{ I. Pustylnik Geodeet 6(30) 1994 20-22 }

Wilhelm Struve's Legacy: Visual Binaries in the Context of Modern Astrophysics

I. Pustylnik

Tartu Astrophysical Observatory, Tõravere, Tartumaa, EE2444 Estonia

Wilhelm Struve pärand: visuaalsed kaksiktähed tänapäeva astrofüüsika saavutuste kontekstis

Täna, enam kui 165 aastat pärast Wilhelm Struve poolt koostatud visuaalkaksiktähtede kataloogi "Catalogus Novus Stellarum Duplicium et Multiplicium" väljaandmist pole tema vaatlusandmed sugugi kaotanud oma teaduslikku väärtust. Kuid tänapäeva astronoomid näevad W. Struve pärandit uues valguses. Struve aegadel olid visuaalsed kaksiktähed kindlad majakad taevavõlvil, neid rakendati kosmiliste kauguste määramiseks, Newtoni seaduste rakendatavuse kindlaksmääramiseks kosmilises mastaabis, astronoomilise instrumentaariumi täiustamiseks. Viimase sajandi vältel aga kaldusid astronoomide huvid järk järgult visuaalkaksiktähtede uurimustelt lähiskaksiktähtede iseärasuste tundmaõppimisele. Ometi viimase kümne aasta jooksul me näeme, kuidas taas ärkas huvi W. Struve poolt avastatud objektide vastu seoses tähesüsteemide ning väikeste täheagregaatide varajase evolutsiooni uurimisega. Käesolevas ettekandes arutame neid probleeme mõningate W. Struve poolt avastatud silmapaistvate kaksiksüsteemide näite varal.

The history of the visual double star research is one of the most fascinating chapters in the history of astronomy. Most of the visual binaries have orbital periods measured in centuries, millenia and even millions of years. To determine the orbital elements of a binary star, even only relatively reliable, one should proceed with observations for at least half of its orbit. Therefore double star astronomy is the branch of astronomy in which the ties between past, present and future are the most tangible. Today we are able to calculate an orbit only because earlier generations of astronomers have observed this particular star and by the same token our observations will be indispensable for the generations of astronomers to come. The unrelenting flow of time, instead of condemning these observations to oblivion, makes them precious and irrevocable.

The astronomy of visual binary stars is by now over 200 years old. The early history of visual binary star research begins with the name of Riccioli, who in 1650 discovered the duplicity of Mizar. In 1781 Christian Mayer working at Mannheim with the eight feet mural quadrant published the list of 89 pairs, observed by him. This industrious observer even claimed that he had discovered orbital revolution of the satellites around more massive components, for which he was derided by his contemporaries.

But the real history of the visual double star astronomy is inseparably connected with the name of Wilhelm Hershel. In 1779 following one of Galileo's ideas he began systematic observations of visual double stars with the purpose “to measure the position of two stars of unequal magnitudes at two opposite points of the earth's orbit” and thus determine stellar parallaxes. Although he did not manage to accomplish this task, with a celebrated publication in 1803 that has made history he proved that physical binary stars were a reality and that the law of universal gravitation was valid also outside the solar system.

While W. Hershel had introduced the concept of double star as physically related pair of stars orbiting each other, Friedrich Wilhelm Struve has been universally acknowledged as the father of a modern science of visual double stars. For more than 150 years up to now all manuals on double star astronomy begin with his name. In this way investigators of visual binaries and historians do not simply pay tribute to an enormous task accomplished by W. Struve: to his two famous catalogues "Catalogus Novus Stellarum Duplicium et Multiplicium" published in 1827 and "Stellarum Duplicium et Multiplicium Mensurae Micrometricae" ten years later comprising over three thousand visual double stars. According to Jackson [1] who analyzed the objects of Struve's catalogue a hundred year after the original observations, 32% of the objects have shown orbital motion and only 5% appeared to be optical pairs, thus testifying to a very high quality of Struve's observations. We owe to Struve also the method of measuring separations between the components with the filar micrometer, known as the double distance method, commonly utilized even nowadays. It is remarkable that more than 500 astronomers after W. Struve have used essentially the same fairly simple technique leaving a patrimony of about one million individual measurements. Over 70000 visual double stars have been discovered in the whole sky, of which less than thousand have today a known orbit.

If we set aside quite a limited number of the so-called astrometric binaries, where invisible components have been detected either through the analysis of periodic oscillations in proper motions of bright components or via the determination of photocentric orbit employing photographic technique, the major technological breakthrough in studying visual binaries came only 150 years after W. Struve with the introduction of the so-called speckle interferometry technique. It is known that resolution loss of large telescopes due to atmospheric seeing can be retrieved in some cases by analyzing interference effects in the image. Such technique gives direct diameter measurements. This new technique implemented during the last twenty years is indispensable in discovering very narrow double stars (separations less than 0.2 arcseconds) thus effectively filling in the gap between the visual double systems and close binary stars. However, despite all its advantages, speckle interferometry technique is still rather complicated and at least for the time being expensive in men and material. In contrast, visual observations are of low cost and complexity. Unfortunately, nowadays observations, of visual double stars certainly do not figure among the fashionable topics of astronomical research. This is how the situation is described by one of the investigators of visual double stars, an italian astronomer M. Scardia [2] working at La Silla with 38 cm refractor: "The observers of double stars are becoming progressively rarer: their number can be already counted on our fingertips. This speciality which requires years of tough apprenticeship, suffers from the dissaffection of the young, who dazzled by astrophysics, prefer theoretical research or shorter experimental ones requiring less observing engagement, which are able to offer secure funds and fast careers, because it is "fashionable"." The story, of course, is not true any more. It was exactly one hundred year ago when the observational program of visual binaries at a great 76 cm Pulkovo refractor was radically reduced when Belopolski started his spectroscopic measurements of radial velocities.

I proceed now to the second part of my talk in which I would like to concentrate on Wilhelm Struve's legacy of visual double stars seen in the context of modern astronomy. In Catalogue Novus W. Struve discussed the properties of more than fifty multiple systems he had discovered, in other words, with three or more components. Intuitively he felt that these were not just random combinations. He pointed to several wide multiple systems where bright components of the 2nd, 3rd and 4th magnitudes were separated by several minutes of arc, Struve believed that these stars are related in some way. Similarly he noticed the connection between the Orion trapezium, stars. Today early stages of stellar formation is one of the topical problems in modern astrophysics. Let me remind you some of the most essential facts concerning the origin of stars and stellar agregates gained during the last half of this century.

Due to the spectacular successes of stellar nuclear evolution theory we know now that all high luminosity (Mv=-3m) main sequence early-type stars of spectral classes O-B2 are in fact very young stars. Their age does not exceed 30 million years. The same is true for Wolf-Rayet, S Dor (or P Cyg) type stars and supergiants of luminosity class I (of all spectral types). These high luminosity objects are the most reliable tracers of early stages of stellar evolution because they invariably indicate the tendency to be concentrated in the sites of the contemporary stellar formation and being luminous they are the first objects to be detected outside, our Galaxy, i.e., in LMC, Andromeda galaxy and other galaxies of Local System. Since the pioneering work of V. Ambartsumian [3] who first introduced the concept of stellar OB associations very many of these agregates have been discovered. For instance, Jefremov, Ivanov and Nikolov [4] claimed that they identified 210 these groups in M31 with an average diameter of 80 pc. Unlikely the stellar clusters stellar associations are distinguished by the enhanced partial stellar density of the specific, that is O-B type stars. According to Baade [5], Ambartsumian [6] OB associations in its turn form agregates of a higher hierarchy - superassociations with the characteristic size of 500-650 pc. Moscow stellar astronomer Jefremov [7] has introduced the concept of stellar complexes. According to him stellar formation is concentrated in huge cells - stellar complexes, whereas young clusters and associations are just the densest and the brightest (and also the youngest) portions of it. According to these views the Local System (stars of the Gould belt) has originated from one gas supercloud of nearly ten million solar masses and a diameter of one kiloparsec. Emelgreen and Emelgreen [8] proposed a scenario, in which these superclouds are formed due to gravitational instability of differentially rotating gaseous galactic disk influenced by magnetic field. An Armenian astronomer A. Oskanyan [9] has studied young sites of stellar formation in some OB associations. These include compact HII regions, compact IR sources, mazer sources of hydroxyl, mazer sources of water vapour. Their age is extremely short, less than one hundred thousand years. He examined specifically Trapezium type IR multiple sources and compared them with their optical counterparts. The underlying idea was to study whether the average sizes of these differ (it is assumed the IR sources are just borne stellar objects embedded in opaque dust envelopes). During the later stages of evolution dust envelopes should be dispersed and central stars become observable in optical region). In Table I some of the data compiled by Oskanyan are summarized. These data are remarkable from the standpoint of this conference because these multiple systems have been discovered by W. Struve. The following data are indicated in the consecutive columns of Table I: the name of stellar association, the ADS number from Index Catalogue of Visual Double Stars [10] of the multiple system identified in the association, then follows the number in W. Struve catalogue, right ascension and declination for the epoch 2000 and the number of components in the multiple system.

Table I

Naturally, the identification of individual systems with the association cannot be proved on 100% confidence level in view of the patchy structure of interstellar medium, besides, distances to associations are not known with a sufficient accuracy. Obviously these multiple objects have been borne in a process of collective star formation. Some researches even argue that there is no fundamental difference between open clusters and multiple stellar systems.

What is the mechanism behind their formation? There are two basic competing theories of double star formation: fission or bifurcation of an original protostellar self-gravitating and rotating cloud and a capture process in multiple stellar encounters. Fortunately enough, they lead to different predictions as to the respective mass functions. The fission or bifurcation process tend to produce double systems with components of nearly equal masses, whereas stars of high masses capture small bodies obeying the so-called Salpeters's mass function. To discriminate between these two theories Abt, Gomez and Levy [11] have investigated a sample of 74 main sequence B type stars, mostly spectroscopic binaries with the orbital periods ranging from a couple of days to several hundred thousand years.

Interestingly enough, one of the objects from their list, namely ADS 16381 again has been discovered by Wilhelm Stmve as a triple system, Nr 2960 in his Double Star Catalogue. These authors have drawn a conclusion from their sample that frequencies of the secondary components fit Salpeters's mass function, therefore systems like ADS 16381 have been formed through a capture process. However this general conclusion is valid only for relatively young stars, whose age is of an order of one free-fall time for a cluster. Or in other words, for stars which had had enough time on average for one capturing encounter only. For older systems the picture may be much more complicated because for older generations stars suffering many encounters luminosity function for the secondary components may be quite different.

References

1. Jackson J., 1955, M.N.R.A.S..115, p.2.

2. Scardia M., 1990, The Messenger, No. 61, p.36.

3. Ambartsumian V., 1968, The Problems of Evolution of the Universe, Jerevan (in Russian).

4. Jefremov J.N., Ivanov G.R., Nikolov N.S., 1987, Astrophys. & Space Sei.,119, p.119.

5. Baade W., 1963, Evolution of Stars and Galaxies. -Cambridge, Mass., Harvard Univ. Press.

6. Ambartsumian V., 1964.IAU Symp. No.20,-Canberra, Austral. Acad. Sei., p. 122.

7. Jefremov Ju.N., 1989, Sites of Star Formation in Galaxies: Star Complexes and Spiral Arms, Nauka Publ. Company, Moscow, p. 18.

8. Emelgrecn B.G., Emelgreen D.M., 1983, M.N.R.A.S.,203,p.31.

9. Oskanyan A.B., 1990, Astrofizika,33, p.47 (in Russian).

10. Index Catalogue of Visual Double Stars, 1961.0 Publications of the Lick Observatory, vol. XXI, Part II.

11. Abt H.A., Gomez A.E. & Levy S.G., 1990, Astrophys. J. Suppl.,74, p.551.