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

Abstract

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 $\omega (=\Omega /{\Omega }_{\mathrm{c}})\ge 0.94$, polar gravity $\log g_p=2.091_{-0.039}^{+0.042}$ (dex cgs), mass $3.10_{-0.32}^{+0.37}$ solar masses, and luminosity $\log (L/Lsun)=3.149_{-0.028}^{+0.041}$. 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.

arXiv comment12 pages, 10 figures, Accepted by MNRAS

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