The asymptotic giant branch of Magellanic Cloud clusters

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Abstract

Thirty-nine clusters in the Magellanic Clouds have been surveyed for carbon and M-type asymptotic giant branch (AGB) stars. We identified and obtained near-infrared photometry for about 400 such stars in and around the clusters. The cluster classification scheme of Searle, Wilkinson, and Bagnuolo (SWB), which we show to be closely correlated with cluster age, is a key element in the analysis of our data. The principal results of our investigation are as follows. In a C-M diagram the cluster M stars shift steadily redward in J-K as one goes from clusters of SWB type I to VI. This is due to the increasing age of the clusters along the sequence. There are two peaks in the color distribution of the M stars which could indicate two epochs of enhanced cluster formation: one about 100 Myr ago, the other about 1 Gyr ago. These correspond to similar enhancements deduced for M stars in the Bar West region of the LMC. For LMC clusters in the 1 Gyr old group we estimate the spread in [Fe/H] to be on the order of 0.2-0.3 dex. This implies that chemical enrichment was fairly uniform throughout the LMC at that epoch. The brightest stars in SWB type VII clusters have luminosities and colors that are closely comparable to the brightest stars in Galactic globular clusters; with one possible exception they have no luminous AGB stars. We deduce that the SWB VII clusters in the Clouds are as old as Galactic globular clusters and have a range in metallicity of about a factor of 10. Luminous carbon stars are present only in SWB IV-VI clusters. They are easily distinguished from M stars by their color and luminosity. The effects of age and metallicity are apparent in luminosity functions for C stars: for those in SWB type VI clusters the luminosity function is several tenths of a magnitude fainter than that for earlier type clusters: both cluster and field SMC C stars are intrinsically fainter than their LMC counterparts. The shape of the luminosity functions for C stars from the various types of clusters are consistent with each being drawn from the same sample of field stars in the Clouds, so that the latter is clearly a composite population with a range in age and metallicity that must closely overlap that for the clusters. For the SWB-type clusters in which C stars are present, they are almost always brighter than the M stars in the same clusters. The transition luminosity between the two types of stars gets systematically brighter as one passes from the latest to the earliest type clusters. The existence of such a transition luminosity is an important prediction of the theory of carbon star formation and evolution. The youngest clusters in which C stars are found have an age of about 100 Myr implying a maximum initial mass for these stars of 3-5 M_sun_. In clusters younger than 100 Myr, the brightest stars are M giants; even though these are the brightest stars in the entire sample, they still cannot account for the missing luminous AGB stars. Therefore, the hypothesis that luminous C stars turn back into M stars appears to be ruled out. Convective overshooting or high mass-loss rates are promising hypotheses for the absence of luminous C stars. The former could also explain the fact that we see luminous C stars in clusters with turnoff masses as low as 1.0 M_sun_ in the SMC. In intermediate-age clusters, ~40% of the bolometric luminosity is contributed by C and M type AGB stars, a somewhat smaller fraction than predicted by Renzini and Buzzoni. However, because the C star luminosity function is shifted to fainter magnitudes than predicted, the age range in which AGB stars make a significant contribution is shifted to substantially older ages. If the contribution of AGB stars is removed from a cluster's light, the resulting distribution of integrated J-K colors shows a jump at types IV-V. This corresponds to the age at which the helium core switches from being degenerate to nondegenerate and is in reasonable agreement with theory.

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