Heavy element abundances in low gravity globular cluster stars: NGC 362 and NGC 6388

Giant branch stars in the globular clusters NGC 362 and NGC 6388 were analysed for key s- and r-process element abundances. The NGC 362 stars showed comparable enhancements in the light and heavy s-process elements for the sample of 13 giant stars with values of <[ l s/Fe ]> = 0.32 +/- 0.10 dex and <[ h s/Fe ]> = 0.46 +/- 0.09 dex. The small spread in these values, as well as in the derived metallicity values (<[ Fe/H ]> = -1.21 +/- 0.09 dex), confirms the homogeneity of these elements in NGC 362. The two stars analysed in NGC 6388 showed enhancements in the light and heavy s-process elements with values of <[ l s/Fe ]> = 0.58 +/- 0.13dex and <[ h s/Fe ]> = 0.39 +/- 0.07 dex. The light s-process elements are more enhanced in this metal-rich cluster compared to the heavy s-process elements. The derived metallicity was in good agreement between the two stars with the mean value for the cluster of <[ Fe/H ]> = -0.60 +/- 0.02 dex. Due to the similar metallicity of NGC 6388 with 47 Tuc, these two stars were compared with the results from Worley et al. where giant branch stars in 47 Tuc were analysed. The heavy element enhancements between the two clusters were found to be very similar indicating that the relative enhancements of ls and hs elemental abundances is metallicity dependent. When comparing the three clusters, there is a distinct trend of decreasing ls abundance with decreasing metallicity. The hs abundances are essentially the same within their respective uncertainties. This is reflected in the [hs/ls] ratio for the three clusters. The ratio is very similar for NGC 6388 and 47 Tuc (<[ h s/l s]> = -0.18 +/- 0.06 dex and <[ h s/l s]> = -0.13 +/- 0.05 dex, respectively), but for NGC 362 the ratio is much higher (<[ h s/l s]> = +0.14 +/- 0.03 dex). The small spread of derived neutron-capture elemental abundance values for all three clusters confirms that these asymptotic giant branch stars are not undergoing third dredge-up and so are not producing the observed s-process enhancements internally.

Based on observations made with European Southern Observatory (ESO) Telescopes at the Paranal Observatories under programme 71.D-0540(A).

E-mail: cworley@oca.eu