Old open clusters and the Galactic metallicity gradient: Berkeley 20, Berkeley 66, and Tombaugh 2

TL;DR

This study analyzes three distant old open clusters near 12-16 kpc from the Galactic center, determining their ages, metallicities, and distances using CCD imaging and synthetic CMD techniques, contributing to understanding the Galactic metallicity gradient.

Contribution

It provides new age, metallicity, and distance estimates for Berkeley 20, Berkeley 66, and Tombaugh 2, especially in the less-studied 12-16 kpc range, using synthetic CMD modeling with different evolutionary tracks.

Findings

Berkeley 20 has a metallicity about half solar and is 4.3-6 Gyr old.

Berkeley 66 is younger (~3 Gyr) with slightly lower metallicity.

Tombaugh 2 is about 1.4-1.8 Gyr old with half solar metallicity.

Abstract

To study the crucial range of Galactocentric distances between 12 and 16 kpc, 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 6 Gyr from stellar models with overshooting and between 4.3 and 4.5 Gyr from models without it. The distance modulus from the best fitting models is always (m-M)0=14.7 (corresponding to a Galactocentric radius of about 16 kpc), 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, (age of 3 Gyr), and closer, (m-M)0=13.3 (i.e., at 12 kpc from the Galactic centre), than Be 20, and suffers from high extinction, 1.2 < E(B-V) < 1.3, variable at the 2-3 per cent level. Finally, the results for To 2 indicate that it is an intermediate age cluster, with an age of about 1.4 Gyr or 1.6-1.8 Gyr 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)0=14.5, implying a distance of about 14 kpc 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.