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_⊙.

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 67.C-0514 and 073.D-0837, and on data acquired using the Large Binocular Telescope (LBT).The reduced VLT FITS spectrum is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/568/L13