Detailed chemical abundances of the globular cluster Terzan 6 in the inner bulge

Abstract

We used near-infrared spectroscopy at medium-high resolution (R = 8000 − 25 000) to perform the first comprehensive chemical study of the intermediate luminosity bulge globular cluster Terzan 6. We derived detailed abundances and abundance patterns of 27 giant stars, likely members of Terzan 6, based on their accurate Hubble Space Telescope proper motions and line-of-sight radial velocities. From the spectral analysis of these stars, we determined an average heliocentric radial velocity of 143.3 ± 1.0 km s⁻¹ with a velocity dispersion of 5.1 ± 0.7 km s⁻¹ and an average [Fe/H] = −0.65 ± 0.01 and a low 1σ dispersion of 0.03 dex. We also measured some depletion of [Mn/Fe] with respect to the solar-scaled values and enhancement of for [Ca/Fe], [Si/Fe], [Mg/Fe], [Ti/Fe], [O/Fe], [Al/Fe], [Na/Fe], and, to a lower extent, for [K/Fe], consistent with previous measurements of other bulge globular clusters and favoring the scenario of a rapid bulge formation and chemical enrichment. Some spread in the light element abundances suggest the presence of first- and second-generation stars, typical of genuine globulars. Finally, we measured some depletion of carbon and low ¹²C/¹³C isotopic ratios, as in previous studies of field and cluster bulge giants, indicating that extra-mixing mechanisms should be at work during the post main sequence evolution in the high metallicity regime as well.

Based on observations collected at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Also based on observations at the Very Large Telescope of the European Southern Observatory at Cerro Paranal (Chile) under program 089.D-0306 (PI:Ferraro).