Baltic Astronomy, vol. 10, 481-587, 2001. KINEMATICS OF HIPPARCOS VISUAL BINARIES. I. STARS WITH ORBITAL SOLUTIONS * A. Bartkevicius and A. Gudas Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania Received June 15, 2001. Abstract. A sample consisting of 570 binary systems is compiled from several sources of visual binary stars with well-known orbital elements. High-precision trigonometric parallaxes (mean relative er- ror about 5%) and proper motions (mean relative error about 3%) are extracted from the Hipparcos Catalogue or from the reprocessed Hipparcos data. However, 13% of the sample stars lack radial ve- locity measurements. Computed galactic velocity components and other kinematic parameters are used to divide the sample stars into kinematic age groups. The majority (89%) of the sample stars, with known radial velocities, are the thin disk stars, 9.5% binaries have thick disk kinematics and only 1.4% are halo stars. 85% of thin disk binaries are young or medium age stars and almost 15% are old thin disk stars. There is an urgent need to increase the number of the iden- tified halo binary stars with known orbits and substantially improve the situation with their radial velocity data. Key words: stars: binaries: visual, kinematics - Galaxy: popula- tion - orbiting observatories: Hipparcos 1. INTRODUCTION The importance of the investigation of binary stars for under- standing stellar formation and evolution is well-known and described by many authors (cf. Duquennoy & Mayor 1991, Larson 2000). Bi- nary stars are the only source for the direct determination of stellar * Based on the data from the Hipparcos astrometry satellite, ESA 482 A. Bartkevicius, Α. Gudas masses and the mass-luminosity relation. The frequency of binary stars, distributions of their orbital elements, correlations between themselves and other stellar parameters tell us important informa- tion about stellar birth conditions and dynamical evolution of the Galaxy. Due to the relative faintness and rarity of Population II stars in the solar neighborhood, visual binaries of this population are poorly investigated. Catalogues or lists of this type of binaries are not avail- able. Some information is presented in the Population II star Cat- alogs (Bartkevicius 1980, 1984; Bartkevicius & Bartkevicienè 1993) and the rest is scattered in separate publications. Only quite re- cently Allen et al. (2000) presented a catalogue and kinematical investigation of 122 high velocity metal-poor wide binaries. The number of publications concerning kinematics of visual bi- naries is very limited, especially for the stars with known orbits. We can only mention Eggen's (1959) note analyzing the distribution of several dozens of these stars in the plane of the galactic space veloc- ity components U, V and W, V. Eggen (1965) also calculated space velocity components for 228 visual binary stars with known orbits and investigated the dependence of distribution of stars in the U, V plane on the ultraviolet excesses 5(U — B). Kulikovskij (1956) and Kurochkin (1978) showed that velocity dispersion increases from early- to late-type wide binaries, as well as for single stars. Saadat (1961) and Bakos (1974) investigated the relation between velocity dispersions and the age of visual dou- ble stars. Starikova (1981) analyzed the kinematical characteristics of visual binary stars from the Fundamental Star Catalogue FK4. Poveda et al. (1994) found total space velocity dispersion of 35.2 km/s for the young nearby wide binaries and 53.7 km/s for the old ones. Only one publication (Allen et al. 2000) is completely devoted to the kinematics of Population II visual binaries. There galactic orbits for 122 high-velocity metal-poor wide binaries are calculated, and a population group is assigned to each. ESA's astrometric satellite Hipparcos (ESA 1997) provided us with invaluable material for the investigation of binary stars. Not only very precise trigonometric parallaxes and proper motions for many known visual binaries were obtained, but also many new bi- naries were detected. Some new orbital binaries were detected, as well as orbital, astrometric and photometric parameters of known binaries were substantially improved by reprocessing the Hipparcos Kinematics of Hipparcos visual binaries. I. 483 Transit Data (TD) and combining them with existing ground-based speckle interferometry and visual observations (Soderhjelm 1999), combining the Transit and Intermediate Astrometric Data (IAD) with the spectroscopic orbit data (Pourbaix & Jorissen 2000) and combining the Hipparcos Catalogue with many ground-based astro- metric catalogues (Gontcharov et al. 2000). Some time ago the investigation program of the Hipparcos Pop- ulation II single and binary stars was undertaken by Bartkevicius (1994) using the multicolor Vilnius photometric system (Bartkevicius & Lazauskaité 1997) and Coravel-type radial velocity speedometers (Bartkevicius & Sperauskas 1999, Sperauskas h Bartkevicius 2001). This publication is the first from a series devoted to the investiga- tion of old visual binaries possessing Hipparcos measurements. Stars with known orbits are analyzed. Binaries having old disk or halo kinematical characteristics are picked out. 2. SELECTION OF STARS Primarily, 235 stars with orbital solution from the Hipparcos Catalogue Double Star Annex DMSA/O (ESA 1997) are included into the sample. The main sample is supplemented with the fol- lowing: (1) 200 systems from Soderhjelm (1999) where improved orbital elements by combining Hipparcos astrometric measurements with ground-based binary observations are given, (2) 40 double- lined spectroscopic binaries which are ground-based visually resolved (VB- SB2), with orbital elements obtained by fitting astrometric and spec- troscopic data simultaneously (Pourbaix 2000), (3) 23 Ba, CH and Tc-poor S stars - spectroscopic binaries with improved orbital and astrometric parameters, obtained by combining the orbital parame- ters of spectroscopic binaries with Hipparcos TD and IAD (Pour- baix & Jorissen 2000), (4) 187 stars, selected by Martin & Mignard (1998) and Martin et al. (1998) for the determination of mass from the Hipparcos data, (5) 95 binary systems from Gontcharov's et al. (2000) Catalogue of Astrometric Binaries (ABC1). Finally, the catalogues of metal-deficient stars MDSP, MDSPSl (Bartkevicius 1980, 1984) and the Population II star catalog (Bartkevicius & Bartkevicienè 1993) are scanned and a few systems are added. Af- ter removing the redundant cases, 570 binaries remain in the final sample. The selected stars are listed in Table 1 in which the columns give the Hipparcos number, the magnitude V and the color index B-V 484 A. Bartkevicius, Α. Gudas for the combined light of the system and spectral types taken from the Hipparcos Celestia Catalogue (ESA 1998). For metal-deficient stars, the spectral type is taken from the MDSP catalog. The next column gives the source of the star selection and the last column gives the kinematical population classes of stars, which will be discussed below. 3. DATA 3.1. Parallaxes Trigonometric parallaxes are taken from the Hipparcos Celes- tia Catalogue (ESA 1998) and from the improved Hipparcos results (Soderhjelm 1999, Pourbaix k Jorissen 2000, Martin h Mignard 1998, Mignard et al., 1998, Fabricius & Makarov 2000, and Falin & Mignard 1999). For 8 stars with σπ/π> 0.25, spectroscopic paral- laxes are also used (Table 2). They where determined from absolute magnitudes My, intrinsic color indices (B-V)o, color-excesses Eß-v and extinctions Ay derived from MK spectral classes, V and B-V values according to the Straizys (1992) tabulation. Standard errors for these parallaxes are estimated adopting OMv— 0-7. Individual values of the parallaxes with their standard errors (in mas= 0.001 arcsec) and the data sources are listed in Table 3. The weighted mean parallaxes and the standard errors of the mean from Ν sources, are listed in Table 4. The distributions of the mean values of parallaxes and their relative errors are shown in Figs. 1 and 2. It is evident that the parallaxes of the selected sample are of very high precision. Their mean relative error is about 5%, and the precision of 96% of the sample stars is better than 20%. 3.2. Proper motions Proper motions are taken from the Hipparcos Celestia Catalogue (ESA 1998) and the Tycho-2 Catalogue (H0g et al. 2000). We have not used the Hipparcos proper motions for 34 stars having stan- dard errors of proper motions or their equatorial components larger than 20 mas. The distribution of differences between Hipparcos and Tycho-2 proper motions is shown in Fig. 3. The individual values of proper motion components with their standard errors and a source code are presented in Table 5. The weighted means and their errors Kinematics of Hipparcos visual binaries. I. 485 200 Ν 150 100 50 0U ' ^ • ' I r-r-H , _ 0.00 50.00 100.00 150.00 200.00 π (mas) Fig. 1. The mean values of the parallaxes. Values for 9 stars with 7Γ > 200 mas are not shown. are listed in Table 6. The distributions of proper motions and their relative errors are given in Figs. 4 and 5. Despite a few deviating cases, the whole material is of high precision. The average relative error is about 3%, and 96% of the stars have accuracy better than 15%. 3.3. Radial velocities Radial velocities for 497 stars are taken from the General Cata- logue of Mean Radial Velocities (Barbier-Brossat & Figon 2000), Bib- liographic Catalogue of Stellar Radial Velocities (1991-1994) (Malar- oda et al. 2000), and Bartkevicius (2000a) compilation of radial velocities of Population II and late-type stars for the period of 1995- 2000. The remaining 73 stars lack radial velocity determinations. Unfortunately, Malaroda et al. do not give standard errors of in- dividual measurements, and Barbier-Brossat & Figon sometimes in- stead of a standard error give its quality class. This quality class is converted into the standard error using the precepts from Wilson's Radial Velocity Catalogue (1953). If no standard error or quality class is given, the standard error is ascribed according to the num- 486 A.