The discovery of a very massive star in W49

Context. Very massive stars (M> 100 M⊙) are very rare objects, but have a strong influence on their environment. The formation of this kind of objects is of prime importance in star formation, but observationally still poorly constrained. Aims. We report on the identification of a very massive star in the central cluster of the star-forming region W49. Methods. We investigate near-infrared K-band spectroscopic observations of W49 from VLT/ISAAC together with JHK images obtained with NTT/SOFI and LBT/LUCI. We derive the spectral type of W49nr1, the brightest star in the dense core of the central cluster of W49. Results. On the basis of its K-band spectrum, W49nr1 is classified as an O2-3.5If* star with a K-band absolute magnitude of –6.27 ± 0.10 mag. The effective temperature and bolometric correction are estimated from stars of similar spectral type. After comparison to the Geneva evolutionary models, we find an initial mass between 100 M⊙ and 180 M⊙. Varying the extinction law results in a larger initial mass range of 90–250 M⊙.