Embedded star clusters in the W51 giant molecular cloud

We present sub-arcsecond (0.35-0.9 arcsec), near-infrared J-, H- and K-band photometric observations of six fields along the W51 giant molecular cloud (W51 GMC). Our observations reveal four new, embedded clusters and provide a new high-resolution (0.35 arcsec) view of the W51 IRS2 (G49.5-0.4) region. The cluster associated with 648.9-0.3 is found to be a double cluster enclosed in a nest of near-infrared nebulosity. We construct stellar surface density maps for four major clusters in the W51 GMC. These unveil the underlying hierarchical structure. Colour-colour and colour-magnitude diagrams for each of these clusters show clear differences in the embedded stellar populations and indicate the relative ages of these clusters. In particular, the clusters associated with the H II regions 648.9-0.3 and 649.0-0.3 are found to have a high fraction of young stellar objects and are therefore considered the youngest of all the near-infrared clusters in the W51 GMC. The estimated masses of the individual clusters, when summed, yield a total stellar mass of similar to10(4) M-circle dot in the W51 GMC, implying a star formation efficiency of 5-10 per cent. These results, in comparison with the CO observations of the W51 GMC, suggest for the first time that star formation in the W51 GMC is probably triggered by a galactic spiral density wave.