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C.J. Lada (CfA), J. Alves (ESO), E.A. Lada (Astronomy Dept, U. Florida), E.A. Bergin (CfA)
Stars and planets form within dark molecular clouds. However, despite 30 years of study little is understood about the internal structure of these clouds and consequently the initial conditions that give rise to star and planet formation. This is largely due to the fact that molecular clouds are primarily composed of molecular hydrogen, which is virtually inaccessible to direct observation. Here we report the application of a powerful observational technique that takes advantage of measurements of background starlight extincted by trace amounts of dust to probe the internal structure of these objects. We use deep infrared imaging observations of the dark cloud Barnard 68 to derive the most finely sampled and highest signal-to-noise density profile ever obtained for a dense molecular cloud. We find the cloud's density structure to be extremely well described by the equations for a pressure confined, self-gravitating isothermal sphere, which is critically stable according to the Bonnor-Ebert criteria. For the first time, the internal structure of a dark cloud has been specified with a detail only exceeded by that characterizing a stellar interior. As a result we are able to precisely specify the physical conditions of a dark cloud on the verge of collapse to form a star. For example, we are able to derive an accurate measurement of the gas-to-dust ratio in a dense cloud core and find it to be close to the standard interstellar value. Finally, we present millimeter-wave observations of rare isotopes of CO and CS which provide compelling evidence indicating that this cloud is supported primarily by thermal pressure.