Double Star of the Month - July 2009
In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.
Both systems featured in this month's column are close to the Sun but vary considerably in difficulty of observation. One is a real test for a medium aperture whilst the other can be seen in a small telescope.
Mu Herculis = STF2220 (17 46 27.72 +27 43 21.0) was found by William Herschel in 1781 and is a wide and very unequal pair of magnitudes 3.42 and 9.78. The current position angle and separation is 248 degrees and 34".9 values which have increased only marginally since Struve measured the pair in 1831. The large proper motion of A and the small change between A and B over time mean that the two stars form a physical system. In 1854 Alvan Clark found that the companion was double. It turns out to have a period of 43.2 years and is a severe test of resolution and light gathering power. The components are mags 10.2 and 10.7 and the separation varies between 0".5 and 1".5. At the time of writing the stars are 1".1 apart and will close until 2018 when a separation of about 0".6 is reached. This is a good opportunity to see this system as a triple star. The revised parallax from Hipparcos is 120.33 mas putting the group at a distance of about 27 light years. The spectral type of A is G5IV and that of the close pair appears to be dwarf M, not withstanding the fact that Chambers in his revision of Smyth's Bedford Catalogue, assigns to it a colour of cerulean blue.
As a nearby solar-like star, mu Her A is a good candidate for hosting a planetary system and in 1994 two independent series of radial velocities were taken. What they showed was a slow drift which indicated a possible period of 30 years. In 1998, a star of V magnitude 12.7 was seen 1.4 arc seconds from A using adaptive optics on the 100-inch reflector at Mount Wilson. This object is close to the sub-stellar mass limit and further observations will be needed to establish its physical connection to A. The WDS lists another star, mag 11.5 at 256", but it far from clear that this also belongs to the system.
About 4 degrees north preceding the 3rd magnitude star alpha Arae is BSO 13 (17 19 02.95 -46 38 11.4). Picked up as early as 1824 in the mural circle at Paramatta (Sydney) it is a similar system to eta Cas - a nearby, long period, unequal pair with a G-type primary and M secondary, in this case G8V and M0V. The stars, which are only 28.7 light years distant, are now moving slowly apart in their 693 year orbit and are currently at 257 degrees and 9".9 so they are visible in 75-mm aperture with ease, although larger apertures will show the colours to greater effect. Hartung notes deep yellow and orange. The WDS gives magnitudes of 5.61 and 8.88 but van den Bos in a series of measures in Johannesburg in the 1920s consistently estimated the magnitude differences as 2.5 or so, so there may be some real variation in the brightness of the companion.
Bob Argyle - Double Star Section Director