An  Astrometric  Study  of  the  Binary  Star  a Oph

George Gatewood,                                                                                                                                                                               August 2005, AJ


The work reported here combines data from the Multichannel Astrometric Photometer (MAP),  the European Astrometric Satellite, Hipparcos, and infrared speckle observations of the nearby sub giant binary star system a 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 magnitudes yielding an absolute visual magnitude of 1.248 0.025, and an absolute K magnitude of the secondary of 4.3 0.2 magnitudes.  Tables list the MAP data and an ephemeris which indicates that the next 4 years are ideal for further interferometry. 


In our 1944-45 Observatory Report, Director N. E. Wagman announced that a study of a Oph (Ras Alhague) indicated an astrometric motion with a period of nine years.  The plate series, on which this discovery was based, was continued at Allegheny Observatory and a second plate series was started at Sproul Observatory.  These were summarized more than two full orbits later (Lippincott & Wagman 1966), improving on the parallax, orbital period and date of periastron.   Lacking any estimate of the separation of the components, the system masses could not be computed.  Numerous attempts to resolve the system in visible light have failed (McAlister & Hartkopf 1984 & Mason et al. 1999).  First resolutions of the system were in the L (3400 nm) band (McCarthy 1983) and more recently in the K (2154 nm) band (Boccaletti et al. 2001) where the magnitude difference is observed to be 3.5 0.2 magnitudes.  The reference frame for these reductions was based on the UCAC2 (Zacharias et al. 2001).


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