The rotation of α Oph investigated using polarimetry

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 α 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 vesin i. However, we cannot fully separate the effects of rotation rate and inclination, leaving a range of possible solutions. We determine a rotation rate (ω = Ω/Ωc) between 0.83 and 0.98 and an axial inclination i > 60°. The rotation-axis position angle is found to be 142 ± 4°, differing by 16° 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 Tp = 8725 ± 175 K, polar gravity log gp = 3.93 ± 0.08 (dex cgs), and polar radius Rp = 2.52 ± 0.06 R⊙. Comparison with rotating-star evolutionary models indicates that α Oph is in the later half of its main-sequence evolution and must have had an initial ω of 0.8 or greater. The interstellar polarization has a maximum value at a wavelength (λmax) of 440 ± 110 nm, consistent with values found for other nearby stars.