NGC 6388: chemical composition of its eight cool giants.
WALLERSTEIN G., KOVTYUKH V.V. and ANDRIEVSKY S.M.
Abstract (from CDS):
Eight cool giants in the unusual globular cluster NGC 6388 have been investigated in order to derive their elemental abundances. Effective temperatures from 3500 to 3850 K were derived using the method of line-depth ratios. Surface gravities were derived in two ways. Spectroscopic gravities, derived by the requirement that the abundance of iron be the same from Fe I and Fe II lines, were rather low, ranging from -0.3 to 0.0. Photometric gravities, derived from the assumed stellar mass of 0.7 M☉ and the luminosity and Teff of the stars, fell between +0.25 and +0.70. Mean [Fe/H] values were -0.8 for spectroscopic gravities and -0.6 for photometric gravities. A test using spectra of the same resolution of the two coolest red giants in the globular cluster M4 obtained at the Apache Point Observatory were analyzed for comparison with the definitive analysis of Ivans et al. ((ref???)1999). The very cool metal-poor red giant HD 232078 was also analyzed for comparison with the analysis of Gonzalez & Wallerstein ((ref???)1998). Both comparisons showed that our methods yield the same abundance scale as previous works. We have compared the composition of stars in NGC 6388 with those of K giants with similar [Fe/H] in 47 Tuc and the Galactic bulge. The observed value of [O/Fe] is near zero, which is less than in 47 Tuc and bulge stars of similar metallicity. The α-elements behave similarly to oxygen and show only small excesses at about the same level as do the α-elements in the globular clusters associated with the Sgr system. It is unclear whether these differences are responsible for the unusual color-magnitude diagram of NGC 6388.
Galaxy: Globular Clusters: Individual: NGC Number: NGC 6388 - Stars: Abundances