2022MNRAS.513.1129L

We report high-precision observations of the linear polarization of the F1III star θ Scorpii. The polarization has a wavelength dependence of the form expected for a rapid rotator, but with an amplitude several times larger than seen in otherwise similar main-sequence stars. This confirms the expectation that lower-gravity stars should have stronger rotational-polarization signatures as a consequence of the density dependence of the ratio of scattering to absorption opacities. By modelling the polarization, together with additional observational constraints (incorporating a revised analysis of Hipparcos astrometry, which clarifies the system's binary status), we determine a set of precise stellar parameters, including a rotation rate ω(= Ω/Ω_ c_)≥0.94, polar gravity log(g_ p_)= 2.091 ^+0.042_–0.039 (dex cgs), mass 3.10 ^+0.37_–0.32 M_☉, and luminosity log(L/L_☉) =3.149^+0.041_–0.028. These values are incompatible with evolutionary models of single rotating stars, with the star rotating too rapidly for its evolutionary stage, and being undermassive for its luminosity. We conclude that θ Sco A is most probably the product of a binary merger.