1999ApJ...513..339T

Recently, Normandeau and coworkers reported the discovery of a Galactic chimney emerging out of the Galactic plane above a cluster of O stars associated with the W4 H II region. Two compact molecular clouds have survived the evacuation of the chimney and show evidence of dynamical effects from the energetic radiation of the nearby O stars. The age of the star cluster indicates that the clouds have been subject to intense UV radiation for several Myr. We have obtained high-resolution images of the compact ``heads'' of these clouds in the CO (2-1) line using the James Clerk Maxwell Telescope (JCMT). We have also obtained CO (3-2) over a more limited area for one of the clouds. Our observations reveal a striking arrow-shaped CO globule, with the symmetry axis of the ``arrow'' pointing toward the most luminous cluster star. A large velocity gradient analysis, carried out using both the JCMT data and CO (1-0) observations from the Five College Radio Astronomy Observatory, yields a temperature of 30 K and a density of 10⁴ cm^–3 for the molecular gas at the apex. The total mass of molecular hydrogen in the region surveyed in C¹⁸O (5.3 pc²) is ∼800 M_☉. The velocity gradients about the symmetry axis of the arrow are well represented by a simple model of symmetric advance about the cloud center of an ionization front driven by the UV radiation of the nearby O star. The overall morphology and kinematics of the globule are in good agreement with the hydrodynamical simulations of photoevaporation of cometary globules, although for our more massive cloud the survival timescale is much longer.