2003A&A...412...91C

We have calculated theoretical isochrones for the photometric Δa system to derive astrophysical parameters such as the age, reddening and distance modulus for open clusters. The Δa system samples the flux depression at 520nm which is highly efficient to detect chemically peculiar (CP) objects of the upper main sequence. The evolutionary status of CP stars is still a matter of debate and very important to test, for example, the dynamo and diffusion theories. In fact, the dynamo or fossil origin of the magnetic fields present in this kind of star is still not clear. Using the stellar evolutionary models by Claret (1995A&AS..109..441C), a grid of isochrones with different initial chemical compositions for the Δa system was generated. The published data of 23 open clusters were used to fit these isochrones with astrophysical parameters (age, reddening and distance modulus) from the literature. As an additional test, isochrones with the same parameters for Johnson UBV data of these open clusters were also considered. The fits show a good agreement between the observations and the theoretical grid. We find that the accuracy of fitting isochrones to Δa data without the knowledge of the cluster parameters is between 5 and 15%.