The main sequence of the metal-poor globular cluster NGC 6397

Details The main sequence of the metal-poor globular cluster NGC 6397 The main sequence of the metal-poor globular cluster NGC 6397

Jun 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980aj.....85..680a&link_type=abstract

Astronomical Journal, vol. 85, June 1980, p. 680-694.

Astronomy and Astrophysics

Astronomy

44

Globular Clusters, Main Sequence Stars, Stellar Spectrophotometry, Abundance, Astronomical Maps, Astronomical Photography, Color-Magnitude Diagram, Galactic Evolution, Helium, Hubble Diagram, Interstellar Extinction, Milky Way Galaxy, Stellar Magnitude, Stellar Models, Tables (Data)

Scientific paper

Main-sequence photometry of the nearby metal-poor globular cluster NGC 6397 is presented. Photographic photometry using a Racine wedge was performed for 1135 cluster stars, to a limiting V magnitude of 20.7. Examination of the color-magnitude diagram of the cluster reveals the main-sequence turn-off to lie at V = 16.7 + or - 0.1 mag and B - V = 0.52 + or - 0.03 mag. From these values, an instrinsic V magnitude of 4.30 and B - V of 0.36 are calculated, which lead to values of 12.40 + or - 0.05 for the distance modulus and 0.16 + or - 0.02 for the reddening. Considerations of the apparent magnitude of the main-sequence turn-off point, distance, helium content and metal abundance are used together with the theoretical model of Iben and Rood (1970) and the isochrones of Demarque and McClure (1977) to estimate a cluster age of 17 + or - 3.5 billion years, allowing upper limit of 58 + or - 12 km/sec per Mpc to be set on the Hubble constant and a lower limit to be set on the age of the universe. The extreme age of NGC 6397 together with its age difference from 47 Tuc is taken to suggest that the Galaxy underwent a slow collapse formation phase, and that heavy element abundances decrease with age.

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