September 2014 - Double Star of the Month

H 1 48 (21 13 42.46 +64 24 15.1) is a very rare example of one of William Herschel's close discoveries which retains its original designation instead of being absorbed into the catalogue of F. G. W. Struve, as many of his pairs were. It is also remarkable as being a fairly short period system which has both high eccentricity and high inclination. It is characterized by periods of rapid angular motion at very small separations and then stretches of decades when it is visible to the medium aperture. The brightness of the stars (7.2 and 7.3) means that it is never an easy object in small telescopes but may well be visible in 15-cm after 2020 or so when the separation slowly increases to 0".9. The ephemeris for the 81.7 year orbit by Marco Scardia and colleagues gives a separation of 0".72 and PA 243° for 2015.0. The pair can be found 1 deg south preceding 6 Cephei which is, in turn, 3 degrees north of alpha Cephei.

BSO 15 (21 48 15.75 -47 18 13.0) is a naked eye star in Gruis about 4 degrees preceding and slightly south of alpha. It was found to be double by Thomas Brisbane in the early 1830s. The primary is a GO dwarf of V = 5.6. Hipparcos places this star at a distance of only 52 light years and as a consequence it has a fairly substantial proper motion of almost one-third of an arc second per year. The mag 8.8 companion is not connected and is being rapidly left behind by the proper motion of A. The pair was first accurately measured by John Herschel in 1836 when he found B at a separation of 30.3" in PA 14°. Last year the writer measured the pair again and found a distance of 78".9 in PA 350.2°, in good agreement with the prediction given in the USNO Linear Elements Catalogue. This pair has not been observed by either Hartung or Haas and the writer didn't note any significant colour in either component. Recent observations by the infra-red Herschel telescope show there is a large proto-planetary dust ring around this star stretching from about 100 to 180 AU.

Bob Argyle - Double Star Section Director