Molecular Rings and the Thickness of Star-Forming Clouds

TL;DR

This study uses JCMT observations to analyze the structure of molecular clouds around star-forming bubbles, revealing they are flattened rings with implications for star formation processes.

Contribution

It provides the first detailed 3D structure of molecular clouds around bubbles, showing they are flattened and challenging existing star formation models.

Findings

Molecular clouds are flattened with a thickness of a few parsecs.

Column densities are below thresholds for 'collect and collapse' star formation.

Flattened clouds may inhibit star formation triggered by expanding bubbles.

Abstract

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.