The late-type stellar content of Magellanic Cloud clusters

Abstract

New broad-band infrared photometric data have been obtained for 48 late-type giants in clusters in the Magellanic Clouds (MC). Visual spectrophotometry was obtained for a subset of these stars. These observations are combined with published data for MC cluster stars and then compared with similar data for MC field giants and with predictions of various evolutionary schemes for cool, luminous, carbon and oxygen rich stars. The MC cluster C stars are found to have a range in spectral energy distributions which is quite similar to that of MC field C stars. The luminosity function of the cluster C stars has a mean M_(bol) = —4.76 with a dispersion of ±0.36, also quite similar to the values for MC field C stars. However, the dispersion of the cluster C stars in M_K_0 at a given (J— K)_0 is between 2 and 3 times less than it is for the field C stars. This can arise if the present sample of clusters has a significantly smaller spread in age and/or metallicity than the progenitors of the field C stars. The LMC field contains M giants which are redder and more luminous than any so far found in LMC or SMC clusters. This is attributed to the presence in the LMC field of a significant population of stars which are younger and/or more metal rich than the stars in the cluster sample. Differences which are found to exist between the M star populations of the LMC and the SMC clusters are also attributed to age and/or metallicity effects. In all but one of the MC clusters which have both M and C stars, the faintest C star is brighter than the brightest M star. Such a “transition” luminosity appears to be correlated with the location of the cluster in the one-dimensional classification sequence of Searle, Wilkinson, and Bagnuolo, and it can be a useful criterion in the evaluation of theories of carbon star evolution. Finally, although the spectrophotometric data suggest that the LMC “halo-type” globular, NGC 1841 and 2257, have metallicities similar to one another and to that of M3, the locations of the NGC 1841 stars in a C-M diagram appear to be anomalous in the sense that its brightest stars have luminosities greater than the tips of giant branches of metal poor galactic globular clusters.