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 α Oph. The A5 IV primary is found to have a mass of 2.84+/-0.19 M_solar, while the approximately K2 V companion has a mass of 0.78+/-0.058 M_solar. 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.