A study of the F-giant star theta Scorpii A: a post-merger rapid rotator?

We report high-precision observations of the linear polarization of the F1III star theta 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 omega(= Omega/Omega(c)) >= 0.94, polar gravity log(g(p)) = 2.091(-0.039)(+0.042) (dex cgs), mass 3.10(-0.32)(+0.37) M-circle dot, and luminosity log(L/L-circle dot) = 3.149(-0.028)(+0.041). 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 theta Sco A is most probably the product of a binary merger.