A Near-Infrared Photometric Study of the Metal-poor Inner Spheroid Globular Clusters NGC 6139 and NGC 6287

Abstract

Moderately deep J, H, K, 2.2 μm continuum, and 2.3 μm CO images are used to investigate the photometric properties of stars in the metal-poor inner spheroid globular clusters NGC 6139 and NGC 6287. The data sample stars as faint as the subgiant branch (SGB) in NGC 6139 and the main-sequence turnoff (MSTO) in NGC 6287. The photometric properties of bright giants are used to estimate the distance, metallicity, and reddening of each cluster. The brightness of the red giant branch tip, as calibrated from the Bergbusch & VandenBerg isochrones, suggests that μ₀ = 15.4 ± 0.1 for NGC 6139 and μ₀ = 14.6 ± 0.1 for NGC 6287. The steep giant branches and weak 2.3 μm CO indices confirm the extreme metal-poor natures of these clusters and suggest that [Fe/H] = -2 for NGC 6139 and [Fe/H] = -2.2 for NGC 6287. Techniques to correct for reddening variations and foreground star contamination are discussed and applied to the NGC 6287 data to construct a K luminosity function (LF) that is complete almost to the MSTO. Comparisons with LFs derived from the Bergbusch & VandenBerg models suggest that the age of NGC 6287 is 12 Gyr, although the number of SGB stars sampled in the existing study is too small to rule out an age as old as 16 Gyr. It is noted that near-infrared LFs may prove useful for establishing a relative age scale for heavily obscured globular clusters.