Astrophys. J., 573, 174-183 (2002/July-1)
Resolving the 47 Tucanae distance problem.
PERCIVAL S.M., SALARIS M., VAN WYK F. and KILKENNY D.
Abstract (from CDS):
We present new B-, V-, and I-band photometry for a sample of 43 local subdwarfs with Hipparcos parallax errors less than 13%, in the metallicity range -1.0<[Fe/H]←0.3, which we use to perform main-sequence (MS) fitting to the Galactic globular cluster 47 Tuc. This sample is many times larger than those used in previous MS-fitting studies and also enables us to fit in two color planes, V/(B-V) and V/(V-I). With this enlarged subdwarf sample we investigate whether the current discrepancy in empirical distance estimates for 47 Tuc, arising from recent MS-fitting and white dwarf fitting results, is due to inaccuracies in the MS-fitting method. Comparison of published photometries for 47 Tuc has revealed systematic offsets, which means that the (B-V) main line used in previous studies may be too blue by ∼0.02 mag, which would have the effect of making any derived distance modulus too large by around 0.1 mag. Preliminary work has also highlighted discrepancies between results obtained in the two color planes, V/(B-V) and V/(V-I). We have derived main lines in V/(B-V) and V/(V-I) from the data of Kaluzny et al., which we have recalibrated from the ``secondary'' standards in 47 Tuc of Stetson ((ref???)2000). Using an assumed cluster reddening of E(B-V)=0.04, our best-fit apparent distance modulus is (m-M)V=13.37+0.10–0.11 in both color planes, which implies a cluster age of 11.0±1.4 Gyr and leads to a dereddened distance modulus of (m-M)0=13.25+0.06–0.07. Comparison with previous work shows that our apparent distance modulus is ∼0.2 mag smaller than those derived in previous MS-fitting studies. The difference is accounted for by our preferred cluster reddening and the recalibration of the cluster photometry, which has made the main line redder by an average of 0.02 mag in (B-V). Our derived distance modulus is also now plausibly consistent with the short distance recently derived from white dwarf fitting. Independent support for our MS-fitting distance comes from consideration of the red clump in the cluster, from which we derive a dereddened distance modulus of (m-M)0=13.31±0.05, which is in agreement with the MS-fitting result.
Cosmology: Distance Scale - Galaxy: Globular Clusters: Individual: Name: 47 Tucanae - Stars: Distances - Stars: Evolution
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