X-shooter observations of main sequence stars in the globular cluster
  NGC 2808: first chemical tagging of a He-normal and a He-rich dwarf
  We present the first chemical composition study of two unevolved stars in the
globular cluster NGC 2808, obtained with the spectrograph X-shooter@VLT. NGC
2808 shows three discrete, well separated main sequences. The most accepted
explanation for this phenomenon is that their stars have different helium
contents. We observed one star on the bluest main sequence, (bMS, claimed to
have high helium content, Y~0.4), and one on the reddest main sequence (rMS,
consistent with a canonical helium content, Y=0.245). We analyzed features of
NH, CH, Na, Mg, Al, and Fe. While Fe, Ca, and other elements have the same
abundances in the two stars, the bMS star shows a huge enhancement of N, a
depletion of C, an enhancement of Na and Al, and small depletion of Mg with
respect to the rMS star. This is exactly what is expected if stars on the bMS
formed from the ejecta produced by an earlier stellar generation in the
complete CNO and MgAl cycles whose main product is helium. The elemental
abundance pattern differences in these two stars are consistent with the
differences in helium content suggested by the color-magnitude diagram
positions of the stars.
