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

Abstract

view Abstract Citations (36) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 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 Vandenberg, Don A. ; Stetson, Peter B. Abstract 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. Publication: The Astronomical Journal Pub Date: September 1991 DOI: 10.1086/115930 Bibcode: 1991AJ....102.1043V Keywords: Color-Magnitude Diagram; Globular Clusters; Horizontal Branch Stars; Red Giant Stars; Astronomical Photometry; Chemical Composition; Position (Location); Astrophysics; CLUSTERS: GLOBULAR; STARS: HORIZONTAL BRANCH full text sources ADS | data products SIMBAD (8)