High-resolution infrared spectra of bulge globular clusters: Liller 1 and NGC 6553

摘要

Using the NIRSPEC spectrograph at Keck II, we have obtained echelle spectra covering the range 1.5-1.8 l m for two of the brightest giants in Liller 1 and NGC 6553, old metal-rich globular clusters in the Galactic bulge. We use spectrum synthesis for the abundance analysis and find Fe=H = -0.3 +/- 0.2 and O=H = 0.3 +/- 0.1 ( from the OH lines) for the giants in both clusters. We measure strong lines for the alpha-elements Mg, Ca, and Si, but the lower sensitivity of these lines to abundance permits us only to state a general alpha/Fe = 0.3 +/- 0.2 dex. The composition of the clusters is similar to that of field stars in the bulge and is consistent with a scenario in which the clusters formed early, with rapid enrichment. We have difficulty achieving a good fit to the spectrum of NGC 6553 using either the low or the high values recently reported in the literature, unless unusually large or no alpha-element enhancements are adopted, respectively. References: 49