Near-infrared photometry of globular clusters NGC 6287 and NGC 6341 (M92): The formation of the galactic halo

We present Hubble Space Telescope (HST) Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera 3 photometry of the inner halo globular cluster NGC 6287 and the intermediate halo globular cluster NGC 6341 (M92). Our photometric measurements range from the lower red giant branch to approximate to3 mag below the main-sequence turnoff, allowing us to define accurate mean loci of the two clusters. Our relative age estimate between NGC 6287 and M92 using the color difference between the main-sequence turnoff and the base of the red giant branch shows that they essentially have the same ages within +/-2 Gyr. The slightly different chemical compositions between the two clusters do not significantly influence our result. Our simulations using synthetic spectra show that the temperature dependence of the interstellar reddening law for the HST NICMOS photometric system could be a more serious problem in the relative age estimate using the color difference between the main-sequence turnoff and the base of the red giant branch. However, this effect adds no more than +/-0.5-1 Gyr to the uncertainty in our relative age estimate. Our color-magnitude diagram of NGC 6287 shows a well-defined blue horizontal branch population. This is consistent with its age and metallicity. We also present the relative interstellar reddening toward NGC 6287 and distance modulus with respect to those of M92. We find E(B-V) = 0.62 and (m - M)(0) = 14.35 for NGC 6287, which are generally in good agreement with previous estimates. Our photometry suggests that there exists a differential reddening across NGC 6287 with an amount of DeltaE(B-V) = 0.07-0.09 mag, consistent with the previous study of Stetson & West. The Galactocentric distance of NGC 6287 becomes R-GC = 1.6 kpc if R-0 = 8.0 kpc. Based on our new radial velocity measurement for NGC 6287, the cluster's (minimum) apogalacticon distance is R-alpha approximate to 5.5 kpc, suggesting that NGC 6287 is an inner halo cluster. Within this framework, our results are consistent with the idea that the globular cluster formation must have been triggered everywhere at the same time in our Galaxy. In the appendices, we discuss that the interstellar extinction law of the HST NICMOS photometric system, particularly in the F110W passband, depends on the temperature of the stars in the sense that hotter stars suffer more interstellar absorption than cooler stars.