Globular Cluster Giant Branches and the Helium Flash: A Comparison between Observation and Theory

Abstract

Four years ago we began a study of globular cluster giant stars in the infrared. The first part of this program, the measurement of the 1.2–2.2 µm energy distributions with the use of broad band filters and the determination of the strengths of CO and H2O absorption in the stars has been published, or will soon appear in press. M3, M13, and M92 are discussed in Cohen, Frogel, and Persson (1978, hereafter CFP); M71 in Frogel, Persson, and Cohen (1979); ω Centauri in Persson et al. (1980); Pal 12 in Cohen et al. (1980); 47 Tucanae in Frogel, Persson, and Cohen (1981, hereafter FPC1); and NGC 3201 in Da Costa, Frogel, and Cohen (1981). We now have in hand similar data for 167 stars in 11 additional clusters: NGC 288, 362, 5904 (M5), 6121 (M4), 6352, 6397, 6637 (M69), 6656 (M22), 6752, 7006, and 7078 (M15) (Frogel, Persson, and Cohen 1982, hereafter FPC2). Pilachowski (1978) has also published data on several of these clusters and M10. With a rather complete sample of cluster types, we can proceed to examine how physical quantities derivable from infrared data vary from cluster to cluster. Furthermore it should be possible to establish "benchmarks" against which theoretical models of stellar atmospheres and evolutionary tracks can be tested.

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