Neutron-capture element abundances of luminous giant stars in the Globular Clusters 47 Tuc, NGC 6388 and NGC 362.

A spectroscopic study of neutron-capture element abundances has been carried out for giant stars in three globular clusters: 47 Tuc, NGC 6388 and NGC 362. These clusters range in metallicity from [Fe/H] = -0.60 ± 0.02 dex for NGC 6388, [Fe/H] = -0.88 ± 0.09 dex for 47 Tuc and [Fe/H] = -1.21 ± 0.09 dex for NGC 362, as derived in this study. The stars analysed here were all luminous giants located near the asymptotic giant branch for each cluster. The two stars analysed in NGC 6388 both showed enhancements in the light (ls: Sr, Y, Zr) and heavy (hs: La, Nd) s-process elements ([ls/Fe] = 0.58 ± 0.13 dex, [hs/Fe] = 0.39 ± 0.07 dex, [hs/ls] = -0.18 ± 0.06 dex). Six stars were analysed in 47 Tuc and similar enhancements were found ([ls/Fe] = 0.53 ± 0.02 dex, [hs/Fe] = 0.40 ± 0.06 dex, [hs/ls] = -0.13 ± 0.05 dex). The eleven stars analysed in NGC 362 also showed enhancements in the s-process elements. However the heavy s-process element abundances were more enhanced than the light s-process ([ls/Fe] = 0.32 ± 0.10 dex, [hs/Fe] = 0.46 ± 0.09 dex , [hs/ls] = +0.14 ± 0.03 dex). A comparison between the s-process element abundances of each cluster show a general trend of increasing [hs/ls] with decreasing [Fe/H]. The small spread in the abundances of these s-process elements indicates a homogeneous distribution within each cluster, confirming that the stars analysed here are not producing sprocess elements internally. Hence the s-process element abundance distribution represents a pre-existing chemical signature for each cluster. Europium, the representative r-process element, is also enhanced, with a homogeneous distribution, in all three clusters.