The nearby star \alpha Oph ( Ras Alhague ) is a rapidly rotating A5IV star spinning at \sim 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 , \alpha 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 _ { \odot } and 0.85 ^ { +0.06 } _ { -0.04 } M _ { \odot } for \alpha Oph A and \alpha 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 \alpha Oph B , of 0.77 \pm 0.05 M _ { \odot } 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 March to May 2012 .
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 .