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Published January 10, 2011 | Published
Journal Article Open

Establishing α Oph as a prototype rotator: improved astrometric orbit


The nearby star α Oph (Ras Alhague) is a rapidly rotating A5IV star spinning at ~ 89% of its breakup velocity. This system has been imaged extensively by interferometric techniques, giving a precise geometric model of the star's oblateness and the resulting temperature variation on the stellar surface. Fortuitously, α Oph has a previously known stellar companion, and characterization of the orbit provides an independent, dynamically based check of both the host star and the companion mass. Such measurements are crucial to constrain models of such rapidly rotating stars. In this study, we combine eight years of adaptive optics imaging data from the Palomar, AEOS, and CFHT telescopes to derive an improved, astrometric characterization of the companion orbit. We also use photometry from these observations to derive a model-based estimate of the companion mass. A fit was performed on the photocenter motion of this system to extract a component mass ratio. We find masses of 2.40^(+0.23)_(−0.37) M_⊙ and 0.85^(+0.06)_(−0.04) M_⊙ for α Oph A and α Oph B, respectively. Previous orbital studies of this system found a mass too high for this system, inconsistent with stellar evolutionary calculations. Our measurements of the host star mass are more consistent with these evolutionary calculations, but with slightly higher uncertainties. In addition to the dynamically derived masses, we use IJHK photometry to derive a model-based mass for α Oph B, of 0.77 ± 0.05 M_⊙ marginally consistent with the dynamical masses derived from our orbit. Our model fits predict a periastron passage on 2012 April 19, with the two components having a 50 mas separation from 2012 March to May. A modest amount of interferometric and radial velocity data during this period could provide a mass determination of this star at the few percent level.

Additional Information

© 2011 The American Astronomical Society. Received 2010 August 11; accepted 2010 October 12; published 2010 December 21. This work was performed in part under contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program. The Lyot Project is based upon work supported by the National Science Foundation under Grant Nos. AST-0804417, 0334916, 0215793, and 0520822. The Lyot Project gratefully acknowledges the support of the US Air Force and NSF in creating the special Advanced Technologies and Instrumentation opportunity that provides access to the AEOS telescope. The Lyot Project is also grateful to the Cordelia Corporation, Hilary and Ethel Lipsitz, the Vincent Astor Fund, Judy Vale, and an anonymous donor who initiated the project. A portion of the research in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Thanks also to Anthony Boccaletti for clarification of his epoch of the α Oph astrometry. We thank Willie Torres for help checking the notation of our orbital elements, and also to the anonymous referee for several helpful comments. J.D.M. gratefully acknowledges support from the National Science Foundation under Grant AST-0707927.

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