The Anomalous Color-Magnitude Diagram of the Remote Globular Cluster NGC 7006

Abstract

The color-magnitude diagram for NGC 7006 has been obtained to V = 19.3 by a combination of photographic and photoelectric measurements. The horizontal branch occurs at V = 18.80. Interstellar reddening is small. Adopting Mv = +0.5 for the RR Lyrae variables gives a true modulus of (m - M)0 = 18.3, a distance of 45 kpc from the Sun, and 15 kpc from the galactic plane. The metal abundance is low, as judged by AV = 2.6 mag and by the spectroscopic and photometric criteria of Morgan and van den Bergh, and yet the distribution of stars along the horizontal branch imitates that of much higher metal-abundance clusters. This is the first clear violation of the heretofore unique correlation of metal abundance and density gradient along the horizontal branch. It shows that at least one additional parameter besides Z controls the stellar distribution along the branch. Models of Faulkner and of Faulkner and Iben suggest that the second parameter may be the abundance of helium. The NGC 7006 anomaly is present in many intergalactic globular clusters and in the clusters of the Small Magellanic Cloud, suggesting a difference in the second parameter between objects that have or have not partaken in the chemical evolution of the galaxy. NGC 7006 may not be gravitationally bound to the galaxy, in which case it was formed in the Local Group but outside the galactic halo. If future observations and more extensive model calculations can establish an abundance difference, A Y, between objects inside and outside the Galaxy, then we would be able to conclude that some helium was made in the early history of the galactic system. This additional helium is then added to the pristine helium produced, for example, in the primeval fireball of a Friedman-type universe or in any other event that manufactured helium which preceded the formation of the Local Group of galaxies.