SODIUM, ALUMINUM, AND OXYGEN ABUNDANCE VARIATIONS IN GIANTS IN THE GLOBULAR-CLUSTER M4

Accurate, quantitative abundances are presented for Fe, Ca, Na, Al, and O in four red giants in the bimodal-CN globular cluster M4. Two of the giants are CN-strong and two are CN-weak: all have very similar magnitudes and colors and, thus, have similar effective temperatures and surface gravities. This analysis utilizes high-resolution, high-SN spectra and up-to-date model atmospheres. For the low-ionization element Na, detailed non-LTE calculations demonstrate that, for lines used in the abundance analysis, departures from LTE are small (less-than-or-similar-to 0.1 dex). As a group, these four M4 giants have homogeneous Fe and Ca abundances, while Na, Al, and O vary: the CN-strong stars have larger Na and Al but smaller O abundances than the CN-weak stars. When compared to field metal-poor stars, the CN-weak stars resemble the field stars, while the CN-strong stars are Na and Al rich.