May 2018 - Double Star of the Month

Not far from Arcturus in the Spring sky is pi Boötis (14 40 43.56 +16 24 05.9) a beautiful pair of white stars found by Christian Mayer and later called H III 8 by William Herschel and STF1864 by F. G. W. Struve. The stars are magnitudes 4.9 and 5.8 and have shown little motion since discovery. Smyth, Webb, Sissy Haas and me all find the both stars are white and the spectral type are B9 and A6.

The primary star, at least, is over 300 light years away but the quoted error on the Hipparcos parallax is significantly large and is probably not affected by the presence of the visual secondary. In 1984 the primary was found to be a spectroscopic binary and the Washington Double Star (WDS) Catalog notes that the secondary is also a spectroscopic binary. In 2015 I found the stars at 113 degrees and 5".4 apart.

The upcoming Gaia catalogue may well settle the issue of whether the visual pair is physical, and if it is then we have here a quadruple system. A third star of mag 10.6, first noted in 1881 is 163 degrees and 127" distant.

k (not kappa) Lupi (15 25 20.21 -38 44 01.0) is a magnitude 4.6 star located in central Lupus about 2 degrees north of delta.

It was observed by James Dunlop who noted a couple of distant 9th magnitude companions. Dunlop's original paper reads for entry number 183: A star of the 6th mag with two stars of the 10th and the measured separations are 12 and 15". It's possible that Dunlop meant 120 and 150" as the latest WDS positions (for 2016 and 1999, respectively) are 203 degrees and 93" for AB and 134 degrees and 149" for AD.

In 1896 Robert Innes, observing from Cape Town using a 7-inch refractor, found that the B component was an almost equal double (I 87) at a distance of 1".4 since which time the position angle has reduced by 40 degrees to 207 degrees and the separation is now just below 1". The relative faintness of the two stars means that this is now a stiff test for 25-cm aperture. Innes also added a magnitude 11.5 at 17 degrees and 42".

Bob Argyle - Double Star Section Director