March 2018 - Double Star of the Month

Tucked away is an obscure part of Ursa Major is the red dwarf binary STF 1321 (09 14 22.79 +52 41 11.8). Containing stars of magnitudes 7.8 and 7.9 it sits about half a degree west of the centroid of a triangle of stars formed by 15, 18 and θ UMa.

Although discovered by Struve almost 200 years ago the orbital motion has amounted to only 50 degrees or so and the projected period is 975 years. Both stars are a distinct yellow colour and currently separated by 16".8 in position angle 99 degrees.

This is one of the nearest stellar systems to the Sun. Hipparcos measured both stars and came up with a distance of 19 ± 0.6 light years for the A component but most recently an interim measurement from Gaia gives a distance for the B star of 20.52 ± 0.05 light years.

The Hipparcos parallax error is exceptionally large and implies there may be underlying structure. Both stars have been suspected of being spectroscopic binaries but this was disproved, at least at the 0.1 km per second level by Morbey and Griffin. Further searches for faint companions have so far revealed nothing. It will be interesting to see what Gaia finds for the A star - the fact that the parallax has not yet been published may be revealing in itself.

The pair move across the sky at more than 2.5 arc-seconds per year and is fast approaching two faint companions found by Ball (mag. 11.9) and Espin (mag. 14.5).

BSO 18 in Vela (08 42 25.41 -53 06 50.5) contains two stars bright enough to be in the HR catalogue, the primary HR 3467 of magnitude 4.8 and HR 3466 of magnitude 5.6, 76 arc-seconds distance in position angle 311 degrees, so this is a fine pair of white stars for the binocular user.

It is easy to find as it is 25 arc-minutes south following the bright star o Velorum (mag. 3.6) which is itself embedded in the galactic cluster IC 2391, so the whole area is a spectacular telescopic view.

B in turn has a magnitude 9.9 star (D) at 266 degrees and 60 arc-seconds. The bright components of BSO 18 share the proper motion of the cluster and are both at a distance of about 500 light years.

In 1929 W. H. van den Bos found a close companion to the B component some 2.5 magnitudes fainter and 0.5 arc-seconds away. Since then the distance has increased only slightly and the position angle has increased by 40 degrees to 153 degrees. This pair (B 1625 BC) was last measured in 1991 and poses a challenge for 30cm.

Bob Argyle - Double Star Section Director