JCMT observations of cometary clouds in the Galactic chimney near W4

Recently, Normandeau and coworkers reported the discovery of a Galactic chi mney emerging out of the Galactic plane above a cluster of O stars associat ed 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 indi cates that the clouds have been subject to intense UV radiation for several Myr. We have obtained high-resolution images of the compact "heads" of the se clouds in the CO (2-1) line using the James Clerk Maxwell Telescope (JCM T). 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 th e symmetry axis of the "arrow" pointing toward the most luminous cluster st ar. A large velocity gradient analysis, carried out using both the JCMT dat a and CO (1-0) observations from the Five College Radio Astronomy Observato ry, yields a temperature of 30 K and a density of 10(4) cm(-3) for the mole cular gas at the apex. The total mass of molecular hydrogen in the region s urveyed in (CO)-O-13 (5.3 pc(2)) is similar to 800 M-circle dot. The veloci ty 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 morpholo gy and kinematics of the globule are in good agreement with the hydrodynami cal simulations of photoevaporation of cometary globules, although for our more massive cloud the survival timescale is much longer.