September 2019 - Double Star of the Month
STF 2744 is located in northern Aquarius (21 03 03.09 +01 31 55.9) near the border with Equuleus.
It seems to have missed the attention of William Herschel which is surprising considering that the current orbit of 1532 years gives a separation of fully 2" for 1780 and the stars are of magnitude 6.3 and 7.0. That fact and the general run of observations plotted in the United States Naval Observatory (USNO) 6th Orbit Catalogue tend to suggest an orbit of smaller angular size and orbital period. In any case observations of position angle are almost ten degrees away from the predicted position.
In 2014 I found it at 113 degrees and 1".4 and motion is slow so it should still be within the resolution range of 10-cm. Greater aperture would be needed to spot two faint field stars - one of magnitude 12.9 at 99 degrees and 98" and another of 14.3 at 300 degrees and 74". The A component does not appear in the Gaia DR2 catalogue, whilst the distance of B is given as 233 ± 5 light-years.
Lambda Sco, also known as Shaula, is the brilliant white star in the tail of the Scorpion (17 33 36.52 -37 06 13.8).
It first appeared in a double star catalogue when James Dunlop noted a magnitude 9.2 star which is currently at 330 degrees, 94" (2016). Dunlop recorded the distance as 60" but this is clearly an error. In 1897 T.J.J. See found a magnitude 14.9 star 42" distant from the magnitude 1.6 primary which has, unsurprisingly, only one observation in the Washington Double Star Catalog (WDS) since discovery.
Slipher found the bright star to be a spectroscopic binary (SB) in 1903, which has subsequently been shown to have a 6 day period, whilst the SUSI interferometer array in Australia showed that this system rotates around another star in a period of 2.9 years. The brighter member of the SB is an early B star whilst its companion is either a massive white dwarf or a TT Tau star. The SUSI companion is another early B dwarf. Like lambda, the Dunlop companion is a brilliant white star.
Bob Argyle - Double Star Section Director
