UMa

This beautiful pair of yellow stars was discovered by William Herschel on May 2 1780, when he wrote "A fine double star, nearly of equal magnitudes, and 2/3 of a diameter asunder; exactly estimated." From the latest orbital elements, we can deduce that the separation on that date was 2.3''. As Herschel was describing the separation between the disks rather than the disk centres we can see that the images in his telescope must have been about 1.4'' across. The subsequent, rapid orbital motion convinced Herschel that the stars were genuinely connected and in 1827 Savary,12 in France, made

Figure 9.3. The apparent orbit of xi UMa, period = 59.878 years.

the first orbital analysis of any double star using xi UMa for the purpose. He obtained a period of 58.8 years and an eccentricity of 0.41. This compares with today's latest values of 59.9 years and 0.40. The apparent orbit of xi UMa appears in Figure 9.3.

As was the case with 70 Oph (see following), the ease of measurement of the pair and the relatively short period led to a plethora of orbits. At the beginning of the last century the separation of the pair was over 2'' and increasing, so taking spectra of both components became possible in good seeing. Norlund13 found a small periodic perturbation in the residuals of the orbit of star A with a period of 1.8 years. As dark companions were somewhat in vogue at the time it seemed natural to ascribe this as the cause of this effect. At this time Wright at Lick Observatory had already noted radial velocity changes corresponding to this 1.8 year period in the spectrum of A. Eventually an orbit was computed by van den Bos14 which is still used today.

Although spectral plates were also taken of star B from 1902 it was not until 1918 that it, too, was found to be a spectroscopic binary with a period of just under 4 days. Berman15 produced an orbit for Bb which remained the sole analysis until Griffin revisited the system.16 He was able to show that Berman's orbit required little adjustment, the difference in the period

Figure 9.3. The apparent orbit of xi UMa, period = 59.878 years.

being 0.6 second! With each successive orbit, the period can be fixed with greater and greater certainty, if the periastron passage is sharply defined. Since Berman's analysis, the pair Bb had gone through more than 6000 orbits.

The next development came much later during an investigation of the system by Mason et al.17 at CHARA (Georgia State University). By using speckle interfer-ometry measures they were able to obtain very accurate relative positions and these were used in an attempt to tune the orbital elements of the AB pair to give a more precise value of the individual masses (1995). During the course of their observing campaign, Mason et al. observed yet another component, attached to the Bb subsystem but it appeared in only one out of 27 observations.

A later discussion by Daniel Bonneau18 argues that if this new component exists, it would have a mass of about 0.5-0.7 M0 and the orbital inclination of the B system would then be incompatible with both the rotation of B and the coplanarity of the orbit of Bb. Resolution of Bb will only be possible from ground-based interferometer systems although Aa should be resolvable in a 2.5-metre telescope with infrared adaptive optics.

70 Oph

Discovered by William Herschel in 1779, this pair has been a favourite amongst double-star observers of all kinds ever since. Its proximity to the Sun (16.6 light years according to Hipparcos) means that during the orbit of 88 years the separation of the stars varies from 1.5 to 6.5'', and it is thus possible to follow it through its whole orbital cycle with ease. The recent periastron passage in 1984 showed the companion moving almost 20° over the year. Another reason for its popularity is the beautiful contrast between its unequal components which have given it a prominent place in all observing handbooks. Placed near the equator it can be seen from virtually all latitudes.

Thomas Lewis in his book on the Struve stars said, in 1906, "It is a splendid system and quite worth the time spent on it by Observers and Computers, although it is a source of much trouble to the latter." Surprisingly enough, it was only recently that the agreement

180°

WDS18004 + 0232

STF2272

2000

2020

270° /

90°

Figure 9.4. The apparent orbit of 70 Oph, period = 88.38 years.

between the spectroscopic and visual orbit was regarded as satisfactory.

70 Oph was a very popular object with Victorian observers and so measures were numerous. As the pair is an easy object (see the apparent orbit in Figure 9.4) disquiet was expressed about the way that the observed measures were not agreeing with the predicted values from the various and numerous orbits that were being calculated (Lewis lists 22). In 1896, T.J.J. See19 postulated that these disagreements were due to the presence of a third body orbiting one of the stars in the system. In 1906 Lewis dedicated a large amount of time and space in his volume to discussing the pair. He was convinced that the anomalies were due to a third body orbiting star B and even derived a period of 36 years for it. Burnham, in his catalogue, dismissed the idea saying it was merely observational error but the idea persisted. Pavel20 postulated a companion orbiting A with a period of 6.5 years.

In 1932 Berman, using radial-velocity measurements of plates taken at Lick Observatory, found a cyclical trend with a period of 18 years but many years later Berman said that he had ceased to be convinced of this result.21

Reuyl and Holmberg22 at McCormick Observatory found an astrometric perturbation with an amplitude of 0.014'' from a series of plates taken between 1914 and 1942.

Worth and Heintz23 re-visited the visual measures and also produced a trigonometrical parallax for the star. Although there were some problems with measures in the 1870s they could find no evidence for a third body other than a rather unlikely scenario of the passage of a third body through the system at that time.

Heintz computed the orbit afresh in 198824 and summarised the situation at the time. This was that recent radial velocity measures showed no perturbation and modern measures using long-focus photography show no systematic deviations beyond the 0.01'' level.

Batten and Fletcher21 re-examined the radial velocity material measured by Berman and could not find his periodic component in the velocities. However they came to the conclusion that the quality of the early plates means that large residuals "are not of much significance". The re-determination of the spectroscopic period came out at 88.05 years and agrees with Heintz's visual orbit within the quoted error (0.70 year).

It appears that 70 Oph really is what it seems - a beautiful binary star.

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