WIYN survey for carbon stars in the M31 and Cetus dwarf spheroidal galaxies: Evolutionary implications

We report results of a photometric survey with the WIYN telescope for carbon stars in the M31 dwarf spheroidal (dSph) companions And III, V, VI, and VII, as well as in the relatively isolated Local Group dSph Cetus. We find three carbon star candidates in And VII and one carbon star in each And VI and Cetus. Comparing the carbon star content with other Local Group dwarf galaxies, we argue against the presence of substantial intermediate-age stellar populations in the all of the galaxies surveyed with the exception of And VII. We discuss these results in the context of the origin of the Andromeda dSph's and conclude that these are ancient galaxies, most of which ceased star formation long before the main merger events in M31. The M31 dSph's therefore show less diversity in star formation histories than the Galactic dSph companions, or the M31 dE companions, as illustrated by NGC 147, which was surveyed as a calibration object. All of our dSph targets except And V have candidate carbon stars below the tip of the red giant branch, which resemble CH stars found in globular clusters. We estimate that 0.3% of stars in the dSph's are CH stars, presumably as a result of C pollution from a binary companion. Comparisons with CH star frequencies in globular clusters could constrain the impact of dense environments on the frequency of this form of binary star evolution.