A Distance-Independent Age for the Globular Cluster M92

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Abstract
We use deep Strömgren CCD photometry to determine the age of the globular cluster M92 (NGC 6341) from the [(v-y)0, c0]-diagram. Besides being completely independent of distance, this color-color plot has the further advantage that it is not very sensitive to the amount of foreground interstellar absorption. The main disadvantage of this approach is that it relies heavily on the accuracy of the Teff and color scales of isochrones. As these aspects of the models continue to be uncertain, absolute ages cannot be derived in this way to within a few gigayears. For instance, while we obtain an age of 14.5 Gyr for M92 using the latest University of Victoria isochrones, an age as low as 12 Gyr or as high as 17 Gyr would be found if we adopted different zero points for the synthetic color–Teff relations or an ad hoc shift of ≈±200 K were applied to the models. However, much tighter constraints on the age may be obtained by invoking some additional constraints. To be specific, we find from the (v-y, c1)-diagram that the extremely metal-deficient field halo stars are most likely coeval with M92 to within 1 Gyr (1 σ error bar) and that the properties of the well-observed [Fe/H] ≈ -2.5 subgiant HD 140283, which has σπ/π = 0.06 from Hipparcos, imply (m - M)V 14.60 and an age 16 Gyr for M92. This, in fact, offers considerable support for the synthetic colors that we have used.