ESTABLISHING α Oph AS A PROTOTYPE ROTATOR: IMPROVED ASTROMETRIC ORBIT
2011
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.
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