The late-type stellar content of the Fornax and Sculptor dwarf galaxies.

A field of area 0.13 square degrees has been surveyed for late-type stars in each of the Fornax and Sculptor dwarf elliptical galaxies. JHK photometric data have been obtained for most of the stars found. In Fornax, we have positively identified 25 C stars and one M giant. In Sculptor, two relatively blue C stars and a small number of possible M giants have been identified. In contrast to the Magellanic Clouds, there are no M6-M9 giants in Fornax or Sculptor. The mean value and the variance of the bolometric luminosity function for the Fornax C stars are — 4.66 ± 0.47, quite similar to the values for the Magellanic Cloud C stars. The colors of the Fornax C stars overlap those of the C stars in the Magellanic Clouds but are bluer in the mean. The large dispersion in the color-magnitude diagram of the Fornax C stars is interpreted as arising from a significant spread in age and/or metallicity in the stellar population of Fornax. The C stars found in Sculptor are quite similar in color and luminosity to the C stars in the globular cluster co Centauri and are at the faint end of the luminosity distribution of C stars found in the Magellanic Clouds and Fornax. The ratio of cool C stars to M stars, as determined from identical survey techniques, increases dramatically along the sequence Milky Way, LMC, SMC, and Fornax. This increase, together with systematic changes in the colors of the C and M stars, can be understood as arising from a systematic decrease in the mean metallicity of the galaxies on this sequence. The lack of concurrent significant changes in Mbol (mean) or Mbol (max) of the carbon stars may not be consistent with current theories of C star formation and evolution. The new data for Sculptor, as well as those previously published, point to a stellar population, and possibly a star-formation history, qualitatively similar to that of co Cen. A sharp discontinuity between Fornax and Sculptor in some of the properties which characterize the late-type stellar population of these two systems stands in contrast to a rather smooth gradation in the same properties for Fornax and galaxies more massive than it. We speculate that this discontinuity could have arisen if Sculptor were stripped of its gas component at a much earlier time than Fornax. Subject headings: galaxies: individual — galaxies: stellar content — stars: abundances — stars: carbon — stars: late-type — stars: luminosities McCarthy 1981, unpublished observations; see also Table 4). These authors have suggested that the ratio of C to M stars in a galaxy is related to the galaxy’s mean metallicity. Stellar interior calculations (e.g., Sackmann, Smith, and Despain 1974; Renzini and Voli 1981; Iben 1981) are in qualitative agreement with this prediction. The other nearby companions to the Milky Way are the dwarf ellipticals in Sculptor and Fornax. Colormagnitude diagrams for Sculptor (Hodge 1965; Kunkel and Demers 1977) show a giant branch similar to that of a moderately metal-poor globular cluster, but with essentially no blue horizontal branch stars and a large number of RR Lyrae variables (e.g., van Agt 1978). Norris and Bessell (1978) argue that the apparent width of the