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JCMT Observations of Cometary Clouds in the Galactic Chimney near W4
Recently, Normandeau and coworkers reported the discovery of a Galacticchimney emerging out of the Galactic plane above a cluster of O starsassociated with the W4 H II region. Two compact molecular clouds havesurvived the evacuation of the chimney and show evidence of dynamicaleffects from the energetic radiation of the nearby O stars. The age ofthe star cluster indicates that the clouds have been subject to intenseUV radiation for several Myr. We have obtained high-resolution images ofthe compact ``heads'' of these clouds in the CO (2-1) line using theJames Clerk Maxwell Telescope (JCMT). We have also obtained CO (3-2)over a more limited area for one of the clouds. Our observations reveala striking arrow-shaped CO globule, with the symmetry axis of the``arrow'' pointing toward the most luminous cluster star. A largevelocity 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 10^4 cm^-3 for themolecular gas at the apex. The total mass of molecular hydrogen in theregion surveyed in C^18O (5.3 pc^2) is ~800 M_solar. The velocitygradients about the symmetry axis of the arrow are well represented by asimple model of symmetric advance about the cloud center of anionization front driven by the UV radiation of the nearby O star. Theoverall morphology and kinematics of the globule are in good agreementwith the hydrodynamical simulations of photoevaporation of cometaryglobules, although for our more massive cloud the survival timescale ismuch longer.
| The chemical evolution of the solar neighborhood. I - A bias-free reduction technique and data sample
The possible ways of measuring the age-metallicity relation for thegalactic disk in the neighborhood of the sun are discussed. It is shownthat the use of a field star sample chosen on the basis of effectivetemperature introduces a bias which results in a monotonic increase inthe metal abundance of the disk with time. However, if theage-metallicity relation for the disk can be shown to satisfy certaincriteria, the bias introduced in such a sample can be neglected: thegalactic disk apparently satisfies the criteria. It is concluded that asample analyzed through the use of uvby and H(beta) photometry inconjunction with a self-consistent set of theoretical isochronesprovides the least biased, most accurate estimate of the age-metallicityrelation for the disk.
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