HD 10840: A southern chemically peculiar Ap star with extreme rare earth element abundances

Rare earth elements (REEs) often shape the spectra of the chemically peculiar magnetic Ap/Bp stars, whose atmospheres provide a natural laboratory for testing REE atomic data and opacities. We aim to characterise the atmosphere and the surface chemical inhomogeneities of the southern Ap star HD 10840, whose spectra show exceptionally strong and variable REE lines. We obtained a spectroscopic time series of HD 10840 at 13 rotational phases with the SALT/HRS spectrograph, supplemented by four archival UVES spectra. We derived the atmospheric parameters by fitting the spectral energy distribution and the hydrogen line profiles. Average abundances were determined under the local thermodynamic equilibrium assumption, and the surface abundance distributions were reconstructed with the INVERS11 Doppler imaging (DI) code, which we extended to support fixed abundance maps for blending elements and a new estimate of the map uncertainties. HD 10840 has K, łogg=3.6, R=2.19 and šini=35 ̨ms, which together with P=2 imply an inclination i The star shows overabundances of Si and the iron-peak elements of up to ∼1.5 dex and extreme REE overabundances reaching ∼6 dex relative to the Sun, exceeding those of Przybylski's star and making HD 10840 the current record holder. DI yielded surface maps for eleven elements, including six REEs. Silicon, the iron-peak elements, and the REEs share a common distribution, with a compact equatorial spot at phase ϕ≈0.5 and a more extended spot at ϕ≈0 separated by an off-centred belt of reduced abundance; only magnesium is distributed roughly in anti-phase. We also detect radial velocity variation of hydrogen line profiles with rotation phase, which are possibly induced by changes in the atmospheric structure in regions with extreme chemical composition inhomogeneities.