Spectral Comparison of Red Giants in the Second-Parameter Globular Cluster Pair NGC 288 and NGC 362

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Abstract

Abundances for 13 giants in the blue horizontal branch cluster NGC 288 and 12 giants in the red horizontal branch cluster NGC 362 are presented. The overall metallicities are found to be [Fe/H] = -1.39 ± 0.01 and -1.33 ± 0.01 for NGC 288 and NGC 362, respectively. [α/Fe] ratios for the two clusters are found to be nearly identical, although NGC 362 is found to have a 0.1 dex smaller [Ca/Fe] abundance. Contrary to previous investigations, oxygen-poor giants are found in both clusters. Although Na-O, Al-O anticorrelations are found in both clusters, zero-point and slope differences in the anticorrelations are found. These differences are attributed to ab initio higher O, Na, and Al abundances in NGC 288. Although the "mixed" giants in NGC 362 have significantly lower [O/Fe] ratios than those found in NGC 288, the extent of mixing is nearly equal in the two clusters because of this ab initio abundance difference. No significant difference is found in the r-process element Eu between the two clusters; however, the average Ba abundance, an s-process element, is slightly higher in NGC 288 than in NGC 362. Mass-loss indicators Hα and Na core velocities show little difference from previous investigations of other globular clusters. Excessive mass loss and deep mixing induced surface He abundance differences are ruled out as sources of the difference in horizontal-branch morphology for these two clusters.

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