Recently we have demonstrated that high-precision polarization observations can detect the polarization resulting from the rotational distortion of a rapidly rotating B-type star .
Here we investigate the extension of this approach to an A-type star .
Linear-polarization observations of \alpha Oph ( A5IV ) have been obtained over wavelengths from 400 to 750 nm .
They show the wavelength dependence expected for a rapidly-rotating star combined with a contribution from interstellar polarization .
We model the observations by fitting rotating-star polarization models and adding additional constraints including a measured v _ { e } \sin { i } .
However , we can not fully separate the effects of rotation rate and inclination , leaving a range of possible solutions .
We determine a rotation rate ( \omega = \Omega / \Omega _ { c } ) between 0.83 and 0.98 and an axial inclination i > 60 ^ { \circ } .
The rotation-axis position angle is found to be 142 ^ { \circ } \pm 4 ^ { \circ } , differing by 16 ^ { \circ } from a value obtained by interferometry .
This might be due to precession of the rotation axis due to interaction with the binary companion .
Other parameters resulting from the analysis include a polar temperature T _ { p } = 8725 \pm 175 K , polar gravity \log { g _ { p } } = 3.93 \pm 0.08 ( dex cgs ) , and polar radius R _ { p } = 2.52 \pm 0.06 ~ { } \mbox { R } _ { \odot } .
Comparison with rotating-star evolutionary models indicates that \alpha Oph is in the later half of its main-sequence evolution and must have had an initial \omega of 0.8 or greater .
The interstellar polarization has a maximum value at a wavelength ( \lambda _ { max } ) of 440 \pm 110 nm , consistent with values found for other nearby stars .