An astrometric study of the binary star α Ophiuchi

The work reported here combines data from the Multichannel Astrometric Photometer (MAP), the European astrometric satellite, Hipparcos, and infrared speckle observations of the nearby subgiant binary star system alpha Oph. The A5 IV primary is found to have a mass of 2.84 +/- 0.19 M-., while the approximately K2 V companion has a mass of 0.78 +/- 0.058 M-.. The distance modulus is 0.834 +/- 0.024 mag, yielding an absolute visual magnitude of 1.248 +/- 0.025 mag and an absolute K magnitude of the secondary of 4.3 +/- 0.2 mag. Tables list the MAP data and an ephemeris that indicates that the next 4 yr are ideal for further interferometry.