January 2019 - Double Star of the Month

Epsilon (or 8) Mon (06 23 46.10 +04 35 34.2) sits about 7.5 degrees ESE of Betelgeuse. It was found to be double by William Herschel on February 15th, 1781 and he briefly dismissed it with a note saying Double, distance about 12".

This is, in fact, an attractive pair for the small telescope. Admiral Smyth found colours of golden yellow and lilac for A and B whilst John Nanson's observed hues of yellow and pale yellow. It earns the accolade of 'showcase pair' in Sissy Haas' book Double Stars for Small Telescopes.

The stars have shown little motion since discovery indicating a long orbital period, the physical connection being demonstrated by astrometry of both components by Gaia. The DR2 catalogue puts the A star 134.3 light years away with its companion at 130.4 light years, but the formal error on the parallax of the bright component is 8 times that of star B. A is known to be a spectroscopic binary with a period of about a year which may explain the difficulty which Gaia has had in pinning down its distance.

In 2014 I found B at 29 degrees and 12".4.

The small constellation of Caelum sits to the south of Lepus and precedes Columba. The second brightest star is gamma (V = 4.7) which was found to be double (JC 9) by Captain W. S. Jacob in India in 1847 using a 6-inch Lerebours refractor. It is located (05 04 24.40 -35 28 58.7) in a rather sparse area of sky and can be found by moving 13 degrees due south of epsilon Lep.

The primary is a K giant whilst B is spectral class G8. E. J. Hartung found the stars orange and white and noted that 7.5-cm shows them clearly, whilst more recently Ross Gould suggested that they were a test for 10-cm.

In 1985, a paper in the International Bulletin of Variable Stars series suggested that B was variable. The authors found a range in delta magnitude of 3.5 to 5 (presumably in the V band), whilst their own data from direct blue-sensitive plates indicated that the B star was between 1.4 and more than 2 magnitudes fainter than that of A, but there are no confirming observations.

The pair forms a binary system as Gaia DR2 shows the stars to be 186.3 light years away with similar (and substantial) proper motions.

For 30-cm users, move 13 arc-minutes south to acquire HDS 658 - 6.4, 9.7, 196 degrees, 1" - a pair discovered by Hipparcos.

Bob Argyle - Double Star Section Director