Near-infrared observations of the giant HII region W49a: A star birth cluster

W49A is one of the most luminous giant H II regions in our Galaxy. This star-forming complex contains numerous compact and ultracompact (UC) H II regions, extending over an area of approximate to15 pc. It emits about 10(51) Lyman continuum photons s(-1), equivalent to the presence of about 100 O stars, but it is completely obscured in optical wavelengths by intervening interstellar dust. The center holds a "cluster" of about 30 O stars, each within an individual UC H II region emitting free-free emission at centimeter wavelengths. Our deep K-band (2.2 mum) image toward the W49A cluster reveals just two of the individual exciting stars, each associated with a pointlike radio source, but the rest are invisible. These O stars are so recently born as to not yet have emerged from their natal dust cocoons, in contrast to other Galactic clusters embedded in giant H II regions, in which many of the individual massive stars are already revealed. Plausibility arguments are made that suggest that a stellar disc might be common during the entire UC H II phase of massive star birth, since it persists after accretion ceases in some stars. Nebular emission, e.g., from Brgamma, is visible around the periphery of the central region of W49A, along with candidate exciting stars. Star formation there may have preceded that in the center, or its lower density environment may have speeded up the dispersal of the natal dust cocoons. The W49A cluster can serve as a template for the more luminous buried star clusters now being found in normal galaxies and starbursts.