Gaia-ESO Survey: Detailed elemental abundances in red giants of the
  peculiar globular cluster NGC 1851
  Context. NGC 1851 is one of several globular clusters for which multiple
stellar populations of the subgiant branch have been clearly identified and a
difference in metallicity detected. A crucial piece of information on the
formation 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. The
separation of the subgiant branch can be based on age and/or A(C+N+O) abundance
differences. Aims. Our main aim was to determine carbon, nitrogen, and oxygen
abundances for evolved giants in the globular cluster NGC1851 in order to check
whether or not the double populations of stars are coeval. Methods.
High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO
VLT telescope, were analysed using a differential model atmosphere method.
Results. We provide abundances of up to 29 chemical elements for a sample of 45
giants in NGC 1851. The investigated stars can be separated into two
populations with a difference of 0.07 dex in the mean metallicity, 0.3 dex in
the mean C/N, and 0.35 dex in the mean s-process dominated element-to-iron
abundance ratios [s/Fe]. No significant difference was determined in the mean
values of A(C+N+O) as well as in abundance to iron ratios of carbon, alpha- and
iron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O)
values between the two populations do not differ, additional evidence is given
that NGC 1851 is composed of two clusters, the metal-rich cluster being by
about 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851
properties and the detailed abundances of chemical elements favour its
formation in a dwarf spheroidal galaxy that was accreted by the Milky Way.
