Chemical composition of Galactic OB stars. II. The fast rotator ζ Ophiuchi

( Oph, HD 149757, is an 09.5 Vnn star with a very high projected rotational velocity (v sin i > 340 km s -1 ). It is also a classical runaway star due to its high proper motion. We perform a quantitative analysis of its optical spectrum in order to measure important observables of the star such as its mass, effective temperature, luminosity and He, C, N, and O abundances. Comparing these observed values to those predicted by the rotating evolutionary models of the Geneva group we find that none of the two sets of models (those of 2000 and 2003) is capable of reproducing the characteristics of the star. Nevertheless, due to its runaway nature, the reason for this discrepancy may be that the star is not the result of the evolution of a single object, but the product of the evolution of a close binary system.