The rotation of α Oph investigated using polarimetry

Abstract

Recently we have demonstrated that high-precision polarization observations can detect the polarization resulting from the rotational distortion of a rapidly rotating Btype star. Here we investigate the extension of this approach to an A-type star. Linearpolarization 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 v e sin 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/Ω c ) between 0.83 and 0.98 and an axial inclination i > 60 • . The rotationaxis 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 T p = 8725 ± 175 K, polar gravity log g p = 3.93 ± 0.08 (dex cgs), and polar radius R p = 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 (λ max ) of 440 ± 110 nm, consistent with values found for other nearby stars.

Polarimetric and radiative transfer modelling of HD 172555

“…Earlier Marshall et al (2016) found λmax to be between 35 nm and 600 nm with a most probable value of 470 nm from 2-band observations of a few stars at Southern declinations. Lastly, in investigating the rotational properties of α Oph, Bailey et al (2020b) found 440 ± 110 nm for that star. Multi-band data was taken for HD 167425 and HD 162521.…”

“…The Serkowski Law gives a redder, more tightly defined λmax, but K is not particularly well defined. The past studies of the interstellar medium (Marshall et al 2016;Cotton et al 2019a;Bailey et al 2020b) have found values of λmax more closely aligned with the Serkowski-Wilking Law fit, and so values of λmax equal to 472.6 nm and K equal to 0.79 are adopted.…”

“…A similar problem exists in aperture polarimetry of other non-variable phenomena, for instance in determining the inclination and rotation rate of rapidly rotating stars (Cotton et al 2017a;Bailey et al 2020b). That case is more easily dealt with however, since the wavelength dependence of polarisation intrinsic to the system is well defined (Bailey et al 2020b). Whereas in debris disc systems we do not yet understand their architectures or compositions well enough to so easily constrain the parameters; consequently, observations of the target system alone are not sufficient.…”

Phase-locked polarization by photospheric reflection in the semidetached eclipsing binary μ1 Sco

“…HIPPI-2 and Mini-HIPPI were both used in the study of the Spica binary system (Bailey et al 2019). HIPPI-2 has also recently been used in the study of the rapidly rotating system α Oph (Bailey et al 2020b), the red supergiant Betelgeuse (Cotton et al 2020a) and the polluted white dwarf G29-38 (Cotton et al 2020b).…”

“…It has been used in Earth atmosphere research as well as planetary atmosphere modelling (Bailey et al 2018;Bott et al 2018). In Cotton et al (2017) and Bailey et al (2020b) / was used for modelling the polarization of a rotationally distorted star, and the supplementary materials of the first work included verifications of the methods by comparison with stellar polarization calculations of Harrington (2015) and rotating star polarization models of Sonneborn (1982).…”