Photometric studies of composite stellar systems. V - Infrared photometry of star clusters in the Magellanic clouds

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Abstract

The results of an infrared photometric study of the integrated light of 84 clusters in the Large and Small Magellanic Clouds (LMC and SMC) are presented. These clusters span nearly the complete range of cluster ages in the Clouds. In contrast to uvgr and UBV cluster colors which vary smoothly with age, the infrared colors display wide variations among the Searle, Wilkinson, and Bagnuolo groups IV-VI, i.e., in the "intermediate age" domain of ∼1-8 × 109 yr. Very red J - K and H - K colors for these clusters are shown to be due to the presence of luminous (M_bol < -4) carbon stars which are absent in the youngest and oldest clusters, and which have no effect upon the visible colors. An analysis of the CO and H - K data shows that on average half of the bolometric luminosity for 20 intermediate-age clusters comes from carbon stars on the asymptotic giant branch. This analysis agrees well with the recent carbon star surveys of Aaronson and Mould, Frogel and Cohen, and Lloyd-Evans. The effects of luminous carbon stars upon the infrared colors of the parent clusters are strong enough that metal-poor, intermediate-age stellar populations may be detectable in the integrated light of more distant galaxies. There is no difference, on average, between the clusters of the LMC and the SMC in the proportion of the light at 2 μm (or bolometrically) due to luminous carbon stars. This result is in apparent contrast to that of the Blanco et al. surveys of the carbon and M star populations in the general fields of the two galaxies; these surveys have revealed a sharp rise in the C/M star ratio going from the LMC to the SMC. The explanation appears to lie in the incompleteness of the Blanco et al. surveys for warm M stars in metal-poor populations. Carbon stars do not appear to be present in clusters ≤ 1 × 109 yr old; the infrared colors of these young clusters are dominated by M giants and supergiants which display strong CO band absorption at 2.3 μm. This lack of carbon stars agrees with the findings of Cohen et al. and Richer that a true paucity of massive, luminous (M_bol ≤ -6) carbon stars exists in the general fields of the Clouds. This situation is in conflict with current theories of stellar evolution on the asymptotic giant branch. The V - K colors of the youngest, i.e., 107 - 108 yr, clusters are compared with the single-burst model of an evolving stellar population due to Struck-Marcell and Tinsley. At a given U - V the observed V - K colors are up to 2 mag redder than the model predicts. The discrepancy is due to the neglect of red supergiants in the model; these luminous stars dominate the infrared colors and bolometric luminosities, and should be detectable in stellar populations like those in the arms of spiral galaxies. The infrared colors of the oldest clusters of the Magellanic Clouds resemble those of globular clusters in our galaxy and in M31.

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