscopy. Although the feasibility of deducing astrometric radial velocities from geometric projection effects was realized already by Schlesinger (1917), only with Hipparcos has it become practical. Such a program has now been carried out for the moving clusters of Ursa Major, Hyades, and Coma Berenices. Realized inaccuracies reach about 300 m/s (Dravins et al. 1997).

Discrepancies between astrometric and spectroscopic radial velocities reveal effects (other than stellar motion) that affect wavelength positions of spectral lines. Such are caused by stellar surface convection, and by gravitational redshifts. A parallel program (Gullberg & Dravins 1997) is analyzing high-precision spectroscopic radial velocities for different spectral lines in these stars, using the ELODIE radial-velocity instrument atHaute-Provence.

After having studied in great detail the observational HR diagram (log Teff, Mbol) composed by 40 main sequence stars of the Hyades (Perryman et al.,1997, A&A., in press), we have tried to apply the same method to the observational main sequences of the three next nearest open clusters: Coma Berenices, the Pleiades, and Praesepe. This method consists in comparing the observational main sequence of the clusters with a grid of theoretical ZAMSs. The stars composing the observational main sequences had to have reliable absolute bolometric magnitudes, coming all from individual Hipparcos parallaxes, precise bolometric corrections, effective temperatures and metal abundances from high resolution detailed spectroscopic analyses. If we assume, following the work by Fernandez et al. (1996, A&A,311,127), that the mixing-lenth parameter is solar, the position of a theoretical ZAMS, in the (log Teff, Mbol) plane, computed with given input physics, only depends on two free parameters: the He content Y by mass, and the metallicity Z by mass. If effective temperature and metallicity of the constituting stars of the 4 clusters are previously known by means of detailed analyses, one can deduce their helium abundances by means of an appropriate grid of theoretical ZAMS’s. The comparison between the empirical (log Teff, Mbol) main sequence of the Hyades and the computed ZAMS corresponding to the observed metallicity Z of the Hyades (Z= 0.0240 ± 0.0085) gives a He abundance for the Hyades, Y= 0.26 ± 0.02. Our interpretation, concerning the observational position of the main sequence of the three nearest clusters after the Hyades, is still under way and appears to be greatly more difficult than for the Hyades. For the moment we can say that:

1. ‒ The 15 dwarfs analysed in detailed in Coma have a solar metallicity: [Fe/H] = -0.05 ± 0.06. However, their observational main sequence fit better with the Hyades ZAMS.

2. ‒ The mean metallicity of 13 Pleiades dwarfs analysed in detail is solar. A metal deficient and He normal ZAMS would fit better. But, a warning for absorption in the Pleiades has to be recalled.

3. ‒ The upper main sequence of Praesepe, (the more distant cluster: 180 pc) composed by 11 stars, analysed in detail, is the one which has the best fit with the Hyades ZAMS. The deduced ‘turnoff age’ of the cluster is slightly higher than that of the Hyades: 0.8 Gyr instead of 0.63 Gyr.

The ‘1996 Edition’ of the Catalogue of [Fe/H] determinations by Cayrel de Strobel et al. (1997, A&A S 124,1) and two recent papers by Castro et al. (1997, AJ Vol.114, N.1) and by Feltzing and Gustafsson (A&A in press) have made possible to increase in the theoretical HR diagram (log Teff, Mbol the number of SMR stars.

The SMR Stars of this new enlarged sample had to have reliable absolute magnitudes, coming all from Hipparcos parallaxes, precise bolometric corrections, effective temperatures and metal abundances from high resolution detailed spectroscopic analyses.

With the help of an appropriate grid of isochrones computed by Lebreton (1997, Perryman et al. A&A, in press), ‘turn-off ages’could then be attributed to the slightly evolved stars (subgiants) of the sample. The (log Teff, Mbol) diagram constituted by the new sample of SMR stars, shows that the conclusions in a former paper by Cayrel de Strobel (1987, A&AJ 8,141) remain valid: the SMR stars areold stars in spite of their higher than solar metallicity. The result, that metal-rich stars were in the mean old stars, was interpreted in the 1987 paper as due to a more chemical uniformity of the nowadays interstellar medium of the Galaxy with respect of the older much more active interstellar medium.