Molecular Rings and the Thickness of Star-Forming Clouds
  The winds and radiation from massive stars clear out large cavities in the
interstellar medium. These bubbles, as they have been called, impact their
surrounding molecular clouds and may influence the formation of stars therein.
Here we present JCMT observations of the J=3-2 line of CO in 43 bubbles
identified with Spitzer Space Telescope observations. These spectroscopic data
reveal the three-dimensional structure of the bubbles. In particular, we show
that the cold gas lies in a ring, not a sphere, around the bubbles indicating
that the parent molecular clouds are flattened with a typical thickness of a
few parsecs. We also mapped 7 bubbles in the J=4-3 line of HCO+ and find that
the column densities inferred from the CO and HCO+ line intensities are below
that necessary for "collect and collapse" models of induced star formation. We
hypothesize that the flattened molecular clouds are not greatly compressed by
expanding shock fronts, which may hinder the formation of new stars.
