Massive star formation in the W49 giant molecular cloud: Implications for the formation of massive star clusters

We present results from JHKs imaging of the densest region of the W49 molecular cloud. In a recent paper (\cite{AH03}, ApJL, 589, L45), we reported the detection of (previously unknown) massive stellar clusters in the well-known giant radio HII region W49A, and here we continue our analysis. We use the extensive line-of-sight extinction to isolate a population of objects associated with W49A. We constrain the slope of the stellar luminosity function by constructing an extinction-limited luminosity function, and use this to obtain a mass function. We find no evidence for a top-heavy MF, and the slope of the derived mass function is -1.6 ± 0.3. We identify candidate massive stars from our color-magnitude diagram, and we use these to estimate the current total stellar mass of 5-7×104 M⊙ in the region of the W49 molecular cloud covered by our survey. Candidate ionizing stars for several ultra-compact HII regions are detected, with many having multipe candidate sources. On the global molecular cloud scale in W49, massive star formation apparently did not proceed in a single concentrated burst, but in small groups, or subclusters. This may be an essential physical description for star formation in what will later be termed a ``massive star cluster''.