2023MNRAS.520.1193H

We report new, extremely precise photopolarimetry of the rapidly-rotating A0 main-sequence star ζ Aql, covering the wavelength range ∼400-900 nm, which reveals a rotationally-induced signal. We model the polarimetry, together with the flux distribution and line profiles, in the framework of Roche geometry with ω-model gravity darkening, to establish the stellar parameters. An additional constraint is provided by TESS photometry, which shows variability with a period, P_phot, of 11.1 h. Modelling based on solid-body surface rotation gives rotation periods, P_rot, that are in only marginal agreement with this value. We compute new ESTER stellar-structure models to predict horizontal surface-velocity fields, which depart from solid-body rotation at only the ∼2 per cent level (consistent with a reasonably strong empirical upper limit on differential rotation derived from the line-profile analysis). These models bring the equatorial rotation period, P_rot(e), into agreement with P_phot, without requiring any 'fine tuning' (for the Gaia parallax). We confirm that surface abundances are significantly subsolar ([M/H] ≃ -0.5). The star's basic parameters are established with reasonably good precision: $M = 2.53\pm 0.16\, \mbox{M}_{\odot }$, log (L/L_⊙) = 1.72± 0.02, $R_{\rm p}= 2.21\pm 0.02\, \mbox{R}_{\odot }$, T_eff = 9693 ± 50 K, $i = 85{^{+5}_{-7}}^\circ$, and ω_e/ω_c = 0.95 ± 0.02. Comparison with single-star solar-abundance stellar-evolution models incorporating rotational effects shows excellent agreement (but somewhat poorer agreement for models at [M/H] ≃ -0.4).