CCD PHOTOMETRY OF THE GLOBULAR CLUSTER NGC 4147

• Published in: Publications of the Astronomical Society of the Pacific 1987-12, Vol.99 (622), p.1248-1260
• Main Authors: FRIEL, EILEEN D., HEASLEY, J. N., CHRISTIAN, CAROL A.
• Format: Article
• Language: English
• Subjects: Astronomical magnitude; Astronomical photometry; Astronomy; CCD photometry; Color magnitude diagram; Earth, ocean, space; Exact sciences and technology; Galaxy groups, clusters, and superclusters. Large-scale structure of the universe; Giant branches; Globular star clusters; Horizontal branch stars; Main sequence stars; Metal abundance; Stellar systems. Galactic and extragalactic objects and systems. The universe; Subgiant stars
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/132110

We present results of CCD photometry in B and V for the globular cluster NGC 4147 obtained at the CFH 3.6-m telescope. The color-magnitude diagram reaches fainter than V = 22, some 3 magnitudes below the main-sequence turnoff. The morphology of the C-M diagram resembles that of a cluster of intermediate metallicity with a normal blue horizontal branch. Rough estimates of the integrated light from additional CCD data in B and I suggest, however, that the population revealed in the C-M diagram from the outer regions may not be representative of the population in the central regions of the cluster. Determination of the age of the cluster from fitting theoretical isochrones (VandenBerg and Bell 1985; Green et al. 1987) is difficult because of uncertainties in the distance modulus of the cluster. At the preferred distance modulus of (m—M)ν = 16.5, assuming an abundance of [Fe/H] = –1.8, and reddening E(B–V) = 0.02 mag for NGC 4147, VandenBerg and Bell (1985) tracks agree well with the observed diagram and imply an age of 17 Gyr. By contrast, the revised Yale isochrones indicate an age of some 2 Gyr younger, for the same parameters. If models with enhanced oxygen abundances of [O/Fe] = + 0.5 are used, the ages deduced would also be 2-3 Gyr younger. Although determinations of the absolute age of NGC 4147 require knowledge of the absolute distance scale and choice of theoretical models, these alternative age estimates are consistent with those made for other clusters using the same models.