CN Abundance Inhomogeneities among Bright Red Giants in the Globular Clusters NGC 288 and NGC 362

Abstract

ABSTRACTElement inhomogeneities are studied in the second-parameter pair of globular clusters NGC 288 and NGC 362. Spectra obtained with the CTIO 1.5 m telescope of relatively high luminosity red giants (MV < -1.0) in each cluster have been used to measure a λ3883 CN band index denoted S(3839), and two indices centered on CH absorption in the λ4300 G band. Giants in both clusters exhibit a comparable spread in CN band strength, with only a small difference in mean G band absorption between CN-strong and CN-weak giants. By contrast, the S(3839) index anticorrelates with the [O/Fe] abundance measured by Shetrone & Keane. Part of this anticorrelation can be explained by the effect of reduced oxygen on the equilibria of CO and CN molecule formation in red giant atmospheres. However, the difference in S(3839) between CN-weak and CN-strong giants is sufficiently large that a factor of ∼10 enhancement in nitrogen in CN-strong stars also appears to be necessary. Additionally, the 3883 Å CN band strength correlates with [Na/Fe] abundance. These CN-O-Na relationships among the red giants of NGC 362 and NGC 288 are also a characteristic of other globular clusters of the Milky Way. At a given S(3839) and [Na/Fe] abundance, the CN-strong red giants of NGC 288 have a higher oxygen abundance than red giants in NGC 362. As such, NGC 288 and NGC 362 do not appear to follow a correlation between minimum [O/Fe] abundance and the blueward extent of the horizontal branch in the color-magnitude diagram.