Double Star of the Month - June 2008

In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

The two systems this month are both bright and relatively easy objects to see in small telescopes now but both have highly eccentric orbits resulting in large ranges of apparent separation during the orbital cycle.

The apparent orbit of 44 Bo├Âtis (STF 1909 - 15 03 47.68 +47 39 14.5) is a very elongated ellipse with the primary star somewhat nearer the eastern end than the western end. At

present the companion is perched at the eastern end of the ellipse having been virtually stationary for the last few years. It will soon be closing noticeably and accelerating to pass by the primary at a distance of 0.20 arc seconds in 2019. At this point the angular motion will be very pronounced - more than 1 degree per week. The pair was discovered by Herschel in 1781 at PA 60.1 degrees but he made no note of the separation. A measure made a few weeks ago (i.e. early July 2008) showed the companion to be at precisely 60.0 degrees.

The current orbit by Soderhjelm gives a period of 206 years. The stars are F7 and K4 dwarfs and the visual magnitudes 5.20 and 6.10. The secondary is one of the brightest W UMa eclipsing systems known with a period of 6 hours.

xi Sco (16 04 21.63 -11 22 24.8) was found to be triple by Herschel the year after he found 44 Boo. The wide pair was separated by about 6.7 arc seconds and since then the angle has decreased 50 degrees and the separation has increased somewhat. Herschel gave no separation for AB - although his measure for PA put the pair at 188 degrees. In Lewis' book (1906) several orbits were listed, all of them with a period near 100 years and very low eccentricity. However, the equality in brightness of the two stars led to differences of 180 degrees in some position angles and it was left to Aitken to show that the period was nearer 45 years and was highly eccentric. The currently accepted period is 45.9 years and the is pair is now approaching maximum distance (1.13 arc seconds in 2021) so this is a good chance to see it with 15-cm aperture. xi Sco is missing from the Hipparcos catalogue but it is still possible to estimate its distance because it shares a common proper motion and radial velocity with the 12 arc second pair STF1999 some 5 arc minutes SE so that the whole system is quintuple. STF1999 has a parallax of 33 mas but with a significant uncertainty. The WDS gives magnitudes of 5.16, 4.87, and 7.3 for the three stars of xi Sco.

Bob Argyle - Double Star Section Director