Trumpler 20 - an old and rich open cluster ⋆ † ‡

We show that the open cluster Trumpler 20, contrary to the earlier findings, is actually an old Galactic open cluster. New CCD photometry and high-resolution spectroscopy are used to derive the main parameters of this cluster. At [Fe/H]=−0.11 for a single red giant star, the metallicity is slightly subsolar. The best fit to the color-magnitude diagrams is achieved using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although for red giants this high reddening yields the color temperature exceeding the spectroscopic Teff by about 200 K. Trumpler 20 is a very rich open cluster, containing at least 700 members brighter than MV = +4. It may extend over the field-of-view available in our study at 20×20.