May 2019 - Double Star of the Month
BU 800 (13 16 51.05 +17 01 01.9) was re-discovered in 1881 using the 15.5-inch refractor at Washburn Observatory by Burnham who recorded a separation of 1".27 in position angle (PA) 121.5 degrees, with estimated magnitudes of 7.1 and 10.2. In fact Herschel had found it almost a century earlier and catalogued the system as H 2 46.
This is a very interesting physical system. By 1898 the separation had doubled with little change in angle leading Burnham to suspect that it was an orbital pair with the plane in the line of sight. By 2015 the stars were 7".7 apart and, according to the 770 year orbit of Hale (1994), they should start to close around 2040.
Hale's orbit suggests that the inclination of the plane to the line of sight is 93 degrees; almost edge-on. Gaia DR2 confirms that they are close together in space at distances of 35.83 light-years (A) and 35.80 light-years (B), respectively. That evidence plus the relatively large and similar proper motions confirms that this is a long period system.
For the small telescope user this will be a challenge. The modern magnitudes are 6.7 and 9.5 but 12.5-cm suffices to see the stars which are orange and red in colour according to Hartung. Haas notes that it is 40 arcmins south of the globular NGC 5053. Recent observations with the CHARA array using baselines of 331 metres show no close companions to A.
Zeta Centauri, a V=2.6 magnitude B star, forms an equatorial triangle with alpha Cen and beta Cru. A wide-field view of zeta will also include two double stars, one 0.5 degrees SW, HJ 4619 (13 52 02.91 -47 51 56.6) and another 1 degree SW, CPO 61 (13 51 32.35 -48 17 35.7).
Both are easy for the small telescope. HJ 4619 was noted by John Herschel on July 2nd, 1834 on Sweep 434 without comment apart from a note on the position angle and separation. It transpires that these stars are unrelated - Gaia DR2 notes that the V=6.9 magnitude primary is 748 light-years away whilst its V=8.4 companion is only 208 light-years away. The error on the DR2 parallax for this star suggests a higher multiplicity. I measured them at 198.3 degrees, 23".24 in 2016.
CPO 61, on the other hand, (7.4, 7.4, 130.6 degrees, 30".59, 2016) is almost certainly a binary pair. The slightly brighter primary is slightly closer (222.0 light-years) compared to the B star at 223.4 light-years.
Bob Argyle - Double Star Section Director