December 2018 - Double Star of the Month
Both pairs in this month's columns are long period binaries accompanied by distant and faint, but co-moving companions.
STF 326 (02 55 39.06 +26 52 23.6) was unknown to me until recently. Observations of it on the web indicate that the stars are yellow-orange and reddish. It is 1.5 degrees ESE of 41 Arietis, itself a wide pair found by Herschel (3.6, 8.8, 237 degrees, 123"), but also a more complex system according to the Washington Double Star catalogue (WDS).
Despite having moved just 5 degrees in position angle (PA) since 1831, STF 326 was allocated a hyperbolic orbit in the 1960s - suggesting that the stars make one close approach and then fly off into different directions in space. The existing astrometry hardly supports this theory but the stars have certainly closed since discovery and are now at 221 degrees and 5".5 with the K2 primary at magnitude 7.7 and the M0V secondary at V = 10. They are thus rather faint but the fine colours make this a system worth looking out for.
There is a background star (C) at 171 degrees and 41" (magnitude 11.9), but the 13.9 magnitude comes at 266 degrees and 44" is LDS 883 D. It is at the same distance as the AB pair, and moving with the same substantial proper motion- 0".3 per year.
Gaia DR2 puts them all at 73.5 light years.
BU 1004 is in the constellation of Eridanus (04 02 03.44 -34 28 55.7) and located about 3.5 degrees west of 41 Eri. It was found by Burnham in 1881 and with magnitudes of 7.3 and 7.9 it must have been an easy object in the Mount Hamilton 12-inch.
Since then the position angle has reduced by 100 degrees and the separation has changed from 1".7 to the current value of 1".2 at PA 50 degrees, making it a rather tricky object from the UK due to the very low altitude.
J. Docobo finds an orbital period of 410 years and predicts that the stars are now close to minimum separation and may reach a maximum of 1".8 by around 2280.
In the last century W. J. Luyten found a faint star, probably a white dwarf, moving through space with a similar proper motion to AB. LDS 3551 B is visual magnitude 18 and lies 64" distant in PA 313 degrees.
DR2 pins all three stars down to 151 light years give or take 0.1 or 0.2 light years so this is a physical triple star.
Bob Argyle - Double Star Section Director