Shivering Cloud

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Science  21 Mar 2003:
Vol. 299, Issue 5614, pp. 1817
DOI: 10.1126/science.299.5614.1817b

Although it is uncertain how or why molecular clouds form in space, many clouds are thought to be important nurseries of star formation through collapse or mergers. A particular group of clouds, the Bok globules, are small, cold molecular clouds that appear to be stable against collapse and do not contribute appreciably to star formation. Lada et al. wanted to know why one particularly well-studied Bok globule, Barnard 68, was so stable, so dark, and so averse to stellar production.

They determined the dynamical state of Barnard 68 using high-resolution molecular-line observations from the 30-meter Institute de Radio Astronomie Millimetrique (IRAM) telescope in Spain. Although turbulence is the common stabilizing force against gravity in most clouds, the spectra confirm that Barnard 68 is stabilized by thermal pressure. More intriguing is the unusual pattern in the emission lines from C18O, N2H+, and C32S that suggests that the outer layers of the cloud are pulsating and that gas in the interior is rotating more slowly than gas in the exterior, which appears to create an odd differential rotation. The oscillations of the outer layers may be caused by the passage of a supernova shock wave, like a bell set to ringing by the ping of a hammer. Nonetheless, although a supernova may have sent a shiver through Barnard 68, it remains cold to star formation and dynamically stable. — LR

Astrophys. J., in press; available at arXiv: astro-ph/0211507 (2003).

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