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With the aim to constrain multiple populations in the metal-poor globularcluster NGC6397, we analyse and discuss the chemical compositions of a largenumber of elements in 21 red giant branch stars. High-resolution spectra wereobtained with the FLAMES/UVES spectrograph on VLT. We have determined non-LTEabundances of Na and LTE abundances for the remaining 21 elements, including O,Mg, Al, alpha, iron-peak, and neutron-capture elements, many of which have notpreviously been analysed for this cluster. We have also considered theinfluence of possible He enrichment in the analysis of stellar spectra. We findthat the Na abundances of evolved, as well as unevolved, stars show a distinctbimodality, which suggests the presence of two stellar populations; oneprimordial stellar generation with composition similar to field stars, and asecond generation that is enriched in material processed throughhydrogen-burning (enriched in Na and Al and depleted in O and Mg). The clusteris dominated (75%) by the second generation. The red giant branch show asimilar bimodal distribution in the Stroemgren colour index c_y=c_1-(b-y),implying a large difference also in N abundance. The two populations have thesame composition of all analysed elements heavier than Al, within themeasurement uncertainty of the analysis, with the possible exception of [Y/Fe].Using two stars with close to identical stellar parameters, one from eachgeneration, we estimate the difference in He content, Delta Y=0.01+-0.06, giventhe assumption that the mass fraction of iron is the same for the stars.Finally, we show that winds from fast rotating massive stars of the firstgeneration can be held responsible for the abundance patterns observed inNGC6397 second generation long-lived stars and estimate that the initial massof the cluster were at least ten times higher than its present-day value.

Tracing the evolution of NGC6397 through the chemical composition of its stellar populations