How age and O/Fe differences affect the location of a globular cluster's giant branch relative to its turnoff in the (V, B - V) plane

Details How age and O/Fe differences affect the location of a globular cluster's giant branch relative to its turnoff in the (V, B - V) plane How age and O/Fe differences affect the location of a globular cluster's giant branch relative to its turnoff in the (V, B - V) plane

Sep 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991aj....102.1043v&link_type=abstract

Astronomical Journal (ISSN 0004-6256), vol. 102, Sept. 1991, p. 1043-1049.

Other

33

Color-Magnitude Diagram, Globular Clusters, Horizontal Branch Stars, Red Giant Stars, Astronomical Photometry, Chemical Composition, Position (Location)

Scientific paper

On the basis of theoretical isochrones for [Fe/H] = -2.26 and -1.26, and assuming 0.0<[O/Fe] ≥ 1.15 and 0.0 ≥ [O/Fe] ≥ 0.75 respectively, analytic expressions are derived to quantify the B-V displacement of the lower red-giant branch from the turnoff as a function of both age and [O/Fe]. As a result, the color-difference method developed by VandenBerg et al. [AJ, 100, 445 (1990)] for the precise determination of relative globular-cluster ages is now extended to allow for the possibility that [O/Fe] varies at a given [Fe/H]. To illustrate the implications of our results, we have reanalyzed the color-magnitude diagrams of NGC 288 and NGC 362. This comparison, within ˜±2 mag of their turnoffs, confirms previous results by ourselves and others that NGC 288 is about 20 percent older than NGC 362 if both clusters have the same [O/Fe] value. Alternatively, for the age difference to be near zero, the present calculations predict that NGC 288 would have to be more oxygen-rich than NGC 362 by δ[O/Fe] ≍ 0.6 (assuming that no other parameter is varying). Horizontal-branch considerations indicate a preference for the former interpretation over the latter.

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