Multicolour CCD photometry of old open clusters: King 5 and Be 20

Abstract

We present multicolour CCD photometry for two poorly studied open clusters (King 5 and Be 20).

Photometry for a field near King 5 was also carried out to estimate the contamination by field stars. The colour magnitude diagrams (CMD) of the clusters show a well defined main sequence extending to the limit of the photometry, V ≈ 20 mag. The reddening for King 5, estimated from the colour-colour diagram, is ∼0.82, whereas that for Be 20 as estimated by comparing theoretical main-sequence (MS) with the observed MS is 0.10. The morphology of the CMDs indicates that these clusters are old. The CMD of Be 20 shows a globular cluster- like horizontal branch. In case of King 5 the comparison of observational CMDs with the standard isochrones of VandenBerg (1985) indicates an apparent discrepancy between the shape of the turnoff and isochrones. The CMDs of King 5 seem to be better understood in terms of stellar models with convective overshoot. The comparison of the CMDs with the stellar models by Bertelli et al. (1994) with convective overshoot produces a good fit for

a metallicity Z = 0.008 and an age = 1 Gyr for King 5 and 5 Gyr for Be 20. An apparent distance modulus (m − M ) = 14.0 and 15.1 has been estimated for King 5 and Be 20 respectively. They correspond to a distance of 1900 ± 100 pc and 9026 ± 480 pc, respectively. The radial density distribution in King 5 indicates that there is an excess of low mass stars in the outer region of the cluster, whereas the density distribution in Be 20 shows a good fit with the empirical King (1962) model. For both clusters, observations have also been carried out to search for variable stars.