Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808: Direct spectroscopic measurements

We present an abundance analysis of 96 horizontal branch (HB) stars in NGC 2808, a globularcluster exhibiting a complex multiple stellar population pattern. These stars are distributed indifferent portions of the HB and cover a wide range of temperature. By studying the chemicalabundances of this sample, we explore the connection between HB morphology and thechemical enrichment history of multiple stellar populations. For stars lying on the red HB, weuse GIRAFFE and UVES spectra to determine Na,Mg, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y, Baand Nd abundances. For colder, blue HB stars, we derive abundances for Na, primarily fromGIRAFFE spectra.We were also able to measure direct non-local thermodynamic equilibriumHe abundances for a subset of these blue HB stars with temperature higher than ~9000 K.Our results show that: (i) HB stars in NGC 2808 show different content in Na depending ontheir position in the colour-magnitude diagram, with blue HB stars having higher Na than redHB stars; (ii) the red HB is not consistent with a uniform chemical abundance, with slightlywarmer stars exhibiting a statistically significant higher Na content; and (iii) our subsampleof blue HB stars with He abundances shows evidence of enhancement with respect to the predicted primordial He content by δY = +0.09 ± 0.01 ± 0.05 (internal plus systematicuncertainty). Our results strongly support theoretical models that predict He enhancementamong second-generation(s) stars in globular clusters and provide observational constraintson the second-parameter governing HB morphology.