The Giant Branch of the Globular Cluster NGC3201

Infrared photometry has been obtained for 26 stars in the field of the globular cluster NGC 3201. For 14 of these stars, optical spectra have also been obtained. These observations show that the giant branch of this cluster possesses an intrinsic width of 0.13 mag (± 1 σ) in (V-K)₀. We conclude however, that variable reddening across the cluster with σ[E(B-V)] = 0.023 is the most likely explanation of this result. In particular, the observed intrinsic width cannot be due to a range in heavy metal abundance, since there is no star-to-star scatter in the strengths of various strong metal lines. There is a range in G band (CH) strength at constant (V-K)₀ that correlates with CO strength: stars with weak CH also have weak CO. Comparisons with published synthetic spectra show that star-to-star variations in carbon abundance of less than a factor of 3 are sufficient to explain the range in G-band strengths. With the exception of two stars, no significant scatter was found in the strengths of the λ3883 CN band. The position of the giant branch in the infrared C-M diagram and the strengths of metal lines in the optical spectra indicate that the overall metal abundance of NGC 3201 is comparable to those of M3, M5, and NGC 6752. This is significantly more metal-poor than the preliminary abundance determined from echelle plates by Pilachowski, Sneden, and Canterna.