The color-magnitude diagrams of the open clusters NGC 2420 and NGC 2506 have been investigated as intermediate links between the solar neighborhood and the Magellanic Clouds .
Two sets of theoretical isochrones which include convective overshoot are zeroed to the sun at solar abundance and to the unevolved main sequence dwarfs of the Hipparcos catalog at [ Fe/H ] = –0.4 , requiring a differential of 0.4 mag between the unevolved main sequences at a given color .
Adopting E ( B - V ) = 0.04 and [ Fe/H ] = –0.39 for NGC 2506 and E ( B - V ) = 0.04 and [ Fe/H ] = –0.29 for NGC 2420 , the respective apparent moduli are ( m - M ) = 12.70 and 12.15 , while the ages of both clusters are approximately 1.9 \pm 0.2 Gyr or 2.2 \pm 0.2 Gyr , depending on the choice of isochrones .
From the composite giant branch of the two clusters , the mean clump magnitudes in V and I are found to be +0.47 and –0.48 ( –0.17 , +0.14 ) , respectively .
Applying a metallicity correction to the M _ { I } values , the cluster sample of Udalski ( 1998 ) \markcite u1 leads to ( m - M ) _ { 0 } = 18.42 ( +0.17 , –0.15 ) and 18.91 ( +0.18 , –0.16 ) for the LMC and SMC , respectively .
A caveat to this discussion and potentially to the claim that clusters of the same abundance and age are identical is the observation that the ( V - I ) colors of the red giants in NGC 2506 are significantly redder at a given ( B - V ) than the giants in clusters of comparable age and/or metallicity .
The distance scale above has been derived using the general cluster relation between ( B - V ) and ( V - I ) .
If the CCD photometry in NGC 2506 is correctly tied to the standard system , M _ { I } for the clump will decrease and the distance moduli should increase by 0.1 mag .