Red-giant branch of the old, metal-rich open cluster NGC 6791

Details Red-giant branch of the old, metal-rich open cluster NGC 6791 Red-giant branch of the old, metal-rich open cluster NGC 6791

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aj....107.1097g&link_type=abstract

Astronomical Journal (ISSN 0004-6256), vol. 107, no. 3, p. 1097-1110

Physics

Optics

70

Astronomical Photometry, Astronomical Spectroscopy, Color-Magnitude Diagram, Images, Interstellar Extinction, Metal Clusters, Metallicity, Radial Distribution, Red Giant Stars, Velocity Distribution, Astronomical Observatories, Cassegrain Optics, Charge Coupled Devices, Gratings (Spectra), Optical Filters, Point Spread Functions, Spectrographs, Telescopes

Scientific paper

V and I photometry out to 12 min from the center of the old, populous, metal-rich, open cluster NGC 6791 has revealed 16 bright stars (I less than or equal to 12.5) with 1.6 less than or equal to V - I less than or equal to 4.1. Follow-up spectroscopy indicates that (1) at least 12 of these red-giant branch (RGB) candidates are bona fide cluster members, and (2) their metallicity, as estimated from the strength of the Ca II infrared triplet lines, is slightly above solar. If the core helium-burning (`clump') stars in NGC 6791 have a mean MV = 0.9, which is the current best estimate for that of their counterparts in the (M/H) approximately equal to 0.0 cluster M67, then the apparent distance modulus of the former is near (m - M)V approximately equal to 13.6. This would imply an age of about 9 Gyr, from appropriate stellar models, versus approximately equal to 4.5 Gyr for M67. Importantly, with this estimate of the distance -- which is unlikely to be in error by more than +/- 0.25 mag -- the color-magnitude diagram (CMD) of NGC 6791 intimates that its RGB does not rise above MI approximately -2.7. In contrast, the RGBs of all Galactic globular clusters with metallicities of one-quarter solar or less are presently believed to reach MI approximately equal to -4.0, though fainter tip magnitudes have been inferred for the very few globulars of higher metal abundance (e.g., NGC 6553) that have been studied. If the tip MI value does not vary with metallicity among (at least) the most metal-rich systems, then that variation can be expected to affect, e.g., estimates of the extragalactic distance scale based on the `surface brightness fluctuation' method. Consequently, NGC 6791 provides an exceedingly important constraint on our understanding of old, metal-rich stellar populations. Encouragingly, theoretical evolutionary tracks whose temperatures and colors are consistent with various empirical constraints appear to provide a fine match to the upper CMD of NGC 6791, if the cluster metallicity is (M/H) greater than 0.0.

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