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Description
To study the crucial range of Galactocentric distances between 12 and 16kpc, where little information is available, we have obtained VI CCD imaging of Berkeley 20 and BVI CCD imaging of Berkeley 66 and Tombaugh 2, three distant, old open clusters. Using the synthetic colour magnitude diagram (CMD) technique with three types of evolutionary tracks of different metallicities, we have determined age, distance, reddening and indicative metallicity of these systems. The CMD of Be 20 is best reproduced by stellar models with a metallicity about half of solar (Z=0.008 or 0.01), in perfect agreement with high resolution spectroscopic estimates. Its age is between 5 and 6Gyr from stellar models with overshooting and between 4.3 and 4.5Gyr from models without it. The distance modulus from the best fitting models is always (m-M)=14.7 (corresponding to a Galactocentric radius of about 16kpc), and the reddening E(B-V) ranges between 0.13 and 0.16. A slightly lower metallicity (Z=~0.006) appears to be more appropriate for Be 66. This cluster is younger (3Gyr), and closer, (m-M)=13.3 (i.e., at 12kpc from the Galactic centre), than Be 20, and suffers from high extinction, 1.2<=E(B-V)<=1.3, variable at the 2-3% level. Finally, the results for To 2 indicate that it is an intermediate age cluster, about 1.4Gyr or 1.6-1.8Gyr for models without and with overshooting, respectively. The metallicity is about half of solar (Z=0.006 to 0.01), in agreement with spectroscopic determinations. The distance modulus is (m-M)=14.5, implying a distance of about 14kpc from the Galactic centre; the reddening E(B-V) is 0.31-0.4, depending on the model and metallicity, with a preferred value around 0.34.
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