First spectroscopic investigation of anomalous Cepheid variables

Context. Anomalous Cepheids (ACEPs) are intermediate-mass metal-poor pulsators that are mostly discovered in dwarf galaxies of the Local Group. However, recent Galactic surveys, including the Gaia Data Release 3, found a few hundred ACEPs in the Milky Way. Their origin is only poorly understood. Aims. We aim to investigate the origin and evolution of Galactic ACEPs by studying the chemical composition of their atmospheres for the first time. Methods. We used UVES at the Very Large Telescope to obtain high-resolution spectra for a sample of nine ACEPs belonging to the Galactic halo. We derived the abundances of 12 elements, C, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Ni, Y, and Ba. We complemented these data with literature abundances from high-resolution spectroscopy for an additional three ACEPs that were previously incorrectly classified as type II Cepheids. This increased the sample to a total of 12 stars. Results. All the investigated ACEPs have an iron abundance [Fe =H] < 1 :5 dex, as expected from theoretical predictions for these pulsators. The abundance ratios of the di fferent elements to iron show that the chemical composition of ACEPs is generally consistent with that of the Galactic halo field stars, with the exception of sodium, which is found to be overabundant in 9 out of the 11 ACEPs where it was measured. This is very similar to the situation for second-generation stars in Galactic globular clusters. The same comparison with dwarf and ultra-faint satellites of the Milky Way reveals more differences than similarities. It is therefore unlikely that the bulk of Galactic ACEPs originated in a galaxy like this that subsequently dissolved into the Galactic halo. The principal finding of this work is the unexpected overabundance of sodium in ACEPs. We explored several hypotheses to explain this feature, finding that the most promising scenario is the evolution of low-mass stars in a binary system with either mass transfer or merging. Detailed modelling is needed to confirm this hypothesis.