Carbon stars in clusters in the Galaxy and the Magellanic Clouds

Abstract

Spectra and infrared (JHK) magnitudes have been obtained for 23 of the brightest giants in the SMC clusters NGC 419 (18), NGC 121 (2) and Kron 3 (3) and for 10 giants in the LMC cluster NGC 1846. In addition, infrared observations have been made of the giant branches of the galactic clusters NGC 2477 and 2660 both of which have been reported to contain carbon stars. We estimate that carbon stars first appear on the asymptotic giant branch of the intermediate age Magellanic Cloud clusters at Mbol = −4.3 to −4.6, although there appears to be a separate class of fainter 13C-rich carbon stars (J stars) in both the clusters and the fields of the Magellanic Clouds. Stars exhibiting bands of ZrO as well as TiO (spectral type MS) were commonly found at the tip of the oxygen-rich giant branch; these stars have obviously begun dredging up carbon and s-process elements (e.g., Zr) and their existence at this phase of evolution confirms the association of the S stars with helium shell-flashing stars in the transition phase between pure M and C spectral types. By examining data on the frequency of occurrence of carbon stars in the fields of the Clouds and the Galaxy, we deduce that dredge-up at helium shell flashes cannot produce carbon stars from stars with masses (at the base of the first giant branch) of ≲ 0.9 M⊙ if [Fe/H] ≈ −1, or ≲ 1.3 M⊙ if [Fe/H] ≈ 0.