July 2019 - Double Star of the Month

STF 2398 (18 42 46.69 +59 37 49.4) is a pair of M dwarfs whose large proper motion has been known for well over a century. It was inevitable that the stars would be shown to be close to the Sun, and such indeed has proved the case.

Gaia DR2 puts the primary star at 11.487 light-years whilst B comes out at 11.490 light-years, with formal errors of 0.002 and 0.004 light-years respectively. The large proper motion of 2.3 arc-seconds per annum means that two fainter and unassociated field stars (C = mag. 12.2 at 158 degs, 215", 2000) and D (13.5 at 110 degs, 100", 2008) are being rapidly left behind.

The current orbital period is 408 years and in mid-2020 the companion can be found at 182 degrees, 10".9. This pair needs 20-cm to see well, but the writer has yet to measure it with the Cambridge 8-inch as the red field illumination tends to swamp the stars; 30-cm might show the colours of orange-red. it can be found 1 degree west and slightly north of omicron Dra, itself a colourful pair worth seeking out (4.8, 8.2, 317 degs, 38") which Burnham calls orange and blue and Haas finds yellow peach and clear grey.

Just 3 degrees north of Antares and a little proceeding is the naked-eye star rho Ophiuchi (16 25 35.03 -23 26 47.0). This beautiful pair, whose components are magnitudes 5.1 and 5.7, is currently 3".0 apart in PA 334 degrees and has been slowly closing since discovery by Herschel with the position angle decreasing by 30 degrees over the same interval.

Whether it is binary is not yet fully established, as the measures of distance by Gaia as given in the DR2 catalogue show that the parallaxes just overlap within the quoted errors but the proper motion of B is significantly larger than that of A.

The stars are 467 light-years away putting it about 40 light-years beyond the rho Ophiuchi dark cloud which lies 1 degree to the south.

Rho has a number of faint companions - C is 7.3 at 0 degrees, 149" and D is 6.8 at 252 degrees, 156"; both distances are slowly reducing. S. W. Burnham divided D into stars of 6.8 and 8.4 which are currently 0".28 apart in a binary orbit which takes 675 years.

Bob Argyle - Double Star Section Director