2002ApJ...576..313K

We present images in H₂ emission (λ=2.12 µm) of seven massive young stellar objects, including four candidate precursors to ultracompact H II regions. These images reveal, for the first time, the presence of dense clusters of young stars in the regions around Onsala 1, IRAS 22134+5834, and IRAS 23033+5951. H₂ line emission is detected in all seven regions, and we find that the H₂ emission arises in physically distinct regimes: in the shocked regions of the outflows and in extended (a few thousand AU) roughly circular structures close to the near-infrared sources. While most H₂ features can be attributed to shocked or fluorescent emission, the exact nature of the excitation calls for further spectroscopic investigation. A detailed comparison of the H₂ images to the outflow CO maps recently reported by Beuther et al. provides evidence that outflows from massive stars can be jetlike or of poor collimation (wide opening angle at the base), as may be the case for low-mass stars. In particular, IRAS 05358+3843 provides a prime example of a jet-driven outflow; it displays most of the ingredients of this kind of object (including a prominent bow shock with its Mach disk tentatively identified in H₂). Some of the outflows are observed to be distorted and reshaped by their interaction with the dense environment found in the vicinity of newly formed high-mass stars. The morphologies of these flows seem to be affected by associated stellar clusters that can partly account for the lack of known well-collimated flows from massive stars compared to that of low-mass stars.