Chemical Abundances of Three Metal-poor Globular Clusters (NGC 6287, NGC 6293, and NGC 6541) in the Inner Halo

Abstract

We present a chemical abundance study of three inner clusters, NGC 6287, 6293, and 6541, finding [Fe/H] = -2.01 ± 0.05, -1.99 ± 0.02, and -1.76 ± 0.02 (internal), respectively, and our metallicity measurements are in good agreement with previous estimates. We also present the radial velocity measurements of the clusters. Our radial velocity measurements for NGC 6293 and NGC 6541 are in good agreement with previous measurements; however, our radial velocity measurement for NGC 6287 is almost 80 km s-1 larger than the previous measurement. The mean α-element abundances of our program clusters are in good agreement with other globular clusters, confirming previous results. However, the individual α-elements appear to follow different trends. The silicon abundances of the inner clusters appear to be enhanced, and the titanium abundances appear to be depleted compared to the intermediate clusters. Our results also appear to oppose to those of metal-rich bulge giants studied by McWilliam & Rich, who found that bulge giants are titanium-enhanced and silicon-deficient. In particular, [Si/Ti] ratios appear to be related to Galactocentric distances, in the sense that [Si/Ti] ratios decrease with Galactocentric distance. We propose that contributions from different masses of the Type II supernovae progenitors that enriched proto–globular cluster clouds' elemental abundances and the different initial physical environments surrounding the proto–globular clusters clouds are responsible for this gradient in [Si/Ti] ratios versus Galactocentric distances of the old halo globular clusters. On the other hand, our program clusters' enhanced s-process elemental abundances suggest that the formation timescale of our program clusters might be as short as a few times 108 yr after the star formation is initiated in the Galaxy's central regions, if the s-process site is intermediate-mass asymptotic giant branch stars.