1 Introduction

The metal-rich globular clusters (GCs) play an important role as age tracers of the oldest disk, thick disk and bulge populations since these systems offer the best posibilities for accurate age determinations. Among the approximately 33 clusters listed in the Harris (1996) compilation with [Fe/H] $~\geq~-0.8$ M 71 (NGC 6838) and 47 Tuc (NGC 104) are among the most easily studied due to their closeness (although the reddening of M 71 is substantial, with some differential reddening evident, its low concentration makes it an easy target for precise photometry) and both have been subjects of several studies. Most notably both clusters have a large spread in the carbon and nitrogen abundances among their members, main-sequence (MS) and red giant stars (RGB) alike (Briley et al. 1994; Briley 1997; Cannon et al. 1998; Cohen 1999; Grundahl 1999; Grundahl et al. 1999a).

After the availability of the Hipparcos parallax database quite a bit of discussion over the distances (and hence ages) of the globular clusters has emerged in the literature. Several of the first Hipparcos-based studies (e.g. Reid 1997 (R97), 1998 (R98); Gratton et al. 1997) concluded that the distances to the metal-poor clusters were significantly larger than previously thought - leading to cluster ages lower by 2-3 Gyr. However some studies, most notably that of Pont et al. (1998) for M 92, concluded that the new parallaxes do not lead to a significantly younger age for this cluster, a result supported by Grundahl et al. (2000). Furthermore, if the longer distance scale is correct, then an apparent dichotomy between the luminosities of the metal-poor field and cluster RR-Lyrae stars appears, in the sense that the luminosities of the cluster RR-Lyraes are brighter than the field RR-Lyraes. See Carretta et al. (2000a) and Catelan (1998) for further discussion of this.

Several possibilities exist for explaining the "long/short'' dichotomy for the cluster distances as there are a number of known problems associated with the main-sequence fitting technique, such as Lutz-Kelker corrections, metallicity bias, binarity among the field subdwarfs, and selection effects (discussed in e.g. R97; R98; Gratton et al. 1997; Carretta et al. 2000b). Perhaps the most important problem lies in the fact that the number of low-metallicity field stars with very accurate parallaxes is quite small - a problem which is currently impossible to circumvent. However, in the case of more metal-rich clusters, such as 47 Tuc and M 71, the steeply rising frequency of subdwarfs with increasing metal abundance ensures that there is a larger sample of field stars with good parallaxes to choose from, compared to the metal-poor case, for the determination of main-sequence distances.

Although M 71 is observationally as easy a target as 47 Tuc there have not yet been any studies as detailed as the Hesser et al. (1987) study for 47 Tuc. The most thorough study so far is that of Hodder et al. (1992). These authors derived an age between 14 and 16 Gyr for the cluster and found an offset of $\Delta~V~=~0.379$ and $\Delta~(B~-~V)~=~0.235$ (sense M 71-47 Tuc) from a differential comparison to the Hesser et al. colour-magnitude diagram for 47 Tuc. Other studies, e.g. Salaris & Weiss (1998), also find the two clusters to have very similar properties within the measuring errors. Chaboyer et al. (1996) found the two to differ in age by 2 Gyr (47 Tuc older) from the $\Delta V_{\rm TO}^{\rm HB}$ method; given the difficulty of locating the turnoff and the lack of precise photometry for M 71 this result is probably still consistent with no age difference. Conversely, Heasley & Christian (1991) found M 71 to be approximately 3 Gyr older than 47 Tuc. Finally, in a homogeneous VI photometric study of 34 nearby globular clusters, Rosenberg et al. (1999) concluded that the two clusters differ in age by 6%$\ \pm\$11%, or 0.9 $\ \pm~1.6~$Gyr (47 Tuc older), if the "typical'' globular cluster age is assumed to be 13 Gyr. In this paper we shall carry out a detailed comparison of the distances and ages for these two clusters, both relative and absolute, based on new Strömgren uvby photometry. This allows for the inclusion of a larger number of subdwarfs as a large data base of homogeneous uvby photometry already exists unlike the situation for BVI photometry.

The outline of the paper is as follows: Sects. 2 and 3 describe the observational material, photometric reductions and calibrations; in Sect. 4 the colour-magnitude diagrams (CMDs) are presented and in Sect. 5 we discuss reddening and metallicity estimates for the clusters. Section 6 presents a brief discussion of the differential ages. In Sect. 7 we deal with the selection of a subdwarf sample and the determination of main-sequence distances. Section 8 discusses the determination of absolute ages for the clusters given the new distance determinations and in Sect. 9 a discussion of our results is given. Finally Sect. 10 concludes and summarises our results.