High resolution ( R \ga 40 000 ) and high S / N spectra have been acquired with UVES on the VLT-Kueyen ( Paranal Observatory , ESO Chile ) for several main sequence turnoff stars ( V \sim 17 mag ) and subgiants at the base of the Red Giant Branch ( V \sim 16 mag ) in three globular clusters ( NGCÂ 6397 , NGCÂ 6752 and 47Â Tuc/NGCÂ 104 ) at different metallicities ( respectively [ { Fe / H } ] \simeq - 2.0 ; -1.5 ; -0.7 ) .
Spectra for a sample of 25 field halo subdwarves have also been taken with equal resolution , but higher S / N .
These data have been used to determine the abundances of several neutron-capture elements in these three clusters : strontium , yttrium , barium and europium .
This is the first abundance determination of these heavy elements for such unevolved stars in these three globular clusters .
These values , together with the [ Ba/Eu ] and [ Sr/Ba ] abundance ratios , have been used to test the self-enrichment scenario .
A comparison is done with field halo stars and other well known Galactic globular clusters in which heavy elements have already been measured in the past , at least in bright giants ( V \ga 11 –12 mag ) .
Our results show clearly that globular clusters have been uniformly enriched by r – and s –process syntheses , and that most of them seem to follow exactly the same abundance patterns as field halo stars , which discards the “ classical ” self-enrichment scenario for the origin of metallicities and heavy elements in globular clusters .