We present UBV photoelectric observations of 352 stars in the field of the open cluster NGC 2539, obtained at the Observatories of Cerro Tololo and Las Campanas. The analysis of these data allows to determine that about 200 stars are probable members of the cluster main sequence. The CC and CM diagrams reveal a well-defined main sequence and a populous red giant branch. We calculated a reddening E(B-V) = 0.10 mag and an apparent distance modulus V-Mv = 11.0 mag, equivalent to a distance of 1380 pc. By means of the fitting of theoretical isochrones computed with mass loss and moderate overshooting, an age of 500 Myr is determined. The abnormal location of the red giants in the HR diagram is analyzed in terms of binarity, metallicity and mass loss. The low contamination of the upper main sequence of the cluster CM diagrams and its populated red giant branch makes NGC 2539 a very good object for testing theoretical models.
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina
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