Introductory Lecture W. G L I E S E and H. J A H R E I S S
Astronomisches Rechen-Institut,Heidelberg,F.R.G. Summary. Thirty-six percent of the 1641 visual components nearer than 22.5 parsecs listed in 1969 were members of systems. Meanwhile for an extension to 25 pc of the Sun about 2000 additional objects are found, however, only 22% of them are known today as members of systems. Angular separations are observed from a few tenths of an arcsec to more than one degree. The widest linear separations at the sphere seem to be larger than 105 astronomical units. The observed frequency distribution of the projected separations for all nearby stars is given and compared with the frequency distribution of more complete subsamples, The 32 presumable systems with the widest separations are discussed with respect to their astrometric and spectroscopic properties.
1. Introduction At present, we are compiling stars and their astrometric, spectroscopic, and photometric data for a 'Third Catalogue of Nearby Stars' which will be extended to 25 parsecs of the Sun. The predecessor (Gliese, 1969) had a distance limit of 22.5 pc; the Catalogue of Stars within Twenty-Five Parsecs of the Sun (Woolley et al., 1970) was restricted mainly to objects with k n o w n trigonometric parallaxes. Meanwhile, van Altena (1985) generously has made available to us a preliminary version of the forthcoming Yale Catalogue of Trigonometric Stellar Parallaxes. Furthermore, spectroscopic data and numerous photometric series became k n o w n to us. Spectral type-luminosity relations and colour-luminosity relations, partly derived by the observers, partly calibrated at Heidelberg allowed the detection of many new nearby members. Particularly the number of faint red dwarf stars has been increased significantly. In spite of this fact one has to bear in mind that still severe selection effects are present already within 20 pc; and an extension of the distance limit from 20 to 25 pc is equivalent to a doubling of the volume of space to be considered.
2. The Relative Binary Frequency In 1969 the relative binary frequency, i.e., the number of components in double or multiple systems relative to the number of all stars considered was - by 61 stars within 0 - 5 pc 59~o, - by 221 stars within 5-10 pc 51%, - b y 1071 stars within 10-20 pc 41~o.
* Presented by W. Gliese.
Astrophysics and Space Science 142 (1988) 49-56. 9 1988 by KluwerAcademic Publishers.
W. GL1ESE AND H. JAHREISS
In these figures spectroscopic and probable astrometric binaries were included. The corresponding percentages are at present the following - by 72 stars within 0-5 pc 63 ~o, - by 258 stars within 5-10 pc 50~o. We hesitate to give a guess for the stars beyond 10 pc, since our compilation is not yet completely finished with respect to radial velocities and other spectroscopic data. Furthermore, the results of several programmes investigating the percentage of spectroscopic binaries among the nearby K and M dwarf are not yet available. In a preliminary report Halbwachs (1985) expects an increase from 4 to 15~o detected spectroscopic binaries among the nearby K dwarfs. For comparison the corresponding relative binary frequency for a 'complete' and well investigated subsample of the LHS (372 stars with # >__1" per year, b > 30 o and Pbl --- + 10 ~ amounts to only 30 ~o. But, one has to recall that on the scale of the Palomar plates separations of less than 10" may cause detection problems (Dawson, 1986). In 1969 we listed 1328 singles and systems with 1641 visual components; in 1979 these numbers were supplemented to altogether 1594 objects with 1972 components. Then 36 ~o of the 1641 stars were members of binary or multiple systems, however, only 22~o of the about 2000 objects added since 1969 are known as members of systems. This is a selection effect as the already known nearest neighbours of the Stm were preferred on observational programmes, particularly programmes for separating very close binaries. Angular distances of a few tenths of an arc sec to more than one degree are detected. 3. Wide Binaries Distances between the components can be expressed in astronomical units (AU) if the parallax of a system is known. Of course, in many systems the components are as close as planets to our Sun; the semi-major axis of the Pluto orbit is 40 AU. But detected were also separations of 100, 1000, or even 10 000 AU between components. Such values are measured as tangential distances at the sphere but we should like to know the real space separation. For the estimation of the radial distance between the components we need the knowledge of the parallaxes of the objects supposed to be gravitationally bound. The problem hereby is the reliability of parallax results. One parsec is equal to 206265 AU. At the catalogue limit a parallax error of 4~/o corresponds to a linear difference of one parsec and this accuracy is attainable today only in a few exceptional cases. As an example with favourable conditions we have our nearest neighbours, the triple system Alpha Centauri A, B and Proxima Cen, obviously a wide multiple system about 1.3 pc away from the Sun. The tangential separation of more than 2 ~ is slightly more than 10000AU. The difference between the two trigonometric parallaxes is 0':0218 + 0':0062 which corresponds to a separation of 7800 AU in the radial direction. In space, Proxima is nearly 13 000 AU apart from Cen A and B, an estimate with a standard error of _+2200 AU. In all other cases the uncertainties in the trigonometric measurements do not allow a reliable determination of separations in the radial direction.
THIRD CATALOGUE OF NEARBY STARS
Therefore, search for wide binaries means a search for common proper motion (c.p.m,) objects. Luyten's N L T T volumes (1979, 1980) listing the stars with yearly proper motions > 0': 18 proved to be one of the best sources for such objects in the solar neighbourhood. Let us insert here a few words of gratitude to W. J. Luyten for his immense number of proper motions measurements which include the large majority of known stars within 25 parsecs of the Sun; 0': 18 per year corresponds to 21 km s - 1 at the limit of our catalogue. Today photometric programmes for N L T T stars are going on fielding data by which many more faint nearby objects are detected. Of course, c.p.m, alone does not decide on gravitational bounding of two stars. Large differences between the parallax data are in contradiction to such a connection. Equal tangential velocities at different distances of the Sun require smaller proper motions with decreasing parallaxes. How to define a lower limit of separations for the term 'wide'? From column 2 of Table I one can deduce a nearly monotonic decrease of the observed frequency distribution of separations with increasing astronomical units. Already Dommanget (1964) had mentioned that the distances between the components are uniformly distributed on a logarithmic scale between a few AU and a few thousand AU. Therefore, there is no distinct limit from which onwards systems should be called 'wide' as a separated class.
4. The Distribution of the Projected Separations In addition to the frequency distribution of the projected separation for all known visual binaries within 25 pc in column 2, Table I gives the corresponding values for the stars TABLE I Frequency of nearby visual binaries with respect to different separations (in AU) between their components AU
Fig. 1. The distribution of projected separations of nearby visual binaries versus logr (AU). Stars within 25 pc (full line); normalized to the latter, stars within 10 pc (dashed line); LHS selected sample dasheddotted line).
within 5 pc, 10 pc and for the stars in the LHS subset in columns 3, 4, and 5, respectively. Likewise, Figure 1 shows the distribution of the 25 pc sample together with the normalized distributions of the 10 pc sample and the selected LHS sample. Compared with the other frequency distribution the 25 pc sample reveals a strong deficit of separations less than 20 AU and two peaks at 20-50 AU and 200-500 AU seem to be indicated. Whereas the 5 pc and 10 pc sample which should be more complete show only the peak at 20-50 AU. The latter is in accordance with Heintz (1969) (the omitted correction loga = logr+ 0 . 1 - l o g n would only slightly change our distribution function) and other finding (see for example, Retterer and King, 1982). Yet, the second maximum is also indicated in the selected LHS sample, i.e., high-velocity stars. So, one may speculate on the presence of a bimodal frequency distribution for the separation of nearby binaries composed by two unimodal functions for low-velocity and high-velocity stars. However, taking into consideration the numbers of objects with different separations in this sample the observed fluctuations along the frequency function must not be overemphasized. 5. The Widest Pairs Our nearby-star material contains 32 systems with projected tangential separations exceeding 5000 AU between their components. They are listed in Table II in order of
THIRD CATALOGUE OF NEARBY STARS
T A B L E II Wide binary and multiple systems in the solar neighbourhood G1, GJ Name, D M
Separation angular linear
Trig. parallax s.e.
1003 1034 331 A ( S B ) , B , C 332 A, B 48 22A, B 1255 A , B , C 199476 549 A , B 549 C 395,394(RVv) +57 ~ 881 879 +75~ +75~ 803 799A, B 154.2(SB) CPD-65~ 629.2A 629.2 B 586A, B 586 C 288 A 288 B 767.1 A, B 765.4 A, B 106.1 A , B 106.1 C UMaA(SB),B(SB) 80 U M a SB 161.1 160.1A, B 615.2A(SB),B 615.2 C 100A, B 100 C 202 201 684A, B 685 559 A, B 551 194 A, B 195 A , B 216A, B VB 1
decreasing linear separations which were c o m p u t e d from the angular separation and the parallax o f the primary. These objects were cited in the literatur as pairs with c o m m o n (or nearly c o m m o n ) p r o p e r motion. In some o f these cases, however, it is very questionable whether or not such two stars a r e really gravitationally bound. E a c h o f these a p p a r e n t systems needs a particular examination. The first entry, the two red dwarfs G J 1003/1034 separated by m o r e than 16 ~ certainly d o riot form a physical system. In other cases the parallax values do n o t agree: G1 549 A B - C , G1 586 A B - C , G1 3 9 5 / 3 9 4 / + 57 ~ 1266, and p e r h a p s GI 161.1/160.1, or the differences in the o b s e r v e d radial velocities (RV) are r e m a r k a b l y large: G1 331/332, + 75~176 G1 1 5 4 . 1 / - 65~ Figure 2 shows a c o l o u r - m a g n i t u d e - d i a g r a m o f the widest pairs. F r o m Table I I only objects with reliable absolute magnitudes M v (s.e. ( M y ) < 0.'30) are plotted. E a c h system is represented by two dots: the p r i m a r y and the wide c o m p a n i o n c o n n e c t e d by a straight line (a b r o k e n line m e a n s a c o m m o n p r o p e r m o t i o n pair which p r o b a b l y is not gravitationally bound). Small dots m a r k single c o m p o n e n t s . However, m a n y o f the primaries a n d a few o f the distant c o m p a n i o n s form themselves a close double or even multiple system. Such objects are shown as big dots ( 0 ) . C o l u m n 4 in Table II gives the total n u m b e r o f objects in the wide systems including also k n o w n spectroscopic binary (SB) c o m p o n e n t s . W e learn from Figure 2 that the majority o f the widest systems is c o m p o s e d o f more than two stars favouring the region of the M a i n Sequence from A to G , mostly as primaries. In addition to objects o f luminosity class V we see a pair o f yellow giants in the u p p e r part o f our diagram: Capella A a n d B a c c o m p a n i e d by a pair o f red dwarfs. Below the
THIRD CATALOGUE OF NEARBY STARS
15 0.0 Fig. 2.
Colour-luminosity diagram of the widest pairs in the solar neighbourhood. Explanation: see text,
Main Sequence there is the wide pair of K subdwarfs - 15 ~ (G1 579.2) with large radial velocities of about + 300 km s - 1. Van Biesbroeck 3 (GI 288B) was recently detected to be a low luminosity white dwarf star (Kunkel et al., 1984). The diagram contains one further white dwarf - 7 ~ with a slightly fainter red companion Ross 476 (G1 515, 514.1). A systematic search among the selected L H S sample for up to now unknown c.p.m. pairs yielded one possible candidate: L H S 228 and L H S 231 with an angular separation of 252.8 arc min have almost identical proper motions 1'.'123 in 12270 to 1':104 in 122.~8. For L H S 228 exists photometry (V = 15.47, B - V = 1.73) and a trigonometric parallax re, = 0':0181 + 0':0034 from U S N O , whereas we have not such data for L H S 231 (mR = 15.4, mpg = 17.6, colour class m). If the combined parallax 0': 022 (from rct and B - V) for L H S 228 should also be valid for L H S 231 a projected separation of 630 000 A U would be obtained. 6. Conclusions The 32 widest (larger than 5000 AU) common proper motion systems known today within 25 pc of the Sun are listed in Table II. Separations between 2000 and 5000 A U are observed for 33 additional systems and linear separations at the sphere between 1000
w . GLIESE AND H. JAHREISS
a n d 2000 A U are f o u n d for 49 systems. It w o u l d be o f i n t e r e s t to c o n s i d e r also t h e c o l o u r - m a g n i t u d e - d i a g r a m o f t h e s e objects. P r o b a b l y the p e r c e n t a g e o f multiple systems with m o r e t h a n t w o c o m p o n e n t s will d e c r e a s e w i t h d e c r e a s i n g separations. A m o n g t h e s e o b j e c t s there will be m a n y pairs with u n c e r t a i n p a r a l l a x data. S p e c i a l investigations of each pair may show whether spectroscopic and/or photometric determin a t i o n s o f t h e l u m i n o s i t y c a n i m p r o v e t h e m e a s u r e d distances, particularly for w i d e c o m p o n e n t s w i t h a p p a r e n t l y large differences b e t w e e n their t r i g o n o m e t r i c p a r a l l a x data. S y s t e m a t i c s e a r c h t h r o u g h the N L T T v o l u m e s will yield n e w c.p.m, objects. A l s o L u y t e n ' s L D S lists s h o u l d be e x a m i n e d . T o g e t h e r with p r o g r a m m e s for b r o a d - b a n d p h o t o m e t r y o f s u c h stars this w o r k will e x t e n d r e m a r k a b l y o u r k n o w l e d g e o f w i d e s y s t e m s in t h e solar n e i g h b o u r h o o d .
R e f e r e n c e s
Dawson, P. C.: 1986, Astrophys. J. 311, 984. Dommanget, J.: 1964, Comm. Obs. Roy Belg. Sdrie B, No. 56. Gliese, W.: 1969, Ver6ffentl. Astron. Reehen-Inst., No. 22. Halbwachs, J. L.: 1985, Bull. Inf. CDS 28, 25. Kunkel, W. E., Liebert, J., and Boroson, T. A.: 1984, Publ. Astron. Soc. Pacific 96, 891. Luyten, W.: 1979, N L T T Catalogue, Vols. 1 and 2, University of Minnesota, Minneapolis. Luyten, W.: 1980, N L T T Catalogue, Vols. 3 and 4, University of Minnesota, Minneapolis. Heintz, W. D.: 1969, J. Roy. Astron. Soc. Can. 63, 275. Retterer, J. M. and King, I. R.: 1982, Astrophys. J. 254, 214. van Altena, W.: 1985, private communication. Woolley, R., Epps, E. A., Penston, M. J., and Pocock, S. B.: 1970, Roy. Observ. Ann. 5.
WEINBERG - In your systematic search for wide binaries, what was the largest angular separation that you considered? GLIESE - The largest angular separation which I included in my list is somewhat larger than 16 ~ however, I am in doubt whether this pair is gravitationally bound. The next object in this list is a system of 4 components separated from a double system with common proper motion and parallax, separation 6 ~16'. LUYTEN - Isn't the separation of Alpha and Proxima Centauri 14000 AU. GLIESE - With the parallax values given in the new Yale catalogue we find about 13 000 AU _+2200 AU. POVEDA - How many binaries do you have with separations larger than 5000 AU? GLIESE - We have listed 32 such systems, however some of them, probably are not gravitationally bound. ABT - You found that when yon proceed to larger distances, the numbers of companions decreases significantly. Is it possible to extrapolate to very near stars to estimate the frequency of companions among the nearest stars? GLIESE - We did not estimate in this way. From 1969 to 1987 the percentage of the known relation binary frequency (number of components in system relative to the number of all stars) for the stars nearer than five parsecs of the Sun increased from 59 to 63 %. HERCZEG - What is the role of photometric parallaxes in constructing the new Catalogue of Nearby Stars. I know you consider them for the 'final' value of ~, but one can make a case that at or over 25 pc distance, good photometric parallaxes (spectral types or colours) might well be more reliable than average trigonometric parallaxes. GLIESE - For many stars only photometric parallaxes are known and will be used. If a trigonometric parallax with a small error (s.c. -< 10%) was measured it will be used without any change. However, for many stars, mainly at distances of 20 ... 25 pc, trigonometric and photometric ( + spectroscopic) parallaxes will be combined to a weighted mean parallax ('resulting parallax').