April 2018 - Double Star of the Month
Just 6 degrees north of Denebola (beta Leo) sits 93 Leo (11 47 59.23 +20 13 08.2) a rather undistinguished white star of magnitude 4.6.
It caught the attention of William Herschel in 1782 who called it H VI 80. Some 40 years later it came to the attention of F. G. W. Struve during the Dorpat survey but the companion star at magnitude 9.0 and distance 77" did not impress him enough to put in the main catalogue, so it was relegated to one of the two Appendix Catalogues of essentially wide pairs.
In 1900, the primary was found to be a spectroscopic binary by Campbell and Wright at Lick Observatory where four spectra of the star showed a range of 38 kms per second.
More recently 93 Leo has been shown to be an RS CVn star, a variable type which involves chromospheric activity on one component of a pair of giant stars, in this case the spectral types are F8III and A6III.
The period was shown to be almost 72 days and then the Mark III interferometer (which was based at Mount Wilson Observatory but closed in 1992) resolved the two stars and defined a visual orbit of great precision even though the separation varied from only 5 to 8 milliseconds of arc.
Hipparcos showed the primary star to be 232 light years away but the companion has recently been observed by Gaia which shows that it is at the same distance and moving through space with the same proper motion as A making this a physical triple system.
Smyth neglects to include in his catalogue. Webb notes a curiously similar pair in the field. This is SHJ 130 (7.5, 10.0, 30 degrees, 71") and is about 13 arc-minutes south-west of 93 Leo.
STF 1604 (12 09 28.54 -11 51 25.4) lies in a barren area of sky on a line between beta Virginis and gamma Corvi and about 6 degrees along the line from the latter star.
There are three components for the small telescope, although the B star is rather faint and is of late spectral type.
AB is magnitude 6.9 and 10 at 89 degrees, separated by 9" (2016). Both components have a significant and similar proper motion and have been approaching the third unconnected star C (magnitude 8.1) since 1831 when the distance AC was 58 arc-seconds. Closest approach was 9".6 in 1983, and I measured the distance at 10".8 in 2017.
The stars were in Virgo when Webb wrote his book but have now sneaked over the border into northern Corvus.
Bob Argyle - Double Star Section Director