Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters

Abstract

We have observed 10 red giant stars in four old Large Magellanic Cloud globular clusters with the high-resolution spectrograph MIKE on the Magellan Landon Clay 6.5 m telescope. The stars in our sample have up to 20 elemental abundance determinations for the α-, iron peak, and neutron-capture element groups. We have also derived abundances for the light odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be more metal-rich than previous estimates from the Ca II triplet, and we derive [Fe/H] values closer to those obtained from the slope of the red giant branch. However, we confirm previous determinations for Hodge 11 and NGC 1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios are comparable to the values observed in old Galactic globular cluster stars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMC clusters do not share the low-Y behavior observed in some dwarf spheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however, are significantly lower than what is seen in the Galactic globular cluster system. Neither does the behavior of [Cu/Fe] as a function of [Fe/H] in our LMC clusters match the trend seen in the Galaxy, staying instead at a constant value of roughly -0.8. Because not all [α/Fe] ratios are suppressed, these abundance ratios cannot be attributed solely to the injection of Type Ia supernova material and instead reflect the differences in star formation history of the LMC versus the Milky Way. An extensive numerical experimental study was performed, varying both input parameters and stellar atmosphere models, to verify that the unusual abundance ratios derived in this study are not the result of the adopted atomic parameters, stellar atmospheres, or stellar parameters. We conclude that many of the abundances in the LMC globular clusters we observed are distinct from those observed in the Milky Way, and these differences are intrinsic to the stars in those systems.