May 2020 - Double Star of the Month

Epsilon Bootis (14 44 55.44 +27 04 29.9) is one of the best-loved and most well-known of visual double stars - and one of the brightest - its components are 1.9 and 4.8 in the Gaia G band - the equivalent of Johnson V.

Its spectacular colours prompted F. G. W. Struve to call it Pulcherrima (most beautiful). It was found by William Herschel and was catalogued by him as H I 1. W. H. Smyth calls it pale orange and sea green but leaves any discussion of it out of his book Sidereal Chromatics. Apertures of 15-cm will show it well, especially if a higher magnification is used.

Gaia DR2 includes both components but does not give a parallax for the bright star. The quoted error in the parallax for B is significantly larger than normal, which may be due to the proximity of A or the fact that B is a spectroscopic binary, although it does not appear in the Ninth Spectroscopic Binary catalogue. DR2 makes the distance of B to be 219 ± 6 light-years. The pair appear to be physical with the position angle having increased about 45 degrees to 347 degrees in 2018. The distance at that epoch is 2".8 which has changed little since 1777. Assuming the orbit is circular then the period will be about 2,000 years.

The galactic equator passes through the north-east corner of Ara and close to the very young cluster NGC 6193, which in turn is just east of the HII region NGC 6188. About 1 degree ENE of NGC 6193 is the wide binocular triple star DUN 211 (16 47 28,13 -48 19 10.1).

The brightest pairing (AB) is stars of magnitude 6.5 and 8.1 in the G band of Gaia. The primary is clearly a red star and is a bright M giant whereas the companion 106" distant in PA 125 degrees is early F. The third obvious component can be found at 144 degrees and 130" and has G = 8.1. However, 10-cm aperture should be sufficient to show the close companion to C found by John Herschel at the Cape (HJ 4885). D is 3".8 away in PA 239 degrees, and both C and D have similar parallaxes and proper motions. Gaia DR2 puts them at around 440 light-years. Star B is unconnected and star A is much more distant than its companions.

Bob Argyle - Double Star Section Director