EDP Sciences
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Volume 412, Number 1, December II 2003
Page(s) 91 - 95
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20031426

A&A 412, 91-95 (2003)
DOI: 10.1051/0004-6361:20031426

Theoretical isochrones for the $\Delta$a photometric system

A. Claret1, E. Paunzen2, 3 and H. M. Maitzen2

1  Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
2  Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
3  Zentraler Informatikdienst der Universität Wien, Universitätsstr. 7, 1010 Wien, Austria

(Received 15 May 2003 / Accepted 20 August 2003)

We have calculated theoretical isochrones for the photometric $\Delta a$ system to derive astrophysical parameters such as the age, reddening and distance modulus for open clusters. The $\Delta a$ system samples the flux depression at 520 nm 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 (1995), a grid of isochrones with different initial chemical compositions for the $\Delta a$ 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  $\Delta a$ data without the knowledge of the cluster parameters is between 5 and 15%.

Key words: open clusters and associations: general -- stars: fundamental parameters

Offprint request: A. Claret, claret@iaa.es

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