A spectroscopic study of the elusive globular cluster ESO452-SC11 and its surroundings

Andreas Koch, Camilla Juul Hansen, Andrea Kunder

Published 2017-03-20Version 1

Globular clusters (GCs) are amongst the oldest objects in the Galaxy and play a pivotal role in deciphering its early history. We present the first spectroscopic study of the GC ESO452-SC11 using the AAOmega spectrograph at medium resolution. Given the sparsity of this object and high degree of foreground contamination due to its location toward the bulge, few details are known for this cluster: there is no consensus of its age, metallicity, or its association with the disk or bulge. We identify 5 members based on radial velocity, metallicity, and position within the GC. Using spectral synthesis, accurate abundances of Fe and several $\alpha$-, Fe-peak, neutron-capture elements (Si,Ca,Ti,Cr,Co,Ni,Sr,Eu) were measured. Two of the 5 cluster candidates are likely non-members, as they have deviant Fe abundances and [$\alpha$/Fe] ratios. The mean radial velocity is 19$\pm$2 km s$^{-1}$ with a low dispersion of 2.8$\pm$3.4 km s$^{-1}$, in line with its low mass. The mean Fe-abundance from spectral fitting is $-0.88\pm0.03$, with a spread driven by observational errors. The $\alpha$-elements of the GC candidates are marginally lower than expected for the bulge at similar metallicities. As spectra of hundreds of stars were collected in a 2 degree field around ESO452-SC11, detailed abundances in the surrounding field were measured. Most non-members have higher [$\alpha$/Fe] ratios, typical of the nearby bulge population. Stars with measured Fe-peak abundances show a large scatter around Solar values, though with large uncertainties. Our study provides the first systematic measurement of Sr in a Galactic bulge GC. The Eu and Sr abundances of the GC candidates are consistent with a disk or bulge association. Our calculations place ESO452 on an elliptical orbit in the central 3 kpc of the bulge. We find no evidence of extratidal stars in our data. (Abridged)