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Context. NGC 1851 is one of several globular clusters for which multiplestellar populations of the subgiant branch have been clearly identified and adifference in metallicity detected. A crucial piece of information on theformation history of this cluster can be provided by the sum of A(C+N+O)abundances. However, these values have lacked a general consensus thus far. Theseparation of the subgiant branch can be based on age and/or A(C+N+O) abundancedifferences. Aims. Our main aim was to determine carbon, nitrogen, and oxygenabundances for evolved giants in the globular cluster NGC1851 in order to checkwhether or not the double populations of stars are coeval. Methods.High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESOVLT telescope, were analysed using a differential model atmosphere method.Results. We provide abundances of up to 29 chemical elements for a sample of 45giants in NGC 1851. The investigated stars can be separated into twopopulations with a difference of 0.07 dex in the mean metallicity, 0.3 dex inthe mean C/N, and 0.35 dex in the mean s-process dominated element-to-ironabundance ratios [s/Fe]. No significant difference was determined in the meanvalues of A(C+N+O) as well as in abundance to iron ratios of carbon, alpha- andiron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O)values between the two populations do not differ, additional evidence is giventhat NGC 1851 is composed of two clusters, the metal-rich cluster being byabout 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851properties and the detailed abundances of chemical elements favour itsformation in a dwarf spheroidal galaxy that was accreted by the Milky Way.

Gaia-ESO Survey: Detailed elemental abundances in red giants of the peculiar globular cluster NGC 1851