Chemical Abundances in 12 Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy

Abstract

High-resolution infrared spectra (� /D� = 50,000) have been obtained for 12 red giant members of the Large Magellanic Cloud (LMC) with the Gemini South 8.3 m telescope and Phoenix spectrometer. Two wavelength regions, at 15540 and 23400 A ˚ , were observed. Quantitative chemical abundances of carbon (both 12 C and 13 C), nitrogen, and oxygen were derived from molecular lines of CO, CN, and OH, while sodium, scandium, titanium, and iron abundances were obtained from neutral atomic lines. The 12 LMC red giants span a metallicity range from [Fe/H] = � 1.1 to [Fe/H] = � 0.3. It is found that values for both [Na/Fe] and [Ti/Fe] in the LMC giants fall below their corresponding Galactic values (at these same [Fe/H] abundances) by about � 0.1–0.5 dex; this effect is similar to abundance patterns found in the few dwarf spheroidal galaxies with published abundances. The program red giants all show evidence of first dredge-up mixing of material exposed to the CN cycle, that is, low 12 C/ 13 C ratios and lower 12 C with higher 14 N abundances. The carbon and nitrogen trends are similar to what is observed in samples of Galactic red giants, although the LMC red giants seem to show smaller 12 C/ 13 C ratios for a given stellar mass. This relatively small difference in the carbon isotope ratios between LMC and Galactic red giants could be due to increased extra mixing in stars of lower metallicity, as suggested previously in the literature. Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxy indicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about 0.2 dex below the Galactic trend. Such an offset can be modeled as due to an overall lower rate of supernovae per unit mass in the LMC relative to the Galaxy, as well as a slightly lower ratio of supernovae of Type II to supernovae of Type Ia.