November 2016 - Double Star of the Month
Iota Cas (02 29 03.96 +67 24 08.7) can be found by extending the line between delta and epsilon Cas by the same distance again. It was originally observed by William Herschel in 1782 as a 7" pair (H 3 4); he missed the closer component B (1".5) on that occasion but found it in 1804 (H I 34). As it has widened considerably since then, B is now easier to see.
Smyth gives the colours as pale yellow, lilac and fine blue whilst Webb restricts himself to yellow, blue and blue. Measures by the writer in 2015 give 232 degrees and 2".9 for AB and 120 degrees and 7".1 for AC. With a small aperture, B and C are delicate objects. The WDS gives magnitudes of 4.63, 6.92 and 9.05.
Orbital motion has reduced the position angle of AB some 60 degrees in 200 years and an orbit for it indicates a period of about 620 years, although one recent report argues that the motion of B relative to A is linear. The separation of AB has not changed in a smooth fashion but rather B appears to execute a loop every 50 years with respect to A. In fact, it is star A which has a faint K-type companion and it was first directly detected in 1982. This star is visual magnitude 8.5 and is separated by about 0".5 from A. Another companion, this time to C, was found in 2006, also at a distance of about 0".4 and this is probably an M dwarf.
Stars in the Dunlop catalogue, denoted by a capital Greek delta (Δ) are mostly very bright and wide and therefore constitute an excellent introduction to the double stars of the southern hemisphere.
Number 7 in that catalogue, DUN 7 (02 39 39.84 -59 34 02.9), is a rather faint member of its class (the mags of A and B are 7.56 and 7.66 in SIMBAD) but for the larger telescope aperture it does boast a close star to B which was discovered by Robert Innes. The primary, A, is a late G or early K giant of visual mag 8.0 and has a mag 8.9 companion about 36" away in PA 97 degrees. Although the Hipparcos parallax for each star differs by an amount somewhat greater than the quoted errors, the similarity of the proper motions indicates that this is almost certainly a physical system, and is located about 700 light years away.
In 1926, whilst measuring BC, W. H. van den Bos estimated the colour of A as between yellow and orange and considered this was equivalent to a spectral type of about K0. He gives the spectrum of B as A5. BC has mags of 8.0 and 8.9 and is currently separated by 0".4 and, although the system has not been measured for 20 years, there is significant angular motion.
Bob Argyle - Double Star Section Director