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.