We use the CCD cameras of the 61-cm UTSO and the 90-cm Dutch telescopes of the Las Campanas and European Southern Observatories respectively in order to perform UBV photometry of visual double stars.

Our sample contains southern visual binaries with A–type (470 pairs) and G–type primaries (170 pairs), which have angular separations mainly between 1″.5 and 5″. The double stars of our sample have been selected from the WDS according to their astrophysical interest and the technological limits of contemporary CCDs.

Owing to the spherical aberration of the Hubble Space Telescope, prospects are now poor for the astrometric detection of low–mass companions of nearby stars with the Wide–Field / Planetary Camera. The installation of a new camera (WF/PC-II) incorporating an optical correction for the telescope will only partially recover the low–mass companion search capability.

After briefly describing the operation of the Hubble Space Telescope Fine Guidance Sensors (HST–FGS) in the Transfer Function (TF) Scan mode, we discuss the reduction and analysis of scan data affected by HST jitter and Optical Field Angle Distortion (OFAD). We present relative positions and magnitude differences for the components of ADS 11300 = Hu 581 = WDS 18229+1458 derived from TF scans obtained on 17 February 1992. Residuals from a newly revised orbit provide a first indication of the accuracy of HST–FGS observations in the Transfer Function Scan mode.

We present an overview of the ST ScI calibration program and the reduction, smoothing, and analysis capabilities we have developed to deal with Transfer Functions. The Transfer Function (TF) is the name for the fringe visibility curve produced by the Koesters prism interferometer in the Fine Guidance Sensors (FGSs). The TF for a single, point, monochromatic source becomes modified — in different ways — when we observe a real source with a typical stellar energy spectrum, a finite disc, or a double system. Thus, we have constructed an in depth calibration program and designed and developed a comprehensive set of TF analysis algorithms. The ST ScI calibration program includes the acquisition of reference TFs at 19 different places in the astrometer FGS (there are field–dependent aberrations beyond the well–publicized spherical aberration of the primary mirror). In addition, for Cycle 2, both stars with finite angular diameters and minor planets will be observed. The software to implement our correlation–based analysis techniques has been extensively tested on artificial binary stars created by combining real TFs and re-introducing photon noise. A wide variety of synthetic binaries — with separations from 10 to 135 milli-arcseconds and magnitude differences from 0 to 4.2 mag — were used so that we can objectively and quantitatively characterize the capability of both the software and the instrument to resolve close pairs and to detect faint companions. Separations to a few milli-arcseconds, position angles to , and magnitude differences to are attainable for primaries brighter than V = 13 mag and Δm < 3 mag. The software is being incorporated into IRAF/STSDAS at the ST ScI.

To better define the global properties of X-ray binaries in the Large Magellanic Cloud, a program of optical identification has been initiated, combining improved X-ray positions from ROSAT with photometry and spectroscopy obtained at CTIO. This initiative is an extension of work by Cowley et al. (1984) and Crampton et al. (1985), who attempted to identify the optical counterparts of “CAL” X-ray point sources found in the Einstein IPC survey of the LMC conducted by Columbia Astrophysical Laboratory (Long et al. 1981). A key element in our investigation of the remaining unidentified sources is the use of significantly more accurate coordinates (typically ±5″ from ROSAT HRI data versus ±30″ for the older IPC positions), thereby greatly reducing the number of stars needing examination. Likely candidates are selected using multicolor photometry (by location in color–color plots or through variability) prior to spectroscopic observing. Although the identifications for CAL 9 and CAL E were reasonably secure (in particular the latter since it had an Einstein HRI position, see Crampton et al.), these sources have been re-observed to test our procedures and to verify the previous results.

Data from the Einstein X-Ray satellite continue to provide useful information for studies of x-ray emitting objects. The Einstein Medium Sensitivity Survey (EMSS), a tabulation of serendipitously discovered point sources from the Einstein database, included a number of objects which were identified, on the basis of spot sampling of optical spectra, as likely binary star systems (Fleming 1988, Silva et al. 1987; Takalo & Nousek 1988). Because the sample is limited primarily by X-ray flux, the physical characteristics of these stars are of considerable interest for understanding the origins of stellar activity among cool stars.

The International Ultraviolet Explorer (IUE) was launched on 26th January 1978 and is still fully operational today, with several more years hopefully to come. After six months, the fully–reduced data is consigned to public–access archives maintained by the project agencies (NASA, ESA, and SERC). Thus, in addition to observations from current and future programmes, there are ~12,000 high–dispersion (Δλ ~ 0.15Å) spectra readily available now for research on stars hotter than about mid-B type. Furthermore, a uniform reprocessing, with optimum schemes, of all past IUE images has begun, to create the Final Archive which will be made accessible on–line through optical disk storage systems and will ensure the value of IUE data well into the future.

The HIPPARCOS Input Catalogue comprises about 118,000 entries of which 16,000 are flagged as related to double or multiple stars. Data analysis begins by fitting Fourier model signals to the observations: the intensity coefficients and the phases at several locations on the spatial modulator are estimated and a first rough recognition of possibly unknown non-single stars is performed. After calibration corrections, a normal point is produced at every transit of a star across the field of view of the telescope and the improved parameters are stored for extensive multiplicity testing and relative astrometry when enough scans are accumulated.

After the processing of one year of observations carried out by HIPPARCOS it is possible to provide quantitative results as to the number of new double and multiple stars to be detected and the real capacity of this mission to perform relative astrometry on double stars. We present and discuss the methods developed to this end and include the first results concerning the detection statistics and the determination of separation and position angle for double stars. About 16,000 stars have been recognized as non-single, including 9,000 already known as double and mutliple before the mission. Also, a subset of 10,500 stars have been successfully solved for their relative coordinates with an accuracy in the range of 3 to 10 mas.

The HIPPARCOS satellite observes 118,000 stars from a predefined list. Some 10% of these are known doubles or multiples, and another 2—3% may ultimately be found to be non-single with separations above 0″.l and magnitude–differences below 4–5. This paper describes briefly the special reduction methods used by NDAC (Northern Data Analysis Consortium) for treating these non-singles. A key feature is the use of the main reduction results to calibrate and collect the data for individual doubles in ‘case history files’. This enables a global solution of both absolute and relative parameters for a double to be carried out in a single step. Using provisional data tapes covering small parts of a 14-month interval, tests have been made of major parts of the double star reductions. These early results are compared with independent HIPPARCOS data and with ground–based speckle interferometry.

In addition to its astrometric capabilities the HIPPARCOS main detector proves to be a good phototometer. The main features of the photometric reduction applied to double stars are outlined. We show how it is possible to discriminate single stars from multiple stars from the photometric signal. Results presented are based on about 16000 stars recognized as non-single, of which 8000 were not known to be double in the HIPPARCOS Input Catalogue. Magnitudes for multiple systems as a whole are derived with a precision usually better than 0.005 mag while the magnitude of each component is recovered up to magnitude difference of 3 mag, but with less accuracy and precision.

The Tycho project will provide positions and (B, V) magnitudes for about 1 million stars, and will be complete for stars brighter than magnitude (B + V)/2 = 10.5. The components of double stars will obtain individual measurements when the separations are wider than 3″.

Now that the HIPPARCOS satellite safely revolves on its “degraded” orbit — after the exploit of the ESA technicians to rescue it from the faulty ignition of its apogeum boost motor — and regularly provides the information expected for reaching the initial aim of the project, one may turn to examine the possible exploitation of the final HIPPARCOS Catalogue.

In the present paper, we describe how double and multiple stars have been introduced in the Input Catalogue and what may be expected from their observation. We also try to evaluate how much the final HIPPARCOS data may influence future researches on these celestial bodies and what kind of ground–based observation programmes should be conducted in order to support the space results.

This paper outlines the details of astrometric parameter determination of double stars from HIPPARCOS observations as prepared in the FAST consortium. Methods of parameter estimation are developed for relative astrometry and absolute astrometry. The capability of the procedures will be demonstrated using simulated observations, in describing the particular steps of a Monte Carlo simulation which have been executed at the Istituto di Astrofisica Spaziale (IAS) and at the Astronomisches Rechen–Institut (ARI).

The immediate object of this effort is the first systematic investigation of the guide star catalog as a new source list for the discovery of possible new visual binary stars. In this first paper of the series we detail on the way used to establish the properties of the catalog, i.e. techniques adopted to measure its completeness as a function of angular separation, magnitude of the primary, and magnitude differences among the components. We have employed the CCDM catalog as the source list of “certified” visual double and multiple stars. The CCDM stars were first matched (rediscovered) in the Guide Star Catalog and then used to build sufficient statistics to establish the relations among the parameters above. These calibrations are essential to properly correct the two– and three–point angular correlation functions, which are our fundamental tools for establishing lists of new probable visual binary and multiple stars.

The study of double stars has since long been recognized as a basic key to the understanding of star formation and stellar evolution. Moreover, close visual double stars have always been systematically neglected in photometric observational programmes although they contain an important part of physically associated systems. It is then timely to organize major observational programmes of these objects for a number of good reasons:

1. 1. The frequency of double stars is continuously reviewed and the rate of their detection is steadily increasing — both from ground–based and space observations — in such a way that they no longer can be discarded in any models of galactic structure;

2. 2. Space observations (HIPPARCOS, HST) significantly improve the quality and the importance of stellar samples and permit to better take into account some of the selection effects;

3. 3. The high–quality astrometric (and also photometric) data that will be provided for such systems by the space observations should be matched with accurate and homogeneous complementary astrophysical information such as colour indices and spectral classification. Such information for close visual double stars is unfortunately almost non-existent but are now being more easily accessible with the use of CCD detectors.

The purpose of my talk is to briefly review the results of two decades of astrometric research using the technique of speckle interferometry. Although speckle, invented by Labeyrie in 1970 (Gezari et al. 1972), is the most well–known and widely–used interferometric technique in the visible and near infra-red, it neither the only technique in use, nor was it the first. Karl Schwarzschild made the first interferometric measurements of binary stars in 1895, using the then–new technique of Michelson interferometry and following a suggestion by Michelson himself that his technique was amenable to binary star astrometry. From 1919 to 1921, Anderson (1920) and Merrill (1922) made a series of measurements of Capella and other bright stars, using a Michelson interferometer of 20-foot baseline, mounted on the Mount Wilson 100-inch. These early measurements have definitely stood the test of time. Figure 1 shows a recently calculated orbit of Capella (Bagnuolo & Hartkopf 1989), using data spanning some 250 full revolutions. Anderson and Merrill’s measurements agree beautifully with observations made 7 decades later. As a result, Capella has probably the most accurately known orbit of any visual binary. Orbital elements calculated in 1981 (McAlister 1981) and again 10 years later differ in period by about 14 seconds, or less than 2 parts per million!

During the last 15 years more than 9,000 speckle interferometric measurements of binary stars have been collected using large optical telescopes (McAlister & Hartkopf 1988). Among them a significant contribution to the world speckle data has been made by the 6-m telscope near Zelenchuk. Up to now this instrument provides the maximal spatial resolution for single–aperture telescopes. First speckle images of the binary Capella were recorded at the telescope in 1977 (Balega & Tikhonov 1977), but we spent 5 more years to create special television techniques for photon counting and digital means for image processing before we started the regular interferometric program of binary observations in the wide range of stellar magnitudes. At first, the measurements were conducted in cooperation with French astronomers from the Centre d’Etudes et de Recherches Geodynamiques et Astronomiques using the optical camera and the television detector developed by Blazit et al. (1977). Since 1983 our equipment has been in use. The program of observations was oriented upon the traditional problems of multiple star speckle interferometry:

1. 1. Determination of stellar distances and masses for different types of binaries whose orbital elements can be derived. This includes already known fast visual and astrometric pairs with undetermined orbits, spectroscopic binaries that can be resolved directly, and newly discovered interferometric pairs which show fast orbital motion. The main attention was devoted to the late–type dwarfs in the vicinity of the Sun.

2. 2. Search for the secondary components whose existence could explain anomalies of stellar spectra or photometry (stars with composite spectra, occultation binaries, etc.)

3. 3. Study of unusual binaries (symbiotic stars, binaries with relativistic components, such as SS 433, etc.)

Differential Interferometry (DI) is the application of high angular resolution interferometric techniques to objects observed simultaneously at different wavelengths. When applied to unresolved double stars it makes it possible to measure the variation of the object photocenter with wavelength, which yields angular and spectral information well beyond the classical resolution limits. Signal–to–noise ratio analysis shows that, if DI experiments are limited by photon and speckle noise, the technique can be applied to a large number of double systems for the measurement of vectorial angular separations and of radial-velocity differences. With 4-m telescopes, there are a few tens of binary systems for which DI should permit the resolution and the measurement of the position angle of the rotation axis of at least one of the components. A preliminary experiment permitted a high SNR resolution of the double star Capella with a 1.52-m telescope and showed the current limitations of DI performances resulting from an imperfect correction of detector geometrical distortions.