Eclipsing binaries in the open cluster NGC 2243 - II. Absolute properties of NV CMa

Abstract

We present echelle spectroscopic data for five eclipsing binary stars and two giant stars in the field of the open cluster NGC 2243. The average cluster velocity is determined to be +60.4 +- 0.6 km/s. Four of the eclipsing binaries are very likely members of the cluster based on their observed radial velocities. The absolute parameters of cluster member NV CMa are determined by analysing photometric and radial velocity data. We obtain 1.089+-0.010 Msun and 1.221+-0.031 Rsun for the primary, and 1.069+-0.010 Msun and 178+-0.037 Rsun for the secondary. Both components of the binary are located on the Main Sequence, about 1 mag. below the turn-off point on the cluster color-magnitude diagram. Using model age-luminosity and age-radius relations we obtain 4.35+-0.25 Gyr for the age of NV CMa. The derived age is, however, very sensitive to the adopted metallicity of the cluster. We demonstrate that a meaningful determination of the ages of objects like NV CMa based on evolutionary models is possible only if their metallicity is know with a relative accuracy of a few percent. The distance moduli calculated for the components of NV CMa agree closely with each other, and imply an apparent distance modulus of the cluster of (m-M)_V=13.25+-0.08.