Stars and planets form within dark molecular clouds, but little is understood about the internal structure of these clouds, and consequently about the initial conditions that give rise to star and planet formation. The clouds are primarily composed of molecular hydrogen, which is virtually inaccessible to direct observation. But the clouds also contain dust, which is well mixed with the gas and which has well understood effects on the transmission of light. Here we use sensitive near-infrared measurements of the light from background stars as it is absorbed and scattered by trace amounts of dust to probe the internal structure of the dark cloud Barnard 68 with unprecedented detail. We find the cloud's density structure to be very well described by the equations for a pressure-confined, self-gravitating isothermal sphere that is critically stable according to the Bonnor–Ebert criteria1,2. As a result we can precisely specify the physical conditions inside a dark cloud on the verge of collapse to form a star.
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We thank M. Lombardi for fruitful discussions and assistance, the Paranal Science Operations team for observing Barnard 68 with FORS1 on Very Large Telescope (VLT) Antu, R. West and E. Janssen for composing Figure 1 top, and R. Hook and R. Fosbury for composing Figure 1 bottom. We also thank M. Petr for helpful discussions during the preparation of the VLT observations. E.A.L. acknowledges support from a Presidential Early Career Award for Scientists and Engineers to the University of Florida.
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Alves, J., Lada, C. & Lada, E. Internal structure of a cold dark molecular cloud inferred from the extinction of background starlight. Nature 409, 159–161 (2001). https://doi.org/10.1038/35051509
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