June 2019 - Double Star of the Month

This month's choices are both fine sights in small telescopes but also of great interest to enthusiasts of stellar multiplicity.

Mu Bootis (15 24 29.54 +37 22 37.1) was noted as a very wide pair by William Herschel in 1780 when he gave a distance of 2' and 8" ("exact est.") for AB. When he revisited the star a year later he noted the secondary was itself a close double (BC) which became H I 17 in his first catalogue. The modern orbit for this pair which suggests a period of 265 years which gives a separation close to 2".1 for 1781 so its odd that Herschel did not see the two stars in 1780.

The distance of AB given by Herschel must be a transcription error as the three stars have common proper motion and there has been little relative change over the last 240 years. During the middle of the C19 the separation closed to less than 0".5 since when it has been widening. For the summer of 2019 the stars can be found at 3 degrees and 2".2, an easy split for 10-cm, as the stars are mags 7.1 and 7.6.

In 1988 using the CFHT on Hawaii, the CHARA team led by H. McAlister found that A was also a close pair whose separation varied between 0".06 and 0".12 in a period of 3.75 years. Although the four stars may seem to be physically connected the astronomer Olga Kiyaeva speculates that because of elemental abundance differences what we are seeing is the close passage of two unassociated pairs.

Gaia DR2 puts A at 116.1 light years but with an uncertainty of 2.4 light years, no doubt due to the interferometric companion, whilst BC are at 120.0 light years with an uncertainty around 0.3 light years.

One of the few constellations which has not yet been visited in this series is Apus, which lies between Pavo and Musca and whose southern border impinges on the northern edge of Octans at the South Celestial Pole.

Perhaps the best pair is I 236 (14 53 13.57 -73 11 24.3) in which the stars are visual magnitude 5.9 and 7.6 and they are currently separated by about 2".2 in PA 123 degrees. In fact there have been no measures since 1996, but there is intriguing evidence that this may be a binary pair with a highly inclined orbit.

Innes, in his Southern Double Star Catalogue of 1927 notes that the first observation was made by Pickering in 1891 in which he estimates a separation of 0".6. Innes independently noted the pair during his early double star searches but Pickering's report to Astronomische Nachrichten at the time mentions only that the primary star had two companions within 30 arc-seconds, and this prompted Innes to claim the close pair for himself.

Just 7 degrees due north is the binary pair HJ 4707. Orbiting in 346 years these stars are currently 1".3 apart in PA 266 degrees, and the magnitudes are 7.5 and 8.0. Very fine was John Herschel's comment when he found the pair in April 1835.

Bob Argyle - Double Star Section Director