Zeta

The history of this fascinating multiple star has recently been comprehensively reviewed by Roger Griffin25 but a brief summary is worth including here. Whilst the duplicity of the star had been taken to originate with Tobias Mayer who observed it in 1756, Griffin has shown that the first suspicion that the star was double comes from an observation by John Flamsteed on 22 March 1680. Flamsteed refers to "the north-following component" which agrees nicely with the position of the brighter of the two stars at that time. The separation of the two stars at the time was about 6''.

In November 1781 William Herschel divided the bright component into two and catalogued the close pair as H I 24. (Herschel allocated a class for pairs of differing separations ranging from I for pairs closer than 2' ' out to VI for pairs divided by 32'' or more.) His notes on the position angle allow a value of 3.5° to be assigned to the system. The close pair was not observed

Figure 9.5. The apparent orbit of zeta Cnc AB, period = 59.56 years.

Figure 9.5. The apparent orbit of zeta Cnc AB, period = 59.56 years.

again until 1825 when Sir James South measured it from France when the position angle was given as 58°. It was only when later measures showed that the position angle was actually decreasing that it became clear the close pair had moved through 305° since 1781! The apparent orbit of this beautiful pair is shown in Figure 9.5.

Over the next 20 years or so, growing numbers of double-star observers made copious measures of both the close and wide pair, and the motion of star C around AB was clearly not proceeding in a smooth curve. The position angle would reduce smoothly and then for several years it would stay constant and then resume its course. In 1874 Otto Struve considered the results of almost 50 years of measures by his father, F.G.W. Struve and himself. His conclusion was that the "wobbling" of C was due to the presence of a fourth star D rotating around it with a period of about 20 years. Towards the end of the nineteenth century, Seeliger produced a comprehensive analysis of the motions in the zeta Cnc system. His astrometric orbit for the pair CD remained in force for over 100 years.

Whilst the existence of star D was in no doubt, a few sporadic efforts were made during the last century to detect it. In 1983, D.W. McCarthy 26 using an infrared speckle interferometer announced that he had detected not only star D but yet another component, in other words, the main sequence component C, a white dwarf and another star. This detection was never confirmed and there the matter stood until the early months of 2000.

Using an adaptive optics system working in the infrared on the Canada-France-Hawaii telescope on the island of Hawaii, J.B. Hutchings et al.27 produced the first direct image of star D. It is a very red object but the effect it has on star C suggests a comparable mass to C, and thus D itself probably comprises a pair of M dwarf stars.

The story does not end here however. In 2000, A. Richichi28 reported on the observation of a reappearance of zeta Cancri in the 1.52-m telescope at Calar Alto on 7 December 1998. Working in the infrared with a broad-band K filter the occultation trace showed four definite stellar sources and slight but significant evidence for a fifth star, located some 64 mas from star C. Referred to as E, it would appear that it is another low-mass M dwarf possibly with a period of 2 years. The component seen by Hutchings and Griffin, D, was also easily visible but if double the separation is likely to be no more than 30 mas, thus requiring a considerably larger aperture to resolve it.

References

1 Lewis, T., 1906, Struve's Mensurae Micrometricae, Mem. RAS, 56.

2 Clerke, Agnes M., 1902, The History of Astronomy during the Nineteenth Century, A. and C. Black, London , 409.

3 Pickering, E.C., 1890, American Journal of Science (3rd series), 39, 46.

8 Pease, F.G., 1927, Proc. Astron. Soc. Pacific, 39, 3.

10 Adams, W.S. and Joy, A.H., 1920, Proc. Astron. Soc. Pacific, 32, 158.

12 Savary, F., 1827, Connaissance des temps pour l'an 1830, 56.

14 van den Bos, W.H., 1928, Danske Vidensk. Selsk. Skrifter, ser 8, 12, 293.

18 Bonneau, D., 2000, Observationsy Travaux, 52, 8.

19 See, T.J.J., 1896, Researches on the Evolution of the Stellar Systems, Nichols, Massachusetts.

21 Batten, A.H. and Fletcher, J.M., 1991, Proc. Astron. Soc. Pacific, 103, 546.

22 Reuyl, D. and Holmberg, E., 1943, Astron. J., 97, 41.

23 Worth, M.D. and Heintz, W.D., 1974, Astrophys.. J., 193, 647.

26 McCarthy, D.W., 1983, IAU Colloquium 76, 97.

27 Hutchings, J.B., Griffin, R.F. and Menard, F. 2000, PASP, 112, 833.

28 Richichi, A., 2000, Astron. Astrophys., 364, 225

Chapter 10

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