Chemical analysis of the bulge globular cluster NGC 6553

This work presents the analysis of a sample of 558 red giant stars located in the direction of the Bulge globular cluster NGC 6553 and observed with the multi-object spectrograph FLAMES at the Very Large Telescope of ESO. According to their metallicity and radial velocity, we identified a sample of 245 stars likely members of the cluster. This represents the largest spectroscopic sample collected so far for NGC 6553. The main goal was to obtain detailed abundances of NGC 6553 with the purpose of studying its chemical composition and investigate its relation with the formation of the Bulge and with other Bulge clusters. In particular, the study aimed at verifying the possible presence of iron spread, which would have revealed a complex nature of NGC 6553, in spite of its standard classification as a genuine globular cluster. Moreover, we investigated, the presence of chemical anomalies through the measure of the Mg and Al abundances that show large star-to-star variations in most of the Galactic clusters. The analysis has revealed that NGC 6553 has an iron abundance of [Fe/H] = −0.13 ± 0.01 dex with no significant internal spread. This result indicates that the cluster was not able to retain the supernovae ejecta in its potential well. The [Si/Fe] abundance ratio is enhanced with respect to the solar value, indicating that the cluster was formed from gas enriched by supernovae ejecta. The [Si/Fe] measured in NGC 6553 is compatible with that measured in other Bulge stars. Finally, NGC 6553 does not exhibit a Mg-Al anti-correlation and no significant spread in either Mg and Al. This suggests that the massive stars populating the cluster in its early stage did not reach in their interior burning temperatures higher than ∼ 70 million K.