Near‐Infrared Observations of the Giant HiiRegion W49A: A Star Birth Cluster

W49A is one of the most luminous giant H II (GH II) regions in our Galaxy.This star forming complex contains numerous compact and ultra-compact (UC) H IIregions, extending over an area of 15 pc. It emits about 10^{51} Lymancontinuum photons per second, equivalent to the presence of about 100 O stars,but it is completely obscured in optical wavelengths by interveninginterstellar dust. The center holds a ``cluster'' of about 30 O stars, eachwithin an individual UCHII region emitting free-free emission at cmwavelengths. Our deep K-band (2.2 um) image toward the W49A cluster revealsjust two of the individual exciting stars, each associated with a point--likeradio source, but the rest are invisible. These O stars are so recently born asto not yet have emerged from their natal dust cocoons, in contrast to otherGalactic clusters embedded in GH II regions in which many of the individualmassive stars are already revealed. Plausibility arguments are made whichsuggest that a stellar disc might be common during the entire UCH II phase ofmassive star birth, as it persists after accretion ceases in some stars.Nebular emission (e.g., from Br gamma) is visible around the periphery of thecentral region of W49A, along with candidate exciting stars. Star formationthere may have preceeded that in the center, or its lower density environmentmay have speeded up the dispersal of the natal dust cocoons. The W49A clustercan serve as a template for the more luminous buried star clusters now beingfound in normal galaxies and starbursts.Comment: latex, 18 pages, 4 figures, accepted by the Ap